Electronics > Beginners
LM358 in Consant Current Circuit Problem
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TheInfernoMan:
Hello,

I am new to this forum and I have a question, regarding to my Schematic.
My wish is to build a Constant Current Dummy Load like the one from Dave, but I have a lot of problems.

I started on the breadboard and I think everything worked okay. Then I designed a PCB and built it up.
There starts the problem. The Current is all, but not constant. At one time, there is no current, then the full 5A, instead my wish of 1A at this time.

So I started debugging my circuit and I think I destroyed one LM358 (I dont know how, but I had 40V on my Circuit, but only powering with 12V Input  :o ).
Now I simplified the schematic as good as I can and attached it to this post. (I used a new LM358 in the debugging circuit)
The original Schematic is also attached (is there a mistake ?)

If I connect the non-inverting Input to Ground and use the LM358 in a Comperator configuration my idea is that no current should flow.
But in Reality there is 1.3A flowing at A1.

Why is this so ? I have no idea.

--
I am really new to electronics and I am German, I'm Sorry for a few mistakes in my language...
Kleinstein:
The LM358 can easily oscillate with too much capacitive load at its output. Something like 100 pF, maybe 1 nF with some luck is Ok, but more is calling for trouble. The 2 FETs are more like in the 10 nF range.

So one would need to have resistors between the OPs output and the gates (one for each FET). This is generally a good idea if there are two MOSFETs in parallel as other wise high frequency oscillations are possible.

In addition there should be a fast feeedback directly around the OP: so a capacitor from OPs output to the inverting input, and a resistor in front of the inverting input.

C1 in the original circuit is at a wrong place (wrong input) and rather large - more like 1 nF would be the right size.

For a first test I would use just one FET, with 40 V it is possible the FETs died from to much voltage.
TheInfernoMan:
So I changed my circuit a little bit, but in this short time I could not desolder my FET's from each other.
The new Circuit is now attached and the problem is likely the same, but now at A1 is only 0.02A instead of 1.3A
But if I slowly rise the Voltage at the non-inverting input the Current Rises instantly to 3A

Should I desolder the FET's from each other and try ?
TheInfernoMan:
If I apply 0.1V to the Non-Inverting Input the flowing Current is like 0.5A, but if I continue rising up the Voltage to 0.15V the Current, which is flowing decreases with more Voltage at the non-inverting Input. And my buck-converter which supplies the non-inverting input makes a quiet buzzer sound. Now and then the Current oscillates (I see it on my Ammeter) from 0A to 0.9A often up and down.
Audioguru:
You missed an important fix to prevent the new oscillation: "In addition there should be a fast feedback directly around the OP: so a capacitor from OPs output to the inverting input".
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