Power-One Power Supply Hackers Page
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below also links to the same .ZIP file.
-- Dave
Power-One has built open frame linear power supplies since the
70's. These workhorses have been built nearly the same way for so
many
years, and there are many available on the new and surplus
markets. The design is simple enough, the technology is
understandable to someone with basic electronics background. In
other words they are ideal for hacking. What can be done with
them? In addition to using them to learn about power supply
theory, I have hacked many power supplies and find these to be
fun and easy. I describe simple hacks from adding a meter or two,
widening the voltage range, coaxing them to do adjustable current
limiting. All these can be done with a few external parts. My
grand finale is to eliminate the ancient uA723 regulator and
replace it with a proper full range, precision voltage / current
limit design with good precision monitors and even a PC to control
and
monitor it.
Modified, they can be used as lab supplies, battery chargers, or
any place a lab supply is needed. They can even be scavenged for
parts or better still, subsections, they can be re-packaged into
a different chassis.
Here is a 24V 2A supply that I hacked years ago and have used as
a lab supply ever since. I set the voltage range to 4V to 25V,
removed a second board and drilled the chassis to make room for
the AC line cord, switch and fuse, binding posts, and a Digital
panel meter and 10 turn pot. I mounted rubber feet on the bottom
to keep it from scratching up the bench and also to isolate its
heat from the bench. If I did it again, I would add a separate
panel for these additions and leave the original board intact. It
provided useful +/-5VA outputs.
Each Power-One supply has everything that a linear power supply
needs: An unregulated DC power source, an output stage capable of
dissipating some power, a regulator containing a reference
voltage, a difference amplifier and a current limiter. After 35+
years
they all still use the DIP
version of the venerable uA723 Voltage Regulator. What else do
they all have in common?
- Raw DC supply: AC Transformer,
diodes and filter capacitors
- AC can be 100, 120 or 240V, 50 0r 60Hz
- Beefy TO-3 power transistors, 2N3055 or equivalent, or power
darlingtons
- Current limiting, generally foldback
- Output voltage adjustable over a narrow range, typically
± 15%
- Some power (efficiency) management, usually a boost supply a
little higher
than the main supply
- Sturdy 3 or 4 sided 1/8" aluminum chassis
- Voltages from 3.3V to 28V and higher. Most common are 5, 12,
15,
24, 28V
- Some are unregulated
- Currents from <1A to 50A
- Single, dual (sometimes +/-) and triple configurations
- Overvoltage protection as an option, sometime on a separate
board
Condor and International Power also build nearly identical
supplies. Other manufacturers may also. When you are searching
for a supply on Ebay, use all the vendor names in your searches.
The differences are subtle, if any.
Caveats:
Needless to say this page is not sanctioned by any power supply
manufacturer. By disassembling and modifying a supply you will of
course void the warranty, and you may never be able to get your
power supply back to its original splendor. You may even blow
something up. These things operate from raw AC line voltage which
can be LETHAL. I *STRONGLY* recommend that you:
Insulate all AC wiring with heat
shrink or electrical tape
Add a power switch and fuse of the correct value
Be careful when operating these or any power supplies
Regarding efficiency, linear supplies are quite inefficient, and
therefore burn holes in the ozone layer, cause more coal-burning
power plants to be built, and speed the decline of civilization.
They are fine for low power applications or occasional use such
as a lab supply. But if you're going to keep any power supply
heavily loaded for long periods of time, please do the
environment a favor and use a nice efficient switcher. Your
electric bill will also appreciate it. While you're at it,
replace your antique incandescent light bulbs with compact
fluorescent bulbs. Each bulb saves more power than a typical
linear supply wastes.
The Supplies
Here is a simplified circuit of a Power-One supply. This is a
+15V, 3A supply similar to the HC15-3. 
Here is the full schematic of
the HC15-3 in ExpressPCB format as well as in .PDF.
Here's a photo of a HC15-3 below.
You can get ExpressPCB's excellent software for free at
www.expresspcb.com.
Most units have current sense resistors to enable current
limiting. These are typically 2-3Watt power resistors, less than
1 ohm, wired from the output transistor's emitter to the
supply output. Some supplies use 2 or more resistors in parallel
to achieve low resistance and higher power ratings. The resistors
are connected to the current sense pins on the uA723. These
inputs are simply the base and emitter of an internal transistor.
When the voltage from pin 3 to pin 2 exceeds about 0.65V, the
internal transistor conducts, and pulls the output down. I
discuss the details of this further on.
Higher current supplies use multiple output transistors in
parallel. To achieve proper current sharing, each transistor has
its own emitter resistor. Without the resistor, the
hottest transistor would have the lowest Vbe which would cause
even higher collector current, causing it to
get even hotter. This effect is called thermal runaway and
ultimately can cause failure of an output transistor as it hogs all
the supply current. Emitter resistor help to balance these currents.
When multiple transistor and emitter
resistors are used, the current limit sense is taken from a
single resistor with the assumption that the currents are all close
to
the same.
Most supplies have an extra 'boost' supply which is
typically a half-wave rectifier and cap that provides a bit more
voltage
than the main supply. This is to provide the few extra volts that
the regulation circuit and output transistor requires without
increasing the main
supply. If the main supply were simply increased, the output
transistor would need to burn the power from that extra voltage
drop.
Lower voltage supplies
simply double the V+ with an extra diode and cap. Some like the
one below use a 'bootstrap' approach where a separate
winding of the transformer provides a 7-10V DC extra voltage.
This voltage is connected in series with the supply output, thus
providing a voltage equal to 7-10V more than the output
voltage.
Most are built around single-sided phenolic PC boards.The usual
mounting method is via the TO-3 transistor mounting screws. The
board can be removed by:
Unsoldering the pins of all T0-3s
with a solder-sucker
Removing the screws that mount the T0-3s.
Carefully seeing where all the insulation hardware goes so you
can replace it later.
Some newer units use socket pins for the TO-3s, so you don't
need to unsolder the transistor to remove the board.
Remote voltage sense is provided on some supplies. Look for an extra
pair
terminals near the voltage output terminals. Remote sense is a
"Kelvin" or 4-wire connection used to compensate for
voltage drop in the power wiring by sensing the voltage at the
load instead of at the power supply. A second pair of terminals
is provided, and these are wired to the remote load. These
terminals are used by the supply to measure the output voltage,
so the + side is typically the voltage feedback to the voltage
divider, and the - side is the V- or GND terminal of the uA723. Low
value resistors are provided between each sense terminal and
its power terminal so that the supply will still operate when the
remote sense inputs are disconnected. The schematic
of the HC15-3 in .PDF shows
this.
Over-Voltage Protect is provided on some supplies. It is also
called a "Crowbar" because it shorts the output of the
power supply in the event of a fault that increases the output
voltage too much. For example, on a +5V supply, the overvoltage
is set to about +6.2V. This is implemented with an SCR and
voltage sense circuit consisting of a Zener diode and resistors.
