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Mosfet driver for op-Amp based constant current source.

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ledtester:
Can you provide a link to the Falstad circuit?

Also, you should become familiar with LTSpice - it's a much better simulator.

https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

4kruby:
Hi,

Thanks. I am planning to learn LTspice.

The Falstad circuit is working fine. I just use the interface to draw the circuit to give a snapshot here. However, I tried the same circuit on a solderless prototype board. The circuit in simulator (Falstad) is working fine. However, in reality, it is not.

Moreover, I tried to avoid the bias resistors (to the gate of the mosfet) with the same result (I tried this since, the mosfet datasheet says that it's gate can handle +/- 20v and giving a gate voltage of 10v means that the operating region will change - in this case the Drain source can allow more current).

I am doubting on the Multimeter probes which I am using to measure the current since when I short the probes in ohmmeter mode, the reading sways between 0.3 to 0.6.
I think I have to use the current sensor like the ACS712 model and measure the voltage at its output. By that method, I think we can get a more accurate current right? Because, as per the datasheet of that current sensor IC, it outputs few millivolts for every ampere. But I doubt whether the multimeter can measure such a small voltage.

Thanks

ledtester:

--- Quote from: 4kruby on January 24, 2022, 05:19:58 am ---...
Moreover, I tried to avoid the bias resistors (to the gate of the mosfet) with the same result
...

--- End quote ---

Honestly, I've never seen those bias resistors on this type of circuit.


--- Quote ---I am doubting on the Multimeter probes which I am using to measure the current since when I short the probes in ohmmeter mode, the reading sways between 0.3 to 0.6.

--- End quote ---

It is very possible that your op-amp is oscillating. This circuit generally needs a capacitor between the op-amp output and the inverting input to quash those oscillations. Being able to look at the op-amp output on a scope would be very helpful.

This video has some good tips -- you can start at 3:35:

Electronic DC Load - Performance Improvements - theBreadboard
https://youtu.be/rh32ylmlz-A?t=3m35s

4kruby:

--- Quote from: ledtester on January 24, 2022, 06:32:13 am ---Honestly, I've never seen those bias resistors on this type of circuit.

--- End quote ---
I am confused. Didn't you mention this in

--- Quote from: ledtester on January 21, 2022, 04:52:14 pm ---Yes. Just use a smaller value shunt resistor - like 0.1R or even 0.05R. Then fix things up on the setting side with an appropriate voltage divider.

--- End quote ---
Or is the voltage divider you mentioned here is for the input of the opamp?


--- Quote from: ledtester on January 24, 2022, 06:32:13 am ---It is very possible that your op-amp is oscillating. This circuit generally needs a capacitor between the op-amp output and the inverting input to quash those oscillations. Being able to look at the op-amp output on a scope would be very helpful.

--- End quote ---
Unfortunately, I don't have a scope. The two basic things I lack are a scope and a bench power supply. What value capacitor can I start with? most probably I should get the anser from the below video I suppose?


--- Quote from: ledtester on January 24, 2022, 06:32:13 am ---This video has some good tips -- you can start at 3:35:

Electronic DC Load - Performance Improvements - theBreadboard
https://youtu.be/rh32ylmlz-A?t=3m35s

--- End quote ---
Thanks. Let me check the video and see if I get any pointers.

ledtester:

--- Quote from: 4kruby on January 24, 2022, 07:04:16 am ---
--- Quote from: ledtester on January 24, 2022, 06:32:13 am ---Honestly, I've never seen those bias resistors on this type of circuit.

--- End quote ---
I am confused. Didn't you mention this in

--- Quote from: ledtester on January 21, 2022, 04:52:14 pm ---Yes. Just use a smaller value shunt resistor - like 0.1R or even 0.05R. Then fix things up on the setting side with an appropriate voltage divider.

--- End quote ---
Or is the voltage divider you mentioned here is for the input of the opamp?

--- End quote ---

I meant the input of the op-amp.

What I meant by "fixing things up" is that when you change the shunt resistance you'll have to adjust the voltage to the non-inverting input of the op-amp and that can be fixed up with a voltage divider.

Usually the voltage presented on the non-inverting input is derived from a stable voltage reference -- like a TL431.

This is the video that precedes the one I mentioned:

Electronic DC Load Design and Testing -- theBreadboard
https://youtu.be/vd5IBFFjnOc

It's a little long, but might be worth watching / skipping / browsing through.

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