| 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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