E-Load Electronic load. The project that will not end....

Adjustable DC Load

eload
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 and then 20A. 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 or more, a fan is a must. The basic circuit is quite simple. Build the electronics on a little Radio-Shack or other proto-board and mount it on the biggest heat sink in your junk drawer.

Rev 1

The original big heat sink (8.5 x 3.2 x 1.2") and IGBT were pulled out of a scrap bin at Analogic in 2001. Once I had these prizes, I had to build an E-Load Here is the original circuit. The 1.25V reference diode, 15K resistor, and ten-turn 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 original proto had a 10K trimpot on board with an adjustment hole in the front. Soon it was upgraded to a nice front-panel ten-turn10K pot.
Load Circuit
Load
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-247 or larger N-FETs in parallel will do fine. the IGBT is a half-bridge type, with only the lower transistor used.  The big aluminum block is a surplus heat sink with its fins facing down. The steel brackets on the end keep it somewhat thermally isolated from the bench. I have run this beast at 24V and 10A: 240W for short periods of time.

Here are the original specs / requirements for this version.
The original reason for self powering was so I could easily test 12V batteries in the field. But a proper battery tester requires that the load shut off below a set voltage. It also needs to keep track of the time, in order to measure the battery Amp-hour capacity. Also, the fan operates from the same supply, so causes ~100mA of parasitic and un-measured current. Also a fan prefers a fixed voltage, not some arbitrary one. Additionally, the FET really wants about 12V-15 max, not 24V or more from the opamp. I soon decided that self-powered wasn't such a good idea.

FET and IGBT SOA limitations in E-Loads

The power semiconductor manufacturers have recently changed how they specify Safe Operating Area (SOA) for FETs and IGBTs. These devices are primarily intended for switching applications, not to dissipate hundreds of watts of continuous DC. So many parts now only specify SOA at 10mS duration or less. Supposedly FETs can develop hot-spots and then fail.

The part I use is an older, half-bridge IGBT module, Toshiba MG100J2YS50. It specifies SOA up to 300W dissipation at DC, 30V @10A. 100V @2A. This thing is truly a beast.I wouldn't run it much higher than 200W.

Back in the good old days when I first built this, FETs and IGBTs specified their SOA at DC. Nowadays you need to buy specific "Linear FETs" to get DC SOA curves.  But for a one-off, as long as you keep power down....  A good Linear L2 FET for a big DC load is the IXYS IXTN110N20 in the beefy SOT227 package.

Another nice part I have used for high current loads is IXFH180N20, in a TO-247, This part can handle about 150-200W DC,  but be very careful about using any thermal pads. at 150-200W, the temp rise of even the lowest thermal resistance insulators will cause a major temperature rise. I screw them directly to the heat sink with thermal grease, and then insulate the entire heat sink since it is at the Drain voltage. Inconvenient but necessary.

Rev 1: Digital set and display

Requiring an external DMM was inconvenient. And a voltage monitor is also useful. Around that time, I designed LeoLed, with a 16b isolated ADC and 12 bit DACs. It allows setting the output current precisely using an encoder. The ADC has programmable ranges, so directly measuring the 0 to 0.5V shunt resistor was a good match. LeoLed has resistor dividers on its inputs so directly measuring up to 100V was no problem.

Rev 2: Variable fan control

Having the noisy fan always on is annoying. I designed a little Fan Control Board that uses a thermistor to monitor heat sink and control the fan speed. It has 2 fan speed settings plus OFF. It was designed for PS--Load, but was equally applicable here. I wired the fan before the power switch so the fan can remain on after the load is turned off. There can be a lot of residual heat in the heat sink after the load is removed.

Fan Board can monitor 1 to 4 thermistors and controls the fan based on the hottest one. This circuit is for +24V input and fans. For 12V input and fan, change R6 and R8. If you just need a single thermistor, only use one thermistor connector and input R-C, and U1.1, U1.2, R1, and D1 an be eliminated. J8 is if you want to monitor the temperature.


sch

fan 3d

fan

Rev 3: DC-DC

I had planned to isolate the +12V power to the load from ground, but hadn't gotten around to it. Without isolation, you either had to be careful about the source grounding, or be careful about the +12V power supply grounding. An isolated 12V AC-DC (not 3 wire AC) could be used. Neither is good. A proper isolated DC-DC is needed. The ADC and DAC are already isolated on the LeoLed board. That's most of the battle.

I've done work on DC-DC common mode noise and finally settled on the 12V to 15V DC-DC that I use on DIY-SMU. I had spare PC boards and could easily band-saw off the small DC-DC power section. It's the thin blue board on the heat sink.

It is not ideal, and generates a bit of 300KHz common-mode noise. Still working on a final solution.

e-load guts

e-load

Rev 4: The Future?

E-Load works well and is meeting my needs for now, but clearly some cleanup and improvements are in order. Maybe if I get not-busy. Then again, it works the way it is.
Maybe just the firmware updates...

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This page was last updated 10/21/23