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Troubleshooting a DIY constant current load
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OM222O:
Hello
I designed and built a constant current source (just like the one dave made) using a 25 turn pot, a shunt resistor, amosfet and an op amp.
The op amp that I had in hand was the MCP6002 (dual channel rail to rail op amp) so I thought why not use it to drive two mosfets, each with their own ballast resistor and voltage feedback.
I used a voltage divider after the 5V regulator in series with the 25 turn pot in order to get a maximum of 2.5A per channel. after building the circuit it works ... at least kind of  :palm:
one of the mosfets gets toasty hot while the other one remains stone cold no matter what. also it's extremely touchy and goes from 10mA to about 2A in about one turn of the pot!  :-// |O
I'm not sure what might be causing this or what the issue might be, so I'm gonna post schematics, PCB and the actual build here, maybe someone can help.
P.S. I forgot the 47u tantalum caps when designing the PCB so I just botch soldered them  :-DD
duak:
OM, I see a few things:

1.) the opamp is probably oscillating trying to drive the gate capacitance of the MOSFET.  Place a 47 to 100 ohm resistors between the opamp output(s) and the MOSFET gate(s).

2.) connect the opamp's V+ terminal to 9V rather than 5V.  This will ensure the MOSFETs will get sufficient gate-source voltage to support the requested drain current.

3.) To reduce the touchiness of the current adjustment, you will need to place a resistor in series with the top or hot end of the potentiometer.

Cheers,

OM222O:
I try adding the gate resistor but the IRLZ44Ns are logic level fets, so 5V is sufficient for a full on state.
As I mentioned, I've already placed a 10k resistor on top of the 10k pot which is 25 turns! I can even get accurate millivolt adjustments using a multi meter. I'm not sure why the jump is so huge and rapid!
why does one of the channels carry no load tho? that just doesn't make any sense to me! it's an identical circuit with identical parts!
aheid:
Maybe it looks worse than it is, but it looks to me like you have some spilled solder shorting the gate to ground, see attached picture, near the opamp.

Have you hooked up a multimeter probe to see what your "vref" and your current shunt voltage is when it goes from 10mA to ~2A in that one turn?
OM222O:
it indeed was not shorting anything. I will need to use the arduino to measure multiple voltages at once if I want to see how Vref and Vgate and the voltage across the shunt behaves for both mosfets. let me write a quick code to do that and I will get back to you.
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