Electronics > Projects, Designs, and Technical Stuff
MOSFET and OPAMP Load tester / constant current load
lordvader88:
Well I made it with Marks basic design so far. I used a temperature trimmed LM336 2.5V reference circuit, and doubled it up to 5V w/ a resistor and pot, instead of unity gain on the 1st 1/2 opamp.
So it's working up to 110mA max so far intentionally, and 0.37A un-intentionally. It's on the breadboard, and it looks clean on the scope overall.
There's some funny business around 1kHz and 10kHz on the mosfet gate, sort of looks like TV remote control style signal. IDK if that's coming from the main PSU, the DUT, or what. It looks cool.
I'll try it on a 5A power brick I have.
For now I using 0-9V for the gate control side, and LM358 and IRF540 with RDs of 0.077mOhm, . I have lower Rds FET's too I think.
But is seems to work good so far. Thanks again. The 1st circuit I tried and hastley made would jump from a few mA to +0.56A for some reason and settings/etc. Not this circuit, and it turns down to 620uA or so.
MarkF:
I only tested mine up to 1KHz. If you want to go faster, change the gate resistor to the MOSFET to 100 ohms.
I had 6.2K for op-amp protection from MOSFET failure but it won't allow high switching rates.
Note. I updated the circuit with a 620 ohm gate resistor.
I tailored the load after what Peter Oakes did. Be aware his load had a meltdown from exceeding power dissipation limits caused by power supply spikes that he was testing. So make sure to operate within the safe operating area.
lordvader88:
I never did anything with AC input yet. Time to get this soldered over the old model.
I'm also looking at this paper on a modular load tester, but it uses 8-9 mosfets, I only have 20-30 total, so screw that.
https://pdfs.semanticscholar.org/319f/3df15130fad0cdc907b1d7a2f67a3ba42b8a.pdf
I want to setup an over-current shut down. So I could use a comparator and or BJT circuit to turn off the gate before OC. Just in case I turn it too high on something I know should not exceed some limit.
I could just add it where I went from 2.5V to 5V, I can use that to set max limit.
lordvader88:
Ok the load tester is working nicely, however it blew-up my homemade PSU again at around 1A. I suspect it's the cheap 2n3055's I have, I guess for now I'll keep them under 0.7A. I don't need anywhere that much current, and the PSU was doing good at static loads.
So I should start feeding in 0-5V waveforms to the opamp controlling the gate, so I guess start with some DC offset and a small waveform ripple,
MarkF:
Another option to limit the max current the load could apply is to modify the voltage divider on the input control.
Setting the max current could be a little tricky.
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