Author Topic: Mosfet driver for op-Amp based constant current source.  (Read 2298 times)

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Online ledtester

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Re: Mosfet driver for op-Amp based constant current source.
« Reply #25 on: January 23, 2022, 07:14:41 pm »
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
 

Offline 4kruby

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Re: Mosfet driver for op-Amp based constant current source.
« Reply #26 on: January 24, 2022, 05:19:58 am »
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
 

Online ledtester

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Re: Mosfet driver for op-Amp based constant current source.
« Reply #27 on: January 24, 2022, 06:32:13 am »
...
Moreover, I tried to avoid the bias resistors (to the gate of the mosfet) with the same result
...

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.

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

 
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Offline 4kruby

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Re: Mosfet driver for op-Amp based constant current source.
« Reply #28 on: January 24, 2022, 07:04:16 am »
Honestly, I've never seen those bias resistors on this type of circuit.
I am confused. Didn't you mention this in
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.
Or is the voltage divider you mentioned here is for the input of the opamp?

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.
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?

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

Online ledtester

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Re: Mosfet driver for op-Amp based constant current source.
« Reply #29 on: January 24, 2022, 08:26:08 am »
Honestly, I've never seen those bias resistors on this type of circuit.
I am confused. Didn't you mention this in
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.
Or is the voltage divider you mentioned here is for the input of the opamp?

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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