Resistive losses in high current circuits like electronic loads ? - Page 1
Forum section: electronics / testgear

author: MathWizard
author_rank: Super Contributor
timestamp: July 01, 2025, 01:26:54 am
content: I've never made anything high current before, so I've gotten carried away with what LTSpice says was possible just from a schematic.I'm modelling and modding the Array 3710 in LTSpice, to make a lower power, lower voltage version. And adding in some wire resistances, and say 0.3 ohms on a binding post and some connector, well it just about ruins my dreams of a high current E-load.It uses low side sensing, and the FETs are in common-drain configuration. I was hoping to get up to 36A max, and had it working with safe power levels on everything. But adding in some extra 10's or 100's of milliohms resistance for between circuit boards, and binding posts, not only do I start to run out of headroom for the gate-source voltages, but say my binding post connection was 0.1 ohm's, well at 30Amps, that's 90 WATTS , and 3V !!!I was hoping to just use my cheap o-pad prototype boards, and lots of 18AWG, and maybe some household 14AWG wire, I have lots of scrapes of that. I figured I'd be running wires between boards with power resistors, and boards with mosfets, etc.The datasheet for the Array says it can do 30A input, so what sort of gold plated connectors and binding posts, and what sort of wire gauges are they using ??Lots of high power ATX computer supplies use bundles of 18AWG, and they can pump out 10's of Amps. I should try and measure some with my 4-wire resistance meter.

author: BillyO
author_rank: Super Contributor
timestamp: July 01, 2025, 04:29:52 am
content: I've never made anything high current beforeAnd you are starting with this?Have you ever heard the expression "learn to walk before you run"?There is another one that works for newcomers .. "Don't over think things"Maybe start with a single MOSFET 5A load with a single op-amp with no MCU and no OLED display, get it working, learn why it works, and then work your way up.Or, jut start with a "Titanic" and watch it sink. Your choice.

author: nctnico
author_rank: Super Contributor
timestamp: July 01, 2025, 08:17:00 am
content: 0.3 Ohm is way too high. Resistance from short wiring is much lower. Get a low resistance meter and measure some samples. You'll likely find resistances in the single digit milli-Ohm range.

author: MathWizard
author_rank: Super Contributor
timestamp: July 01, 2025, 09:20:47 am
content: Well I've built PSU's and an Eload before, but both are under 2A lets say. The new Eload I make can be fairly modular, so yeah I can scale it up and see how it goes.Most stuff I'd want to test w/ an Eload would be under 1A, some things like my bench PSU's are 6A. But I'd like to test some SMPS for computers, and other high current, low voltage SMPS. So 10-20A-30A would be a good start. I'll probably never built 1 good enough to test up to +100A, like my PSU can do, those machines cost thousands of $$.For the sake of the mosfet gate-source voltage and drivers, I'm also trying to mod the Arry such that the low-voltage supplies rails, have their gnd, below the mosfet's and their 0.1R source resistor. So the LOAD+ is still on the FET drain, but now the main sense resistor's are below GND, and LOAD- is at the bottom of the sense resistors. It works in LTSpice, but I'm sure something will have to be altered.

author: BillyO
author_rank: Super Contributor
timestamp: July 01, 2025, 02:43:51 pm
content: In this design I did a while back I use 4 MOSFETS. The source load resistors on each MOSFET are 0.02Ω and the ammeter is also 0.02Ω.So when the load is passing 20A the source voltage of each MOSFET is at ~ 0.5VThe biggest issue I came across in building this thing was getting rid of the heat. Using this heatsink:https://vi.aliexpress.com/item/1005006546391080.html(the larger one) and a fan at each end (one pushing and one pulling), as well as closing off the channel with aluminum tape, I was able to sustain 100W of dissipation on the 4 MOSFETS without the case temperature rising above 75°C. I used a PICAXE microcontroller to run the fans. It starts the fans slowly at 30°C and ramps them up to maximum at 70°C. It will also shut the load down via the 2N7000 if the temperature rises above 85°C.Oh, BTW, I just measured from the positive input terminal (a 4mm banana jack) to the drain of the MOSFETS. 0.005Ω - No gold plating, nothing special at all. The wire from the terminal to the PCB is #14 fine stranded coper wire. One end is terminated with a standard crimp-on banjo terminal and the PCB end goes into a standard screw terminal block. The traces on the PCB are 10mm wide and over the short distance that they will need to carry the highest current, I put on a maybe 1mm coating of solder.