When the voltage increases, the SCR turns on and shorts the
output of the supply. If the supply is functioning it will
current limit. If the supply's output transistor has shorted,
the current limit won't function, and the AC fuse should blow
due to a direct short on the power supply.
Some supplies use circuitry on the board for this. Some use an
external board wired to the output terminals. When hacking a
supply, you typically don't want it to ever crowbar, so I
recommend disabling the overvoltage circuit. This can be done by
simply disconnecting an external overvoltage, or by removing the
SCR on the on-board types. The SCR is typically a TO-220 device
located near the outputs. Unsolder and unscrew it, or clip its
leads.
uA723
The uA723 (same as the LM723) voltage regulator has been around
since the 60's. It is the core, the brain, the technological
center, if you will of each Power-One supply. It contains all of
the building blocks of a voltage regulator: voltage reference,
difference amplifier, power stage, and current limiter. Below is
a block diagram, courtesy of TI. Here is the uA723 full data sheet.
This IC is very flexible and low cost. But it does have
limitations. The voltage reference is nominally 7.15V. The
minimum power supply to make this reference and the rest of the
device operate is about +12V. The maximum voltage is +40V.
The inputs of the difference amplifier have a minimum (common
mode) voltage range of +2V. This means that the output voltage
cannot easily go below +2V without playing games. Most Power-one
supplies operate these inputs between +5V, the lowest standard
voltage output available, and the +7V reference. In fact there is
a spec in the '723 data sheet that says that the + and -
inputs must see no more than +/- 5V. So keep these signals
between 2V and the 7V reference.
The '723 output voltage can be about 2.5V below the V+ and VC
pins. Internally, the '723 uses a current source followed by
two emitter followers and can output a maximum of 150mA. In
Power-One supplies this is used to drive external TO-3 output
transistors that boost this to a couple of amps. Most supplies
use a 2N3055. The minimum beta (current gain) of a 2N3055 is
about 25, so this 150mA could theoretically be amplified to about
25 x 150 = 3.75A in a single TO-3. It is a bad idea to operate
the '723 at this maximum current, and 50 mA is more
reasonable maximum. The maximum power of the '723 is about
.75W, and at 150ma this only allows .75W / .15A or 5V of drop
between the V+ and the output. Most power-one supplies operate at
fairly high voltage drop.
For example, the +15V 3A supply that I have uses another
transistor, a TO-220 TIP29 between the '723 and the 2N3055.
Figure the TIP29 has an additional beta of 25. So the '723
provides 3A / (25 * 25) or only 5mA. Another supply I own uses a
darlington output transistor type 2N6059 to provide the higher
current gain required.
To identify if your supply uses Darlingtons or vanilla NPNs and
assuming you can't just read the number off the TO-3 and find
a data sheet, load the supply with a nominal load (~0.5 A) and
measure the voltage drop from the base to emitter terminals of
the TO-3. If you get 0.6 to 0.8V, it's a single transistor.
If you get 1.1 to 1.5V it's a darlington.
About 2 or 3 amps is the maximum you want to draw from a single
TO-3 without forced air cooling. A 2N3055 is rated for 60V, 10A
and 115W, but 115W is only under ideal conditions: with an
infinite heat sink and at 25C. Lower current, up to 5A, Power-one
supplies have only the aluminum case as the heat sink. Larger
supplies up to 50A often use additional finned heat sinks and
encourage forced air cooling. Needless to say a 50A linear is a
generally bad idea due to the power waste or hundreds of
watts at full power.
I estimate the case heat sink to have a thermal resistance of
about 3 degrees C / Watt without a finned heat sink and without
forced air cooling. To keep the temperature rise to +50C (+25C
ambient = 75C, quite hot!) , the transistor can dissipate 50C /
3C/W = ~17 Watts. At 3A, the transistor can only drop 17W / 3A =
~6V.
If you modify a supply and want to operate at lower voltages, you
will drop much more voltage across the transistor and burn more
power.
Lets say you want to operate a +15V, 3A supply at +5V. At the
same 3A, the output transistor will burn (15V -5V) * 3A = 30W
more. The additional temp rise at 3C/W is +90C. Ouch! Without
forced air cooling you will need to derate the current of the
supply by 1/3 to 1/2. It's a bit counter-intuitive: this
supply will output 3A at its full +15V, but at 5V only lower
currents.
Keep in mind that the transformer / cap / diodes can still
provide the full 3A at any output voltage. For that +15V 3A
supply, they provide an unregulated +20V or so at 3A or 60W. The
unregulated parts don't care if most of the 60W is being
dissipated on the heat sink or in an external load. Well, they do
care a bit since they also are mounted to the same heat sink, and
when it gets hot, so do they.
Another way to manage more heat is to remove the T0-3
transistor(s) and move to a larger, external heat sink. The
device can be removed and simply wired via the traces on the
board to the transistor located within 6-12" away. Once you
do this, the Power-One case is no longer needed: remove the board
and transformer and mount them wherever you want. After removing
all the parts, you could even saw the case in pieces to give you
separate brackets for mounting the transformer and the board
and/or heat sink. If you do this, there are things to watch for:
the 2 (or more) transistor screws are the board mounting. This is
convenient if you have a transistor and insulator, but not
convenient if you moved them elsewhere. Since these transistor
connections are still electrically hot, you need to use insulated
washers to mount the board to anything conductive. Also on some
designs, the 2 screw leads of the TO-3 are used as electrical
connections on the board. Once you remove the transistor, these
connections no longer exist. If you see traces on the board
connected to both screw mounts, solder a wire from one screw
mount to the other.
Increase the Voltage Range
Before you do the voltage range mod or the constant current mod,
realize that both of these will cause a larger voltage drop
across the power transistor at high currents. This is the main
source of power and therefore heat in the supply. Get the
transistor too hot at too high a power dissipation, and it will
fail, usually by shorting its collector-to-emitter. This causes
the supply to output its full unregulated voltage. Since you
don't want this to happen, it is important to watch the
maximum temperature at high loads, and particularly at low
voltages. There are 2 basic ways to keep it cool: one is to apply
forced air via a fan. The second is to limit the current at low
voltages.
The voltage output of a Power-one is set with resistors and a
trimpot. Older units use a big metal trimpot and newer ones use
smaller plastic ones. Depending on whether the supply is rated
for >7V or <7V, the circuit is wired differently. For Vout
>7V, the +7V reference pin is wired directly to the + input. A
resistive divider with the trimpot reduces the output voltage to
7V to be applied to the - input. The trimpot allows the output to
be accurately set and allows for some adjustment range.
(Sometimes a single supply is rated for either 12V and 15V
operation and the trim is used to set the voltage. These are
typically derated for lower currents for +15V operation than for
12V.)
