A simple high power high voltage high current active load using NPN transistors

I built this.  If you are clever with electronics, you can build one for yourself.  If you find this circuit somewhere besides https://votefordavid.wordpress.com/2020/05/08/a-simple-high-power-high-voltage-high-current-active-load-using-npn-transistors/ then be careful that the text may have changed and rendered this dangerous or even deadly!

Money shot: the final product

The rear view:

This is not vaporware. I built it and it works and it works well.  If you have any smart questions, feel free to ask them in comments but beware I don’t check the queue very often. I’ll get to you eventually.

This load is effectively a big resistor of variable value, that will sink as much current as you like.  It can be scaled up to as many transistors-and-resistors as you need to increase power handling.  Heat sinks will need to be large for high power inputs.  I have giant fan-cooled heat sinks and the circuit itself limits input current at low voltage.  At 100V I am only getting about 60 Watts, but I have used this to test a few different power supplies and it sucked down 30 amps at lower voltage pretty easily.

My goal was to be able to go up to 100V DC input, with as much current as could be obtained.  In the end I didn’t get as much high voltage current capacity as I would like, but the low voltage current capacity is capacious indeed and I’m pretty happy with the result.

My meters reading 18.7VDC input at 30A, when my supply is giving 19.1V.  Almost half a volt is dropped across my giant 4AWG input cables, which surprised me a little.


My inspiration came from Hamuro on deeptronic.com https://www.deeptronic.com/electronic-circuit-design/variable-dummy-load-for-power-supply-testing/

The final circuit is approximately:

The basic circuit is extremely simple: a DC input is divided by a potentiometer, and the divided voltage is applied to the base of an NPN transistor.  The collector is tied to the DC input voltage, and the emitter is grounded via a resistor. The transistor allows various amounts of current to flow out the emitter, due to its base voltage controlling its collector current.  Here I have several transistors in parallel, which is a trap for new players: If you don’t have a proper resistor on the emitter, you may get one transistor hogging the current, heating up, going into thermal runaway, and exploding.  You will want some resistor there to be a dummy load, and it will vary based on your needs.

I have two sets of emitter resistors, and these two fat toggle switches select between them.

My desire to go to very high voltage forced a couple of changes.  A lower base resistor would allow more current to flow through the transistor at low input voltages, but it made the base voltage setting potentiometer tricky to control with precision at high voltages.  I added a 10k pot in series with the 100k pot for finer control, which in use turns out to have been a great idea.  I also had to insert a high power resistor in the R14 position.  A short-circuited potentiometer would lead to up to 100V/1000Ω=0.1A*100V= 10 watts of power dissipation in R14!  A lower input voltage would allow a smaller resistor here, or none at all, if you like to live dangerously or don’t mind burning the occasional transistor and potentiometer.  I had to use a high value emitter resistor to prevent current hogging at high voltage.  On a darlington transistor, you want even higher values.  I went with 10 ohms which is should be plenty even for low current at high voltage.  For low voltage I used 0.75 ohms.  All of these are high power resistors because they may have to take up to nearly-ALL of your circuit’s power, depending on how you adjust the base potentiometer!  The low value resistors are paralleled for lower resistance, and the high value resistors are a series string.  These are selected by big toggle switches.  For a simpler single voltage range you could of course eliminate the different set of emitter resistors and the switches to select between them.

The power transistors I chose are MJ11016 NPN darlingtons: https://www.onsemi.com/pub/Collateral/MJ11012-D.PDF Choose your power devices to have suitable voltage and current ratings for your intended input, or things may well explode on you.  These are high power, high voltage TO-3 devices, which suits my application just fine.  They are mounted to heat sinks with about a combined 2.2ºC/W junction-to-ambient combined interface resistance, and when I did the maths it worked out to about 90 Watts maximum from each transistor with the heatsinks still cool enough to be able to touch.  I can get a hundred-ish watts each with things getting toasty, but the fans cool them down again pretty fast after they stop sinking power.

A final note: Depending on what you do with it, this circuit will have lethal potentials floating around.  If you don’t know how to make this safe, please please PLEASE study what you are considering doing.  Ask for help at the EEVBlog community forum https://www.eevblog.com/forum/index.php or somewhere else full of knowledgeable people, when you start to understand and can ask good questions.


Wherein I explain how my life was slightly improved

This weekend I made good progress toward eliminating a first world problem that’s bothered me for ages. I have an old variable DC power supply rated for 0-40V 0-3A that’s good enough for most things I typically need a power supply for. It’s a old Lambda, near relative to those seen on the cover of this 1974 catalog:

It had a few niggling complaints. So:

The meters are like 2″ analog things and not ridiculously accurate. This weekend I did range-changes to a pair of nice Newport 4.5-digit digital panel meters. With these installed, I have 4 digit precision on current and voltage, and nice big bright LED displays to boot. The voltage was easy enough, just convert to the proper range and install a potentiometer/voltage divider to get me to 40.00V indications. Good deal. It’s off by a few millivolts at low voltage but that’s ok enough for me. The current meter was a problem. First I had to find a 0.1 ohms resistor with enough power capacity. Running the output from the power supply through this gives me a scaled 0.1V/A voltage output. The problem is that the meters I have are for 0-10V input. So I had to do some reverse-engineering and figure out a 19M ohms string of resistors to increase the sensitivity to 0-2V. This was more work than it sounds, and calibration gave me fits for quite a few hours. I now have a 0 to 3.000A indication.  It’s off by a few milliamps at high current, but that’s ok for me too.

