Electronics > Beginners
LM358 in Consant Current Circuit Problem
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orolo:
You can solve both problems. Instead of amplifying x5 the load voltage, divide by 5 the potentiometer reference voltage: you can do that with a follower and a divider. That way, you keep the dangerous amplifier out of your feedback loop.

About the constant current, keep it simple and try an RC filter like the following. I don't mind building the circuit myself and testing it, if it gives more trouble.



Yansi:
Yes, that is the correct way to do it. However, you're missing a gate resistor - loading the opamp capacitively migh produce instability, or even HF oscillations of the mosfet. So adding a few hundred ohms series resistor to the gate is the simplest solution.

TheInfernoMan: You shall stop watching simple videos, as obviously, you don't learn anything from them, as things aren't that simple in reality.

I think Dave (EEVBlog) did make a video about designing a constant current load some time ago. Try to look it up. But I'd also recommend googling for application notes from the big semiconductor companies, as there are lot of very detailed appnotes with all the engineering in them, to make a basic up to very well behaved constant current loads.

For example I remember appnote from Jim Williams designing a 100A load, this one: http://cds.linear.com/docs/en/application-note/an104f.pdf
TheInfernoMan:
So first I want to say thanks to Orolo and Yansi.
I built up the simple circuit from you and it works fine.
You can't imagine how happy I am to see a circuit on my desk working  ;D

I would read the document you linked Yansi now, and I think I can learn a lot  :D
I will now redesign my schematic and build a new PCB.

Thanks,
@Orolo Can I send you my finished schematic, that you can have a look on it, I would not spend again hours to solder and then throw it away  :'(

Attached: Now the schematic that works; can I built my new schematic up from that schematic ?
Yansi:
Soldering and throwing away often gives you more valuable experience, than just look at things  ;)

Even if you build something that does not work as intended is an experience.
orolo:

--- Quote from: TheInfernoMan on January 01, 2017, 12:00:36 pm ---@Orolo Can I send you my finished schematic, that you can have a look on it, I would not spend again hours to solder and then throw it away  :'(

--- End quote ---
I'll build your circuit (with a different mosfet, I have to look what I got in the hoard  :) ) -- I'm curious about the gate resistor. Simulating your circuit, it works fine as per your schematic. It gently gets into regulation with no overshoot, no matter how big the gate resistor. However, with other mosfet models, increasing the gate resistor caused overshoot, so I think I'll put a pot there and see what happens.

I'm very glad the circuit worked, but the merit is not mine at all. I took the simple circuit from The Art of Electronics, values included. In the book they don't use a gate resistor, probably because they employ a jfet instead of a mosfet.
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