For voltages <7V (usually 5V) , the Vout is wired directly to
the - input and the reference is attenuated to the output voltage
by a divider and trimpot.
The >7V supplies are easier to hack. One thing you typically
want is a wide output voltage range. I like to use the 15V or 24V
supplies at 2 or 3 Amps to build lab supplies. I want the output
to go all the way to 0, but the '723 won't allow this
easily. The '723 spec is +2V but I like margin and don't
often need to go below +2.5V. To do this, attenuate the reference
from +7.15V to +2.5V by adding two resistors. The resistor ratio
is (7.15 - 2.5) / 2.5 or1.86 : 1.00. 1.00K to ground and and
1.87K to the reference will work. Now the output divider can go
from 2.5V to slightly higher than its rated V. So for a +15V
supply, lets push it up to +16V. The divider here is (16V - 2.5V)
/ 2.5V or 5.40:1 The large value is a pot or trimpot, so pick its
value first and select the small one to match. For 16V, a 10K pot
will draw <1.6ma of output current and <25mW. We probably
want to use a 10 turn pot and these are readily available. One
turn pots don't allow adjustment to 3 1/2 digits easily. Some
people don't like 10 turns since in a panic they take a long
time to turn down, so you could use a 10K, 1 turn in series with
a another 1K in a coarse / fine arrangement. Set the fine pot to
mid scale, adjust the coarse to get close, and then tweak the
fine. The total 16V range here would be 11K, not 10K.
So 10K * 1 / 5.4 is 1852 ohms Use a 1.82K ohm 1%. Mount the
pot(s) and the panel meter(s) on a scrap of sheet metal which can
be mounted to the supply or mounted remotely. Remove the resistor
in series with the PowerOne voltage trimpot and the low side
resistor. Depending on your unsoldering skills you may remove the
trimpot too. Replace the low divider resistor with the 1.82K, and
the pot plus the other resistor with 2 wires. Then run
wires to the external 10K pot. Remember to wire the unused (CCW)
terminal to the wiper for safety in case your pot wiper gets
noisy and intermittently opens up. This will cause the supply to
go to + infinity which is likely to do bad things to a load. This
schematic of the HC15-3 shows the
original circuit plus the changes.
If your new supply range is >19.99V, it cannot be displayed on
a 3 1/2 digit meter at full resolution. So either use a 199.9V
range and only have .1V of resolution, or add a range switch to
the meter so at lower than 20V you can get 0.01V resolution. This
is what I did on my first supply.
Current Limiting
In order to accurately limit current, it is generally required to
measure the voltage across a low value resistor in series with
the load. If this voltage exceeds a preset limit, then the supply
outputs reduced voltage until the current drops to a safe value.
Power-One supplies do this with a single NPN transistor in the
'723. Its emitter is tied to the power supply voltage output
and its base goes to a current sense resistor. In this way, if the
voltage across the sense resistor increases to about +.7V, the
transistor begins to conduct, reducing the output voltage. This
is a crude current limit, intended to protect the power supply
from a short circuited output load. However, this type of current
limit causes the output transistor to dissipate the full voltage
and current which is a large value of power, causing it to
overheat and possibly fail. So an improved method is needed. This
is called foldback current limiting. With this approach, when a
supply goes into current limit, the current is reduced to a safe
value, typically 1/2 or 1/3 of the limit value. removing the load
allows instant recovery.
The '723 can be outfitted for foldback by simply adding a
resistor divider to the current sense input such that as the
output voltage drops, the current threshold also drops. This
basic circuit is shown above. Another problem with this circuit
is that it depends on an uncompensated Vbe voltage which varies
about -2mV/ degree C. And also without varying the low-value
current sense resistor, there is no easy way to reduce the
current limit value. However, Power-One has come up with a clever
solution, shown in the schematic of
the HC15-3. Note the current limit trimpot, and how it is
connected to the base of the output transistor, not directly
across the current sense resistor. In this way, the sense
resistor plus the output transistor's Vbe are being measured.
These are compared to the trimpot voltage plus the 723's
current sense transistor Vbe. Since the Vbe's kind-of cancel
out, the voltage across the shunt is compared to the voltage
across the trimpot. By reducing the trimpot's resistance, the
current limit can be reduced also. The pull-down resistor and
diodes provide current to the trimpot and tailor the foldback
current limiting characteristics. Very clever.
However, for a lab supply, a precision constant current mode is more
desirable than current foldback. In constant current, a control
is used to adjust the maximum output current of the supply, and
the supply will dutifully and accurately output this current at
any voltage from 0V up to the voltage setting. To do this
accurately requires an accurate measurement of current, a
precision setting, and a control loop to take over the power
supply when the current limit is reached. But the Power-One
circuit can be easily hacked to provide a decent if not
super-accurate current limiter. To eliminate the foldback
limiting and allow constant current control, all that is
necessary is to regulate the voltage across the trimpot by
regulating the current through the trimpot. This can be done by
replacing R6 and its 2 diodes with a 1mA constant current source
to ground. Now, the 500 ohm trimpot will have a maximum of 1mA *
500 = 0.5V, the maximum voltage desired across the shunt
resistor. By varying the trimpot down, the voltage across the
shunt is also reduced. This circuit isn't perfectly precise,
but can be set to any desired value from about 10% to 100% of the
supply's current rating. It will drift a bit as the power
transistor heats up though. But it's a lot better than
nothing and can be used for many applications. I built this up
and found that the Vbe of the current measuring transistor was a
stable 646mV. But the Vbe of the 2N3055 varied quite a bit with
current: from about 0.5V at 0 current to 0.8V at 2.5A current.
The 0.5V came as a surprise to me. This caused the minimum
current setting to be about 0.3A, not 0 as I had hoped. The shunt
resistor in the HC15-3 was 0.12 ohms. With this change the
current could be varied from 0.3A to about 2.3A. And it was
pretty stable when in current limit.
For a 1mA current source, the National LM334 can be used. It is a
resistor-programmable current source in a TO-92 package. A single
68 ohm resistor is used to set its current. You may have to mess
with the 68 ohm resistor value to get the current ranges you
want.
This is a crude variable current limiter. A precision one is shown
below.
Output Monitoring
To monitor and adjust voltage, a DMM can be used across the
output terminals. Current monitoring with a DMM can be done two
ways: 1) Wire a DMM in current mode in series with the output, or
2) Monitor the voltage across the shunt resistor and solve ohms
law.
Low cost 3 1/2 digit LCD and LED digital panel meters (DPMs) can
be hard-wired to serve these functions. For voltage monitoring,
use a 5V common ground (not a 9V) compatible DPM. Wire its input
as a 19.99V or 199.9V range and set the decimal point
accordingly. I like the All Electronics PM-128E for this
application. It is flexible and cheap ($12.25). It needs a +5
supply at low current. I cheated on one system and used
the +7.1V reference to power the DPM. It has worked for years,
but I suspect that that DPM was specified to operate from +7V. A
better solution is to use a 5V regulator such as the 78L05,
powered from V+. A TO-92 regulator and bypass cap can be soldered
right onto the DPM.
