[tinfever] needed a high-power benchtop electronic load for an upcoming project, and by their own admission decided foolishly to build their own. And we’re glad they did. The thing is, whilst this isn’t exactly a super-cheap project to build, buying a commercial offering with a capability of 10 kW and up to 30 kW pulsed, is going to cost an absolute fortune.
Built inside a cubic frame using what appears to be standard 2020 aluminum rails and fixturing, the modular construction is nice and clean, with plenty of space around the load boards to allow the cooling air to circulate.
The operating principle is very simple; custom PCBs act in parallel to provide any load needed, by switching in the on-board load resistor. Each load board handles all the details of switching and dumping the power due to the inductance in the system wiring and the wire-wound resistors themselves.
Whilst we know that wire-wound resistors are reverse-wound to minimize inductance, there will still be some, and each load board will contribute a little more when the whole system is scaled up. Also, each load PCB handles its own temperature sensing, and current measurement passing these data off to the control PCB. A front-end connector PCB provides a variety of connection options to interface to the DUT (Device Under Test.) The system controller is based around an STM32 processor which deals with quite a lot more than you might think is needed on a first look.
The sense currents from each load need to be sensed, scaled, and summed to keep the overall load accuracy within the 1% spec. Also, it is on duty for PWM control of the cooling fans, handling the user interface, and any other remote connectivity. There are a lot of details on the project page, as we’re only skimming the surface here. If you’re interested in building an active load, this is a project you really should be digging into.
We shall watch with interest for when [tinfever] scales up this eight-slot prototype to the full specification of 52 stages! When working with power applications, there comes a point when you really need an electronic load, and to that end, here’s one with a very specific use case to get you started.
There is also the option of buying something cheap from the usual sources and hacking on some custom firmware to adapt it a little to your needs.
Wait, hold up, TIL, what’s this about “reverse wound” wire-wound resistors?
Half the turns clockwise and half counterclockwise, I suppose?
Exactly. It’s also called bifilar winding.
https://en.wikipedia.org/wiki/Bifilar_coil#/media/File:Bifilar_2.svg
Ah, yes bifilar, that was the term I was trying to remember. Thanks!
Turns out there’s also: https://en.wikipedia.org/wiki/Ayrton%E2%80%93Perry_winding
Which I’m trying to wrap my head around. (ba dum tisss)
I wondered why the builder didn’t use the normal MOSFET-in-linear mode and their answer seems to be “didn’t want to”. Okay.
I once made something like this when I was young using a rotary switch to drive a diode array to generate control signals for some relays which switched in load resistors. It had a very satisfying ‘chunk’ every time you turned the know a step. I was working at a design shop that used (and tested) lots of relays, so they were pretty much free. I can’t imagine designing things like that today.
You may have already seen it but I wrote up a whole section going through the numbers of why I didn’t use the normal MOSFET linear mode design. It’s in the project details section under the heading “Why resistors?”. Admittedly, I didn’t do as thorough analysis in making that decision as I should have at the start. However, in short, I think using large power resistors saves about $1500 in heatsink costs.
That was a good choice. It also saved you the cost of complicated safety circuitry, a box of analgesic pills and a fire extinguisher.
The answer is in the project description, switching resistors is cheaper and easier. Using paralleled MOSFETs in linear mode can be nightmare. Thermal runaway, temperature dependent current distribution, cooling requirements, availability of suitable transistors, etc.
If you’re already going for low-tech and cheap, might be fun to use a bunch of automotive light bulbs for a lot of the dissipation, and since the type doesn’t matter, it looks like you could get 100W per dollar online. Not constant resistance since they heat and cool, but it’d look neat.
Or for real dumpster engineering, maybe some lengths of salvaged enameled wire in a water bath would be lower than the stupidity threshold needed to get a darwin award. You can do the same things as the bought resistors with the wire to keep the inductance down, and until the water boils, the enamel shouldn’t cook too quickly.
Globes are a good cheap load. You could get 2kW globes quite cheaply for stage lighting.
Using copper as a resistor… it’s temperature coefficient or tempco is a massive 0.4%/°C. So in a water bath from 25°C to 100°C the resistance has increased by a factor of 30%! Compare with nichrome’s tempco of 0.02%.
And putting wire in a water bath is fine until it boils. Then there is only steam touching the wire which barely cools it at all. Cooking oil works better. It only smells like a takeaway rather than turning the lab into a sauna.
True, there’s a number of better invariant alloys. But once you clamp to 100C-ish, it’d be more stable. And I imagine you measure what happens and could adjust the load based on feedback to cancel that, or cancel the coefficient in that range with something with the other sign.
Carbon rods or tubes work. I have a few banks around that were in an old edm machine.
“Stupid carbon rod!”
-Homer Simpson
Pish posh, this all could have simply been done with frozen excited bromide in an argon matrix.
Now I want popcorn.
My first thought would have been to get some old toasters, kit bash some heat guns or something, but then, I’m weird. :D
Granted, [RedactedCo] has their backup generator load tested annually, and the vendor brings out an 800 amp, 3 phase monstrosity that is on it’s own tandem axle trailer, and dumps out a ridiculous amount of heat.
Better the Hackaday Prize than the Darwin Award!
I remember building a 100KW load bank for the military quite a while ago. Separate controllable inductive and resistive loads. Contactors for switching. Had an airplane propeller to cool it. Tested it at full load using an Army diesel generator. Tried to get the heavy gage feed wires to jump around by going from no load to full load but they did not move.
In work we use multiple electric heaters for high power load :D
We just disconnected the heating element from main power and connected it to 2 “banana” plugs, so you can use any voltage (even DC) and still you can plug it into 230V AC to turn on the built in fan…
Room full o’ old hairdryers also work. Same principle, smaller form factor. Room could double as a sauna after an hour or two.
Only downside’s the noise level, when all are going full-throttle, though…