Electronics > Beginners

Mosfet driver for op-Amp based constant current source.

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ledtester:
Sounds like a good plan. You can use multiple 1R resistors in parallel to create a lower value resistor.

Vovk_Z:
It is fine to have 0.5 Ohm shunt for 0.5 ADC rated current source. It'll give 0.5R*0.5A = 0.25 V voltage, just fine. (And 0.125 W dissipated across it - fine too).
So, it is two 1 Ohm resistors in parallel.

flowib:
10PC metal film 1R resistors in parallel will suffice as a shunt, and likely be more accurate as well.

Ideally you use something like a LT1010 or LH0002 to isolate the opamp from the mosfet capacitance.

The LH0002 internal circuitry is just four resistors and four transistors.

I like RFP12N10L for current sources but there are beter fets available, there is the IXTH80N075L2 which is nearly indestructible and designed for this application.

magic:
You don't need buffers unless for a high bandwidth load. Most simple project isolate the capacitance with a resistor if it becomes a problem and/or use an RC feedback network to stabilize the whole loop.

IXTH80N075L2 looks nice on paper but it's too large to be useful: its RthJC is less than 2x its RthCS - you lose 35% of rated dissipation simply by going from Tc=25°C to Ts=25°C ::)
Given its high price, it may be better to parallel a few smaller ones if you need that sort of power.

4kruby:
Thanks for your replies guys,

My knowledge limits my understanding and I need to get some knowledge on the gate capacitance affecting the opamp

For starters,
I tried the circuit as given in the attachment, and got some results. But not accurate enough.
I set the potentiometer to 0.5V.
When I measured the current (I measured it from the 1ohm resistor to the ground), I got only 0.4A.
Why is there error of 0.1A? Is it due to accuracy of the 1 ohm resistor? or is it something else in the circuit?

Also, any improvement in the circuit is welcome.

Thanks.

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