For current metering there are several choices. The simplest is
to use an analog uA meter wired directly across the shunt with a
scaling resistor to handle the 0.5V full scale. The problem with
analog meters is that to get the readout scaled the way you want
may require taking it apart and marking up the scale. Too much
work considering that you still have an inaccurate (5%) current
reading.
There are a few ICs on the market that allow current measurements
across a shunt resistor, and then provide a nice
ground-referenced voltage output. This can then be measured with
a DPM or even the same DPM you use for voltage measurement if a
selector switch is provided. Some lab supplies use this approach.
Problem here is that most of these current measure ICs can't
handle the wide range ( 0 to +20 or so volts) of a lab supply.
Analog Devices recently announced (as of 7/07) one that should
work. A differential amplifier or instrumentation amp (INA) can
be used. Watch out for common-mode rejection and voltage range.
This must accurately measure 0 to 0.5V across the shunt while its
input common mode varies from 0V to +25V or higher. This needs
CMRR in the 80+ dB range or resistors matched to 0.01%. Most
monolithic INAs cannot handle both the high voltages required
plus the need for the inputs and output to go all the way to
ground. Most will need a negative power supply. -5V will do.
The voltage across the shunt can be directly measured by a DPM
with its ground pin connected to the power supply output, but a
floating +5V power supply is needed for this approach. Since many
power-one supplies have a floating +V supply in the +7-10V range,
this may be able to be regulated down to Vout + 5V and may work
with some supplies. The HC15-3 is one example.
Accurate high-side current measurement is one of the gnarly
problems in building a good lab supply. Many commercial designs
put the shunt resistor in the ground path to simplify this
problem.
Power Management and Efficiency
Power management is one of the tougher design issues on a linear
supply. The goal is to minimize power wasted while still meeting
all the specs for AC line voltage and frequency, and load. The
unregulated DC voltage will drop as the AC line voltage drops. It
will also drop as the load current increases due to transformer
and diode losses. At no load, the ripple voltage will be low, but
will increase at high current and at low (47 Hz) line frequency.
A regulator has a 'dropout voltage' which is the minimum
input voltage at which it will regulate. Typically this is due to
the output transistor voltage drops (one Vbe + 1 Vsat ) = ~ 0.7
+1.0 , the shunt resistor drop, plus wiring drop, all at the
maximum output voltage setting. For a +15v supply set to +15.2V,
this is Vcmin:
+15.2V + 1.7V ( transistors) + 0.5V (shunt) + 0.3V (wiring) =
17.7V.
The negative peaks of the ripple voltage on should not drop below
this voltage. On the high end, the '723 has a maximum V+
voltage of +40V. Its V+ is usually a higher (boosted) voltage
than the Vc by 5-10V and is done simply to accommodate the
additional 3Vor so of dropout that the that he uA723 needs.
Power-One supplies such as the HC15-3 are specified to operate
from 104 to 132 VAC, 47 to 60Hz when jumpered for 120VAC. If you
can accept a narrower input voltage and 60 Hz only, you can
operate with a bit less dropout voltage margin.
Adjustable DC Load
To test power supplies, a load of some kind is needed. This can
be as simple as a handful of different value power resistors. The
gold anodized power resistors made by Dale and others work great.
These can be screwed to an aluminum plate or other heat sink to
keep their temperatures low. I keep some 1.0, 5.0 and 10.0 ohm,
10W and 25W values around. Put enough of these in series and
parallel and you can load down a power supply. But I never have
the exact right values and changing the load usually involved
soldering. So I built an electronic load. This load applies a
constant current to a power supply of between 3V and 30V. My
original one used a simple power N-FET bolted to a big heat sink,
an op-amp, shunt resistor and voltage reference. It was self
powered from 20V down to about 5V and designed for up to 4A
loads. Over the years it was upgraded to 10A. Now I use a surplus
IGBT which will dissipate 10A at 12V or more (120W) with
impunity. I think it's rated to switch 75A at 400V. At 120W,
a fan is a must. The circuit is quite simple. Build the
electronics on a little Radio-Shack proto-board and mount it on
the biggest heat sink in your junk drawer.
The 1.25V reference diode, 15K resistor, and 10K pot develop a
stable 0 to 0.5V. The single-supply op-amp and FET apply the
voltage to the 0.05 ohm resistor. 0.5V across 0.05 ohms is 10A
maximum. You can parallel multiple larger resistor values if you
have trouble finding a 0.05 ohm 10W resistor. To build a 5A load,
use a 0.1 ohm, 5W resistor instead. The only trick in building
this is to wire the high current path with heavy gauge (16GA or
more) wire and treat the 0.05 ohm resistor as a 4-wire device:
heavy traces for the high current path, lighter wires soldered to
the leads near the body for the voltage measure path. Here is the
ExpressPCB schematic. and the
.PDF. version. For extra credit,
use the unused op-amp and a thermistor to detect when the heat sink
gets hot and
turn the fan on. The circuit as shown shouldn't be used at
more than 18V or so since the full supply can be applied to the
FET gate, These are usually rated for 20V max. By removing D3 and
always using the external 12V supply, this limitation is removed
and the voltage can go up to anything the FET can handle. Watch
out for maximum power of the FET though. And remember to de-rate
the FET power at high temperatures.
The top binding posts are for the load and the bottom ones for a
current monitor via a DMM. The 3 resistors on the right are just
spares. Note the big black IGBT in the background, this is in
place of the FET in the schematic, but a couple of high power
TO-220 or larger N-FETs in parallel will do fine. The big
aluminum block is a surplus heat sink with its fins facing down.
The brackets on the end keep it somewhat thermally isolated from the
bench. With the fan, I have run this beast at 24V and 10A: 240W for
short periods of time.
A real lab supply design
A real lab supply can output a precision voltage or current down
to 0V and 0A. It has an accurate meter and nice front panel
controls. This is more than a mere uA723 can provide. I had the
brainstorm to replace the '723 with a small board of
precision electronics. I unsoldered the '723 on this supply,
and replaced it with a socket. Then the functions of the '723
were replaced with a radio-shack proto board via a 14 pin ribbon
cable.
Here is the board cabled to an old Power-One supply. Some minor
components changes on the supply are needed to make this work on
any supply. The divider resistors on the supply need to be scaled
or adjusted (or bypassed) to provide +5V full scale. The foldback
current sense
circuit needs to be bypassed. Just remove all the resistors so
that pins 2 and 3 of the '723 connect directly to the current
sense resistor.
