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Mosfet driver for op-Amp based constant current source.

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4kruby:
Hello,

I am trying out a circuit for a constant current load for discharging Lithium batteries using mosfet and op-amp.
I am using LM358 for op-amp and IRFZ44N for the mosfet from a 5v (500mA max) power supply.
First I tried a circuit like this (file : circuit_1):

Here, I got the output of the op-amp as around 3.7v which is not enough to switch the mosfet completely. Moreover, as you can see, I was using the same power source for powering mosfet as well as to connect the mosfet drain. I observed that very little current was flowing and mosfet became very hot.
I have a few questions here:
Can I use a single transistor to drive the mosfet? Most of the circuits I encountered had the mosfet gate connected to the collector of the BJT where the logic will be reversed (mosfet OFF when op-amp is ON).
Is it possible to connect the mosfet to the low-side (emitter) of the BJT and switch it ON?

Thanks.

ledtester:

--- Quote from: 4kruby on January 20, 2022, 02:35:50 pm ---Here, I got the output of the op-amp as around 3.7v which is not enough to switch the mosfet completely.

--- End quote ---
It seems you are using +5V to power the op-amp. The LM358  only has  a voltage swing of 0 to the positive supply - 1.5V. How about using a higher supply voltage for the op-amp? Or a different op-amp (with rail-to-rail output) or a different MOSFET with a lower Vgs.


--- Quote ---Is it possible to connect the mosfet to the low-side (emitter) of the BJT and switch it ON?

--- End quote ---

A schematic of what you're intending would be helpful.

Do you mean the op-amp output is driving an NPN base with the emitter driving the MOSFET gate?

magic:
This circuit is not very accurate because LM358 draws additional 0.5~1mA from the battery which is not measured by the resistor.

If this is not a problem, you can replace the MOSFET with any random TO220 NPN transistor. Base current will not be an error because it comes from LM358 which takes it from the battery. So all current measured by the 1Ω resistor comes from the battery and the only error is the small current flowing through LM358 ground pin, which is not counted, as before.

4kruby:

--- Quote from: ledtester on January 20, 2022, 04:06:59 pm ---It seems you are using +5V to power the op-amp. The LM358  only has  a voltage swing of 0 to the positive supply - 1.5V. How about using a higher supply voltage for the op-amp? Or a different op-amp (with rail-to-rail output) or a different MOSFET with a lower Vgs.

--- End quote ---
I was naïve enough to think that a transistor would do the trick with the 5v supply forgetting the fact that the transistor will drop a good 0.6volts atleast which will then be under the 4.5v gate threshold.

--- Quote from: ledtester on January 20, 2022, 04:06:59 pm ---Do you mean the op-amp output is driving an NPN base with the emitter driving the MOSFET gate?

--- End quote ---
Yes Exactly.

I came to my senses a bit now. A 5v is not sufficient even if a transistor is introduced (unless there is a boost circuit of some sort).
So, the supply voltage has to be increased. In that case, since the Opamp should be able to handle a wide range of voltage, I can altogether use the same circuit with a higher voltage as you suggested and throw away the great idea of including a transistor (unless we need a fast switching of course for the gate current which I need to determine).

4kruby:

--- Quote from: magic on January 20, 2022, 09:14:34 pm ---This circuit is not very accurate because LM358 draws additional 0.5~1mA from the battery which is not measured by the resistor.

--- End quote ---

Well, obviously I did not think about this too. (The assumption of an ideal opamp - which I should try to erase from my brain).
Is there a way I can mitigate this? I mean make the circuit more accurate?

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