These meters were not installed from the factory. There is not room inside the power supply for these, and if there were it would require cutting a bit of steel which I didn’t feel like doing, and then they would have still protruded. I could have run the wires out the side or through a ventilation hole (the case is 100% ventilated panels) but that would be inelegant. I could have run the wires from the front panel binding posts but that’s a horrible bodge (!) So I ran the wires to the screw terminal strip on the rear panel.

There is a strip of rear panel screw terminals for various connections including remote voltage sensing, which is where the voltage control section gets its reference. I moved the remote sense wires internally from the rear to the front panel where I take all my volts from anyway. So there were two unused terminals on the back, which I used as a proportional current monitoring output. Nice. At work, I have a digital panel meter for my main bench supply that just stays on, burning 3W of power whether I’m using the power supply or not. At home, I want it done better, so I did more reverse engineering and figured out where the internal power connections are to be found. I added a wire to a “spare” rear panel screw terminal so now there is a rear panel output with a switched mains supply from the power supply – connected after the power switch and fuse in the power supply! The neutral and ground for the meters both connect at the main inputs, and then my panel meters now run off a *switched* main supply. The turn on and off with the power supply they monitor. Very trick.

I had my choice of several meters. I tried and failed to figure out a range change on a 3.5 digit meter of a different model for the current readout but in the end I’m glad to have 4 digits and both meters are the same model so that’s also nice. These have screw terminals also so the mains connection is daisy-chained between the power supply and meters.

The power supply had a broken V- Out front panel binding post since forever, and at one point I bodged the V- on to the ground binding post, so the output was ground-referenced but at least usable. This weekend I employed a bandsaw (thanks JB!)  and hacksaw to do the fitting/bodging, and I now have a set of larger/easier to use front panel binding posts which are also separated again so the Vout is no longer permanently grounded. Nice.

The voltage wouldn’t reach all the way to 40. I tweaked that so now I can get just over 40V output. I tried to adjust the calibration of the voltage meter on the supply itself and fried a nice Bourns 10 turn pot in the process of failing.

The current limit adjustment on the power supply was flaky. Difficult to turn at best and it would sometimes cut out so I had to jiggle the handle (like an old toilet) to get the power supply to give voltage out. I replaced that bad stiff old 3/4 turn potentiometer with a 10-turn pot for current limiting. Nice.

The voltage had a coarse and fine coaxial pair of 3/4 turn knobs. The fine adjustment knob had a bent shaft and wasn’t great to operate. I pulled that out and installed a separate 3/4 turn fine voltage adjustment. This required drilling a fresh hole in the faceplate, but that was easy enough. I accidentally drilled a pilot hole in the wrong side of the faceplate but I ended up needing to relocate a cable clamp and this made a handy mounting hole, so no worries. I used the power supply a bit for calibrating the panel meters and didn’t like the ability to only adjust in 0.006v or so increments with the 3/4 turn potentiometer, so I installed a 10-turn fine voltage control pot instead. Very nice.

The final step was to install a mounting bracket for the new panel meters. This was fabricated from brace lifted from a vintage piece of test equipment from an old job that closed down around my ears and gave me a bunch of stuff as I was on my way out the door. That holds the top of the meters’ cases, near the front. The rear of the meters was spaced away from the power supply bottom panel with the use of the aforementioned fried potentiometer and a twin that was previously broken, safety-wired in position. The panel meters now are securely mounted under the power supply AND come alive only when the supply is switched on. Nice.


The bodge-grade mounting solution, seen from the side (hidden well under my bench):

All of this reads as much more simple than it was in person to do. Taking the supply down means crawling under the workbench and fighting the heavy thing off of 5 screws that hold it to the underside of my work bench. Then the cover had terrible little screws (which I replaced) and the faceplate also had horrible little screws (which I replaced also). Now the exterior hardware mostly matches. It is a hassle to get this faceplate off but I had to to it several times this weekend. I had the output/heatsink module out a couple of times. I had the cover off many times and the faceplate off many, many times. And then I installed it and . . . the voltage was backwards. My panel meter sensing wires were accidentally connected in reverse polarity and it gave negative voltage readings on the meter. In the end, I decided to not dismount everything once more and just crawled under the bench waist-deep with my arms poked out in front of me like I used to do, back when I was working on airplanes, to switch the wires ’round.

After a day-and-a-half of work my work area looks like a tornado hit, and the power supply has one extra knob and a fresh set of jacks on the front. It sports a pair of big beautiful panel meters and is back under the bench, a bodged-up-laboratory-grade instrument after all the upgrades!

and: the way this power supply is mounted, I had to figure out how to remove my soldering iron (which is also screwed to the bottom of the bench) and remove the iron, TWICE, during all this.  I had mounted it myself but over the last decade or so forgot how I did it.  I was impressed with my own handiwork, so that was neat. Having permanently-mounted stuff under the bench is sure convenient in use but for maintenance it kinda stinks!

If you scrolled all the way down, I congratulate you and present a reward: A shot of the bottom of my bench! You see here: a soldering iron, a power supply with new meters and an extra knob, and a multimeter measuring the output of the supply for comparison. All of this, thanks God, cost me $0 plus some time. 😀