This supply was originally designed to provide +5V at 3A and +30V
unregulated at 2.5A I bought it expressly to build a lab supply
out of it someday. On the +5V side (Output 1) I removed the
overvoltage
SCR and added one resistor to allow the output voltage to go down to
+3V. More and more of my projects run off +3.3V.
Output 2 contained the transformer
winding, rectifier diodes and filter caps only. It used a wire to
bypass all the regulator circuitry and connect the + unregulated
supply directly to the output. Note that all the regulation
components
are
missing. I removed the wire, added a .25 ohm shunt resistor,
bypassed the trimpot, and added a TO-3 NPN transistor. I also
added a 100uF 50V output capacitor. The remaining circuitry is on
the perf board and is shown on this .PDF
schematic. This is the first prototype of this circuit. It
works as follows:
There are two control loops, one for voltage and one for
current. Each control loop uses a measurement circuit, and an
integrator. A power stage drives the output. Diodes are used to
select either the voltage or the current integrator to control
the output, depending which is in range. Either integrator can
pull the output low if the output tries to go above its limit.
The current sense consists of a differential x10 amplifier to
bring the 0.5V signal across the current sense resistor to +5V
full scale. Its common mode rejection needs to be very good, like
80dB in order accurately measure the small voltage across the
shunt while the output voltage varies from 0 to full scale. To
measure CMRR, unload the power supply (0 current) and see what
the meter reads. Then vary the power supply voltage and see how
it changes. The ratio of voltage change to meter change is CMRR.
To build it with readily available parts requires that the ratio
of its 2 pairs of 10K / 100K resistors be matched precisely,
better than 0.1%. You can do this with a 4 1/2 digit DMM which
will resolve the resistors to one part in 10,000. Or wire up a
whetstone bridge driven from a 10-12V power supply, and use your
3 1/2 digit DMM on the mV range for the null measurement.
You also need the op-amp to have good CMRR and be able to handle
the full unregulated voltage. (See the LT1013 below.)
The voltage sense circuit consists of a simple voltage divider. A
goal of both these circuits is to scale both signals to 0 to 5V
(or whatever reference voltage) full scale. That way they can be
measured precisely with a simple ground-referenced DMM or A/D.
That way the current and voltage adjust pots also operate off the
same reference voltage.
It did work OK, but it had a few problems. The current sense
amplifier output cannot go all the way to 0, and neither could the
supply output. This is due to the LM358 inability to pull its
output to ground. Also to switch from voltage to current control
and back, the integrator outputs need to slew the full power
supply voltage of about +40V in order to take control. This takes
time and in response to a large current change, the output will
droop or rise (kick) while this is happening. Also LM358As used
are only rated for +32V operation and I was using them at +40V.
So they are probably not long for this world.So I changed the
op-amps
to LT1013s which can handle 44V. The ability to go to 0V is pretty
important. since when you short the output with a current meter to
set
the current, the loop must regulate with 0 output volts. So I added
2
series diodes between the regulator and the TO-220 driver
transistor.
This did the trick. On this early proto I was wiling to live with
the
current measure not going all the way to 0. A 1K resistor from the
current amp to GND would help a bit here. But at full current (5 V
out)
the op-amp would dissipate 5mA X 40V = 200mW and get hot. Not
terrible,
but not ideal.
One fix for the 0 voltage and current problems is to build a low
current negative (-5V) supply for the op-amps. Another is to bias
the current measurement opamp so 0 current is actually +0.5V or
so, but this complicates the A/D or DPM design. The integrators
and voltage amp can be operated from a lower supply voltage, like
+12V to reduce their slew times. Then this lower voltage can be
amplified up to the required output voltage by an additional
amplifier. But the current sense amp and the output amp cannot
operate from lower voltages. Fortunately there is a nice part,
the LT1013, a reasonably priced, precision, dual, single supply
opamp that can handle up to 44V. Also I want an LED to tell me
when the supply is in current limit mode. The difference between
the voltage at the output of the integrators tells this and a
comparator will do the job.
I don't mind hand-wiring a simple first proto to prove out a
concept, but when it gets complicated with support circuitry, or
I need to build more, not so much. So my plan is to build an
ExpressPCB to do this. The first version will prove out all the
analog stuff and will probably use an LCD DPM to do the
monitoring.
Here are the original schematic,
PDF Schematic and PCB artwork so far. I added a +12V
and
-5V supply, limited the range of the integrator opamps to +12V,
changed the op-amps that run off -5 and +35 to LT1013s which can
handle up to 44V. The design supports either a single DPM with a
switch to allow it to read either Volts or Amps, or 2 DPMs, one
each for current and voltage. It supports an LED to show when it
is in current limit mode. The layout uses just 1/2 of an
ExpressPCB. The right half can be a copy of the design (if you
need 6 boards) or can be anything else you can fit in the area.
ExpressPCB only allows 350 holes on a mini-board, and this design
is right up to that limit. If you add anything you may have too
remove something also.
For the +12V regulator, I used a LM317 adjustable regulator. These
can
handle the higher input voltage. The LT1013 with a -5V supply is
specified for 44 - 5 = +39V. This is fine for a +15V supply but a
+24V supply will sometimes have its raw DC as high as +40V. Then
with a line surge, bad things could happen. A 5V zener in series
between the V+ and the op-amp supply would do the trick.
There is an effect called "integrator wind-up" where a
control loop takes extra time to respond to a change because the
integrator has gone off scale in one direction and then needs to
integrate all the way in the other direction in order to regain
control. With a power supply like this, let's say a load is
applied that causes the current limit to take control. When in
current limit, the voltage integrator goes to its + full scale.
Then if the load is suddenly removed, the current integrator will
integrates up until the voltage integrator comes back down to
take control. This takes time and causes a jump or overshoot in
the output voltage. There are many strategies for eliminating
integrator windup. One is to simply reduce the output voltage
range of the integrators. The integrators need not rise above the
maximum control setpoint of the supply. The integrators are
powered by the +12V supply. It may be desirable to reduce this
voltage in order to reduce wind-up. So having an adjustable
regulator here might be a good thing.
When multiple output transistors / shunt resistors are used, one
could simply sense current from one shunt and multiply it times
the number of shunts. But this approach won't be very
accurate. How to measure the current from several shunt resistors
and sum them up while still maintaining the resistor matching
required for high CMRR? I didn't want to build multiple high
CMRR amps so I came up with a simple resistor network to manage
this. The magic value is 500 ohms. By adding a 500 ohm resistor
to each 10K, the CMRR is still well balanced. To sum 2 shunt
resistors, replace the + side 500 ohm resistor with two 1Ks, one
for each resistor. For 3 shunts, use 3 x 1500 ohms. for 4, use 4
x 2.00K. All these resistors in parallel equal 500 ohms. All
these are available in 1% except for the 500 ohms. But 499 ohms
is close enough. Even better, these multiple resistors can each
be sky-wired directly to the shunt resistors and only their
common terminal needs to be returned to the 10K of the current
measure amp.