author: MathWizard
author_rank: Super Contributor
timestamp: July 06, 2025, 04:25:57 am
content: I have a bunch of 0.1R, 10W resistors, I'll have to try and calculate their power rating for sure, but with about 1W on them in still air, they start getting warm. So with some fans, hopefully they will be closer to ambient.I have 4 plasma TV heatsinks, with 2x IRFZ44N per heatsink, the heatsink gets to about 50C with 40W total. That was in a partially blocked off cereal box, with a good 12V fan.So depending on actual resistive losses, LTSpice says that I can run 18A total, with 22W max power on each FET, and 0.5W max on some of the 0.1R's.However, I'm still worried that if I have 18A on some 50mR connection like between the binding post and hookup wires, then that's 16W which sounds like the smoke detector.In LTSpice, I seem to have high current versions working, with a pretty high bandwidth, compared to where I started from, the Array 3370.In my supplies, I have 15x COS77, and 10 COS277, they are similar to OP07/77/277.In LTSpice with the OP277 in the control loop, and OP07's driving the gates, 1 version with 4 extra OP277's only has an open-loop DC gain of 42.5 dBV, but it's still over 9.3dBV at 100kHz, and I've adjusted for the phase margin and gain margin, they look ok. But I'm pretty new to using that stuff, there's a few methods and pdf's I need to study.With 1uH of inductance added to the output, the open-loop gain doesn't drop to 0dBV until 432kHz. It's about 15dBV at 50kHz. The output current is 50dB.Another version, with no extra op-amps in the control loop, and running very hot, with 36A, 32W per FET, would have an open-loop DC gain of 114 dBV, but at 1kHz it's only 30 dBV and -10 dBV at 100kHz.So IDK really how good bad this would be for testing a bench supply or computer PSU, I need faster op-amps, and probably need 10x the BW of whatever SMPS I'd test.So how good/bad does that sound ?

author: nctnico
author_rank: Super Contributor
timestamp: July 06, 2025, 10:27:18 am
content: Quote from: BillyO on July 01, 2025, 02:43:51 pmAs a ballpark you can't realistically get more then 25W of heat dissipation from a transistor in a somewhat standard housing (like TO220, TO247, TO3) once you factor in all the thermal resistance and have some margin for error and elevated operating temperature. Moving more air gets to a point of saturation quickly as the effect tapers off exponentially. Having a temperature controlled fan is good idea; it may not run full throttle ever.

author: MF-jockey
author_rank: Regular Contributor
timestamp: July 06, 2025, 04:52:03 pm
content: Quote from: MathWizard on July 06, 2025, 04:25:57 amWhere does the high resistance come from? With solid 14AWG wiring, the resistance is less than 10 mOhm at a length of 1 meter. The power dissipation of approximately 3W is distributed over the length and shouldn't cause any problems with some air movement.

author: MathWizard
author_rank: Super Contributor
timestamp: July 07, 2025, 01:27:11 am
content: IDK I'll try some 4-wire measurements on some banana plugs and jacks, and alligator clips. I read on here just regular spade crimps are probably better.I'm just breadboarding the core of it, and I'll start out low, with 2 MOSFET's, and adjust the Vref input for low current. I have a few ranges I'll use, the main one will be say 24V at 7A. I'll build some fan control and temperature control circuits, and some maybe some other protection circuits. I'm sure all that will further reduce my bandwidth, so I'll try and keep it simple for now. And maybe use more switches, and make the high DC gain version, and a higher BW version.I want to try some more baremetal AVR coding, and make some display output and maybe controls too. I have some nice relays from microwaves and other appliances. I might use them for something too.I'm hoping to fit it all in a cableTV box, and the heatsinks I have, have a small cross-section and are long, so should all workout. Maybe the controls circuits would be in another little box on top, and just use the cablebox as the cooling box.

author: MathWizard
author_rank: Super Contributor
timestamp: July 07, 2025, 11:00:11 pm
content: So the 30V 3A PSU I want to test 1st, according to LTSice, has greater bandwidth, and higher gain most of the time, than this Eload I'm designing, when I lower it's gain, to extend it's bandwidth. The PSU only uses 2x uA741 in the voltage control loop.So how bad would it be for trying to test the PSU ?? The original Array, was already under 0dB well before 1kHz, so it would be worse past 10 or 100Hz.These are closed loop gain's, for both set to 15V, 1.5A