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Mosfet driver for op-Amp based constant current source.

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4kruby:

--- Quote from: magic on January 21, 2022, 10:10:20 am ---Base current of NPN will cause error because it is measured by the 1Ω resistor but it doesn't come from the battery. Actual discharge current is ~1% lower than 1Ω resistor current (assuming β=100).

--- End quote ---
I agree since the bjt's are current operated device and base current also will flow through the emitter path. Then if I need accuracy, raising up the supply voltage and using a mosfet is the better choice. The one thing I was considering is the switching frequency of the mosfet. Whether the opamp will be able to cope up with switching the mosfet ON (supplying enough current) at the switching frequency.


--- Quote from: Terry Bites on January 21, 2022, 08:41:08 am ---You just dont have enough headroom. If you have set the input to 4V the mosfet will have 4V subtracted from its Vgs. G is already at 4V. So the mosfet gate need Vgs +4 V to tun on.

--- End quote ---
I don't quite understand this statement. Do you mean that if the Gate threshold voltage is 4.5v, I need 8.5v to turn on the Mosfet? Can you please explain it?

ledtester:

--- Quote from: 4kruby on January 21, 2022, 11:50:03 am ---
--- Quote from: Terry Bites on January 21, 2022, 08:41:08 am ---You just dont have enough headroom. If you have set the input to 4V the mosfet will have 4V subtracted from its Vgs. G is already at 4V. So the mosfet gate need Vgs +4 V to tun on.

--- End quote ---
I don't quite understand this statement. Do you mean that if the Gate threshold voltage is 4.5v, I need 8.5v to turn on the Mosfet? Can you please explain it?

--- End quote ---

This is based on your use of a 1 ohm shunt resistor. The MOSFET source is at I*R volts where R is the shunt resistance and the gate needs to be raised above that.

4kruby:

--- Quote from: ledtester on January 21, 2022, 03:43:09 pm ---
This is based on your use of a 1 ohm shunt resistor. The MOSFET source is at I*R volts where R is the shunt resistance and the gate needs to be raised above that.

--- End quote ---
So if I understand you correctly, if the current is set to 0.5A, and if the gate threshold Vgs is 4.5 volts, then I have to apply 5v as the source will be at 0.5v right?

ledtester:
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.

4kruby:

--- 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 ---
Thanks. I understand it better now.
But I don't have lower value resistors (like 0.1R) - might order them at later point of time.
For now, I guess these modifications should work :
1. Increase the supply voltage of the opamp to 9v or 12v.
2. Determine the current (which should not be changed) - example 1A
3. Measure the opamp output and bias the mosfet gate such that it gets 5.5v (4.5 threshold + 1v source voltage).

Is the above approach good enough?

Thanks.

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