When you are done building this dandy controller, it may be
apparent to you that the power-one supply is not all that
necessary. All we're really using of it is the transformer,
rectifier, TO-3, heat-sink (case) and filter cap. These parts can
be bought or scrounged separately, mounted to sheet metal pretty
easily and the Power-one board can be pretty much eliminated.
Advantages of this approach? Since you need to derate the
Power-one current spec by about 2/3 to 1/2 when building a lab
supply, the transformer, diodes and cap are over-designed,
meaning overly large. The transistor is OK, but the heat sinking
of the power-one case will be too light. Adding a real finned
heat sink and mounting it on the rear of your package will be a
big improvement.
Power Supply Dynamics
The dynamic or transient response of a power supply is design
dependent. Raw Power-one supplies are pretty simple. Their
frequency response is primarily dictated by the output filter
capacitor plus the uA723 compensation capacitor. Complicated a
bit by the fact that the output transistor can charge the output
but not easily discharge it. The supply is designed to provide a
steady DC controlled by a trimpot, so as long as the output
tracks the trimpot during adjustment, all is good. A lab supply
has a much wider range of adjustment. And a programmable lab
supply can jump from one voltage setting to another pretty
quickly.
Another aspect of transient behavior is the response of the
current limit circuit. On a power-one, the speed of the output to
drop during current limit and to recover after isn't real
important. However you don't want the output voltage to
overshoot much when the output current is removed. With a lab
supply intended to operate regularly in both constant current and
constant voltage modes, the circuit should be able to switch
modes cleanly. The circuit that does this is sometimes called a
'crossover' since it automatically switches from constant
current to constant voltage. One simple way to build this is with
two integrators, one for voltage and one for current control.
Then the output of each integrator is simply analog
'or-ed' with two common-anode diodes and a pull-up
resistor. The lowest integrator voltage limits the output
voltage. If the voltage output is higher than the voltage
setting, then the voltage integrator slews down to reduce the
output. Same with the current integrator. Lowest integrator
wins.
This works well except for one important case: When the unit is
in current limit mode, the voltage integrator goes to it's
maximum value, set by the integrator's supply voltage or by
some limiter circuit if present. Then if the current load is
quickly removed, the current integrator slews up to increase the
output voltage. But the voltage integrator can't begin
slewing downward until the output is above the setpoints. The net
result is that it takes time for the voltage integrator to
respond and the output overshoots it's setpoint. The
2-integrator circuit I built causes about a 1V overshoot. This
extra 1V can damage sensitive circuits. Not good. The resistors
in series with the integrator caps help. They add a "P"
or proportional term to the loop so an instantaneous change in an
output ( I or V) causes a quicker response of the integrator
output. A "D" or derivative term can also help here.
But it cannot be corrected for all cases.
A more elegant solution is to use a single integrator with a
'crossover at it's input. The charge on the single
integrator controls the output. The single integrator either
integrates voltage error or the current error with some simple
decision logic. The voltage error normally controls the
integrator. But if the current output is above the current
setpoint, the current error controls it. This can be done pretty
simply with a 'precision diode' or absolute value
circuit.
Packaging a lab supply
Packaging homebrew projects is always a challenge. You want them
to look decent but don't want to put infinite work into the
case. Trying to jam your latest creation into an off-the-shelf
box can be frustrating. One part won't quite fit so you need
to go to the next bigger size box which now is mostly filled with
air. The box looks, well, off-the-shelf. For lab projects or
prototypes, any old box or even no box can be used. In the early
days I built boxes from wood and sheet metal. Plywood for a
quick-and-dirty, oak and plexiglas for something that lives in
the house, and teak for the boat. But unfortunately, the
seventies are over. One advantage of wood, is that it can be
machined with a table saw or router, it can be screwed into. It
can be painted or varnished. Lately I've been looking at
"King Starboard" and other plastics. Starboard is being
widely accepted as an alternative to wood on boats since it can
be machined like wood and never needs refinishing. It's
available in marine stores in mostly white, and on-line in black,
gray, white and off-whites. Plexiglas (lexan is good for
machining, acrylic is too brittle) is also available in clear and
other colors. Contact your local plastics dealer or buy scraps on
Ebay.
My box concept: 2 vertical sides of 3/8" or 1/2"
plastic, machined with slots for the top and bottom, and and
threaded for screws for the front and rear. The other panels are
1/16" or so sheet aluminum either painted or left natural.
The front can be 0.060, 0.090 or 1/8" aluminum to give a sense
of
strength. 1/8" aluminum rack panels are
one way to go. These can be obtained in a nice brushed aluminum
finish for a price. Heights are standard 1U/2U/3U ... sizes. The
front of this design overlaps the sides, top, and bottom to hide
the plastic from the front. I'd round those sharp corners.
Use contrasting black socket head (allen) screws and it will look
pretty decent. Here's a .pdf
drawing of what I have in mind. Visio is great for seeing if
things will fit. Measure the components of your project, draw
them as simple shapes, then arrange them to fit the enclosure, or
change the enclosure size to fit the objects. You can do a nice
2D dimensioned drawing with Visio.
The real deal
Here is the final HC15-3 power supply. This one is designed to
output up to +18V at 2A.
For the low voltage HC15, I was able to build a nice simple 4
op-amp circuit. Two 8 pin DIP op-amps, a +5V regulator, and a -5V
converter fit nicely on a small RS breadboard. The design does
0.0 to 18V at up to 2A. The transformer and board came directly
from the HC15-3. The DPM is a Modutec 0-2V model.
The Mods to the HC15-3 board are as follows:
Remove the uA723 regulator and replace it with a nice
machined-contact IC socket.
Bypass the boost power supply. This requires removing R1 (220
ohms) and adding a wire to connect V+ to VC.
The clever current limit was bypassed: remove the current limit
pot R5 and CR7, then wire U1 pin2 to the + side of the current
limit resistor R2.
The voltage divider was bypassed: remove R9 and R10 and adjust R8
for it's minimum value.
Remove the output transitor Q2, bolt it to the heat sink (with
proper insulating washers) and run wires from the board E-B-C
connections to the transistor on the heat sink.
Check your wiring. Make sure:
The power board + Sense pin is
connected to the uA723 pin 4
Pin 3 is connected to Vout+
Pin 2 is connected to the + (input) side of R2
Without the uA723 installed, power it up and check that V+ and VC
are both about +23V
After turning off the AC power, you'll need to discharge the
cap with a 10 to 220 ohm resistor from V+ to VS- (GND) to safely
proceed.
