Electronics > Beginners

Constant current load Help needed

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xavier60:

--- Quote from: LukeB on September 23, 2018, 01:36:46 pm ---Sorry just realised that the lm317 isnt an opamp but i still like that idea. Was thinking of a different opamp (LM358).

--- End quote ---
The LM311 is a comparator.
The data sheet for the LM358 op-amp states "• Input Common-Mode Voltage Range Includes Ground"
You can learn things from data sheets.
It is a dual op-amp, so you will need to choose one half and use the appropriate pins.
With the different op-amp, the oscillation problem could come back, so check with your oscilloscope again.
The circuit really should have compensation added to slow it down and improve its stability.

MiDi:
A picture of the circuit board could help...

What kind of current sense resistor do you use?
It could lead to oscillation when you use a wirewound one - the inductance of it should be kept low.

You should throw out C2, it shorts the battery when connecting...

A resistance between OP out and Gate should be introduced, op amps are quite sensitive to capacitive loads (it drives the effective gate capacitance Ciss) - try a 1k.


Make shure to set it to <=0A when you apply the battery, else the mosfet is fully on and shorts the battery when connecting.
Alternative is to apply battery and then power the circuit.

MarkF:
You may like what I did.
A LM358 would work here as well.

   

xavier60:

--- Quote from: MarkF on September 23, 2018, 10:27:38 pm ---You may like what I did.
A LM358 would work here as well.

   

--- End quote ---
That's a good example of op-amp compensation and MOSFET Gate resistor.
The component value are not critical.

LukeB:
There's a picture of the PCB. All of this advice is really helpful. I will try to add in the mosfet gate resistor and see if that makes the diff.

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