Instead of a crimp-style DIP cable, this time I built a my own
cable from a 14 pin DIP header and six #24 wires. All you need is
V+ (12), IN- (4),V- (7), ILIM+ (2), ILIM- (3) and OUT(10). When
soldering to a DIP header, put it in a socket to prevent the heat
from melting the body.
The DPM is a 1.999V range unit with a +5V power supply. A voltage
divider measures the V out and divides by 10.0. I use the gain
adjustment on the DPM to calibrate the voltage meter range. Make
the supply DPM agree with an external DMM. Then to calibrate
current, short the output through a DMM set to 2A or 10A. Set the
current limit pot to about 1.5A, and adjust R20 till the DMM and
the DPM agree I used a fixed value for R20 but a 5K trimpot would
be better.
The voltage adjust is a nice 100K 10-turn pot. The current pot is
1 turn only. I was too cheap to use another $10 10-turn pot. The
heat sink is surplus, intended for a big Apex power amplifier
module and re-drilled for a TO-3. The sheet metal was scraps, and
the sides are 1/2" King Starboard in a beige color.
Here's the schematic in .PDF and
in
Express. It's a simple 2 integrator
design that uses a -2.5V reference voltage. The adjust pots
balance against fixed resistors that control the full scale
voltage (R14) and current (R18) values. The integrators use GND
as their + input, and output up to +3.3V. The gating diodes add
+.7V to make +4V maximum. This is multiplied by the 5X amplifier
to make +20V max. The output stage needs 2 Vbe drops (1.4V) +
.25V for the shunt or about 1.7V. So the max output is about 20 -
1.7V, just over the required 18V. In reality the LM358
integrators will output a bit more voltage since their output is
being pulled high by the diode and 10K pull-up.
The Current limit LED works reasonably well tied to the Voltage
integrator and +5V. In current limit mode, the voltage integrator
loses control and goes low. The light is a bit dim when the
voltage out is above about +15V, but otherwise works fine.
Since the max V is only +18V and the max I is 2A, the DPM design
is simple. The ranges can be 19.99V and 1.999A. it means you
can't read over 1.999A though, the meter goes into
over-range. But 1.999 is close enough to 2.000.
03/08 Update: VI Design / Battery tester
The power supply was so successful I decided to go to the next
level: a computer controlled voltage / current source and sink.
This is sometimes called a VI. It can sink or source current, be
adjusted, turned on and off so it can charge or discharge (test)
any battery. I worked at an ATE company, Teradyne and we
built very high quality VIs for semiconductor testing. A proper
VI used in ATE probably has many decades of current ranging from
micro Amps to Amps and and is a very precise instrument. They are
generally 'four-quadrant' devices that can sink or source
current at + or - voltages. This one is 2 quadrant, no negative
voltages. And it has just one current and one voltage range.
A simple LabView program controls it. No Power-One components
this time. 0 to 20V, +3A source or -5A sink. The control loop is
similar to the one on the PowerOne hack, but uses an additional
integrator to control the sink current. I use a small National
Instruments USB data acquisition (DAQ) unit to control and
measure it. The DAQ has 2 0-5V DACs and 14 bit ADCs. Mine is the
14 bit USB6009 (but the 12 bit USB6008 ($149) will also work fine.
The
output stage uses two big TO3 transistors on a heat sink. One NPN
to source current and one PNP to sink it. Here's the
schematic in ExpressPCB format.
and in PDF
If the DAQ had three D/As I would use the third to independently
control sink (negative) current, but with just two, one DAC is
used to control both source and sink at +/- the same setting. So
far it hasn't been much of a limitation. The first DAC
controls voltage.
One limitation of this system is that this DAQ (like most low-cost
USB
DAQs) is grounded to the PC, so the load must essentially be
floating. Not a problem with a battery, but as a power supply or
load, the grounding can cause errors if there are other paths to
ground. Dave's first rule of analog problems: "It's
always the ground."
The control circuitry is built on a large RadioShack board.
Behind the transformer is another small RadioShack board with the
rectifier diode and filter cap. The output connectors and the
shunt resistor are on the right. The NI USB DAQ is on the upper
left. The big white thing on the left is a lithium battery being
tested. The white twisted wires on the bottom is the not-so-safe
AC input. It is fused and insulated however. The transformer is
about 30VAC, 3A. the final unit will have the AC in and a cooling
fan on the rear, hopefully a DMM or two on the front, and the
ability to operate it stand-alone (via knobs) or remotely via the
LabView DAQ.
The DAQ is about 0.5% accurate and I wanted better, so I
calibrated it. Unfortunately NI doesn't support any type of
calibration for these cheapie DAQs, so I was on my own. I
measured zero and full scale voltage on the ADCs and DACs and
calculated the appropriate corrections. Works great and gives be
about +/- 0.1% voltage accuracy. But every channel and voltage
range needs calibration, and when you change DAQs, the numbers
all need to change. I only use one voltage range and 2 inputs so
the task wasn't bad.
Here is a screen shot of the LabView application running. It can
operate as a normal VI, or can charge or discharge a battery,
terminating on a specified condition. The top plot is the
instantaneous voltage and current. The bottom plots are voltage
and current long-term trends. There are two reset buttons to
reset either the time or the accumulated charge. This indicator
reads + when the VI is sourcing current and - when it drains
current. Here is the LabView
code.
For charge termination control, many batteries want to be charged
at a constant current until a voltage limit is reached, then
monitor the current until it drops to a certain value. This works
well for Lithiums, lead acid, and ni-cads. Be careful not to
exceed the current and voltage limits on batteries or they will
be damaged or explode. A proper high current charger or tester
would also monitor temperature and time and terminate if either
of these exceeded limits. Exercises left to the student.
For discharge testing, the VI is set to apply a constant current
load and the LabView program monitors the voltage. LabView
terminates when the voltage drops below a threshold so as not to
damage the battery. LabView also accumulates the current every
second to arrive at the battery capacity. I accumulate mA-Seconds
and scale it to display mA-Hours.
Lithiums are the pickiest of all. Charge voltage is generally
4.20V per cell, not 4.25. Over charge and they die. Discharge
below 3V and they die. Too much charge current and they die. Look
a them cross-eyed and they die. Don't balance the voltages
properly and they die. Or worse, explode.
I have taken apart many old laptop batteries to scavenge their
16450 cells. With careful disassembly and very careful
unsoldering, it can be done. The key word is "old". New
ones are too valuable to take apart, and old ones lose so much of
their capacity that they are not useful. So this has been an
un-fruitful endeavor. Finally I found some nice surplus
oblong-shaped lithium cells from Saft, about the size of three
16450s. They test at about 5000mAH and are currently powering my
bike light. .
12/09 Update: Boat Anchor or Audio Amplifier?
I just obtained a Power-One F-24-12-A. This giant, full rack
width supply is 24V at 12A
or 28V at 10A. It has nine TO3 devices and 4 heat sinks to dissipate
/
waste all the
power it needs. It also specifies a 50CFM fan to fulfill its destiny
as
a space heater. I makes little sense to use a 300W linear
supply
when a switcher would be 1/10 the volume and 85% efficient vs. maybe
60%
efficient. But free was free. The last time I looked there
were a
handful of these
beasts on
Ebay for ~$50.
It would make a pretty poor lab
supply. A proper lab supply of more than
100W uses some kind of pre-regulator to reduce the voltage drop and
power dissipation in the pass element. Many such as the original HP
supplies used SCR stages to pre-regulate and as such are very
efficient as well as clean. I love my HP 6286A. 20V at 10A and
it
is barely warm when
outputting high currents at low voltages.
My thinking is to use just the unregulated section of the F-24-12-A
to
power an audio amp. It would have to be rewired for +/- operation.
Since the unregulated section was about 30-35V, a decent voltage for
an
audio amp, this could work. But alas, the transformer is not center
tapped. It is tapped, but only to supply either 24V or 28V raw. A
jumper on the board is used to select the tap. So it cannot output
full
wave DC to both a + and - supply. Too bad. And the filter caps are
three at 13,000 uF 50V. They would be fine for an audio amp, but
three?
You need either 2 or 4. And the diodes are TO3; first time I have
ever
seen diodes in a TO3 case. To build the diode bridge, there are two
different TO3 diode part numbers, one common anode and one common
cathode.
A toroidal transformer is best for an audio amp. Toroids throw off
much lower magnetic fields which can induce hum in low level audio
stages. But this freebie is staring at me, daring me to give it new
life. I wonder if this transformer's windings are bifilar, two
windings in parallel and can be somehow separated? Sure enough, they
are bifilar and the windings can be easily separated. Check it out.
Use
solder wick to get every speck of solder off the leads. Then use
small
pieces of heat shrink pushed under the transformer insulation to
prevent shorts.
There are two 28VAC windings in parallel, in series with two
3VAC
windings in parallel. The 3V is added to the 28V to provide extra
volts for the 28VDC option. This means that the transformer can
provide
either 28VAC, 31VAC, or if you wire the 3V in series with the 28V,
but
with reversed polarity, 28-3 = 25VAC. And there are two of
each
of these that can be rewired in series for a center tapped
configuration. Cool! The AC options are 25, 28 0r 31V (50, 56, 61
VCT).
DC is 1.414 *
the AC, minus about 1V for the diode. The loaded supply is about 3
volts less. I measured this with a 3A DC load. Then you lose about
4V
in the
filter caps due to ripple and
the output transistors of a typical amp design , the power into an 8
ohm
speaker is about (((VDC - 4) / 1.414 )^2) / 8. This .pdf schematic shows the final
design
with the transformer connection options. Here is the ExpressPCB .SCH File.
VAC
|
Wiring
|
VDC
No Load
(1.4* VAC) - 1
|
VDC w/ Load
|
Power estimate into 8 ohms
(((VDC - 4) / 1.414) ^2 ) / 8
|
25VAC: 50VCT
|
28V - 3V
|
+/- 33 |
+/- 30
|
40W
|
28VAC: 56VCT
|
28V, no 3V
|
+/- 37
|
+/- 34
|
53W
|
31VAC: 62VCT
|
28V + 3V |
+/- 41
|
+/- 37
|
64W
|
The windings are good for about 350W, so a guess at the total audio
output is about 250W. I have wanted to build a multi channel amp
based on the National LM3886 for years. The LM3886 is a decent 50W
part, so I'll probably go with the middle voltage option. I'll saw
the
PC board and the bracket just to the right of the 2 caps.
That will give me a transformer, the diodes and two caps, just
the raw DC supplies, and will remove all the regulator stuff.
The board is mounted to the chassis via the TO3 hardware, so leave
the
diodes and maybe the next row of TO3s just for mounting.
The unit came apart fairly easily. The 4 heat sinks and 7 of the 9
TO3s
just unscrewed. The 3 big caps just unscrewed with an allen wrench.
The
two TO3 diodes were a challenge to unsolder, They have big
leads
and the board holes are a tight fit. After sucking away most of the
solder on top, I had to pry the part off one lead at a time. Two big
soldering irons, one on each lead would have helped.
I used a Sharpie to mark where the copper cuts and jumps need to go.
The heavy copper layer was too thick to easily cut with an Xacto so
I
used a utility knife to make 2 cuts and then peeled off the copper
strip between the slices. For wire I used #16.
Here is the board prior to being cut in two.
And after, showing the board top and bottom. After cutting the
traces
and adding the wires, make sure to continuity check every
connection.
Here is the unit all built up. I kept the screws of the right-most
TO3
transistors to use as convenient mounting screws (1) and output
terminals (3). You can determine if the output is to be grounded or
not
at the chassis. Use an insulating shoulder washer for the ground
screw
to keep is isolated. Remove the insulation to ground it. Typically
in
an audio amp up want to ground the power supply at the amplifier,
not
the power supply. This requires using the original TO3 insulators
under
these terminals.
Mark the TO3 diodes to maks sure you get the right one back in the
right place. You can use a DMM on diode range to identify their
polarity. And DO NOT FORGET
to
install C3 backwards from the marking on the PCB: + side goes to GND. I should
have
crossed out the original "+" on the board.
With no 3V windings connected, the AC voltage is 28VAC. Output
voltage
is +/- 37V, no load, and +/- 34V with a 3A DC load. Ripple is about
1.5Vp-p
at 3A. It's all ready to power a nice audio amp.
For the chassis, after removing the heat sinks, board and
transformer,
I marked it and cut it with a band saw. A jig saw or a hack saw plus
patience would also do it. The heat sinks are mounted to pressed-in
studs which remove with a light hammer blow. With the lighter load
of
an audio amp, the diodes should generate little heat and so no heat
sink is required. The chassis alone is a fine heat sink. You could
also
remove the left end plate since it is unlikely to be used for
mounting,
and it provides no added strength. You could also kill the chassis
bracket completely and just mount the board and transformer to your
chassis. You'd need heavy spacers and longer screws for the
transformer. You'd need to replicate the mounting holes, but the
original chassis could be used as a fine template.
The three unused terminals on top of the transformer are for the
original boost supply. This 8.5VAC winding could be used to power
some
additional logic or analog circuits. I removed the diodes and the
cap
that they were connected to.
Here are the leftover parts, not including the other half of the
cut-off chassis. The chassis, transistors, big cap, and remaining
1/2
board could still be built up as a lab supply, but would need a
transformer and diodes. By the way, the 6 removed T03 series pass
transistors are 2N3773s, 140V NPNs. 4 of them would make an
nice
audio amp quasi-complementary output stage. The .22 ohm ballast
resistors and heat sink hardware can also be used.
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This page was last updated 7/24/10