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LM317 Dummy load circuit
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VEGETA:
Dear friends,

I have attached my circuit for this project but it does not work perfectly, thus I seek your feedback.

I used LM317 module (from Banggood or Aliexpress), it comes with filter caps and so on. I removed feedback resistor in order to drive the adjust pin directly.

My circuit can do from 0-650mA and then it jumps to more than 2A directly! sometimes I can regulate it to 1-1.7A but most of the times it jumps directly from 650-700mA to 2-2.5A.

I measured the op-amp output voltage and it jumps too, same as LM317 output voltage which is obvious.

I needed a negative rail in order for LM317 to make it to 0v output which is a must, so I have put diodes in series to achieve it. They get too hot and I managed to burn a few of them.

Do you have any solution?

I know I can use a simple NPN for the job instead of LM317 but I don't have a powerful NPN (all tiny size) nor a MOSFET and I also want to make it using the LM317.

Looking forward to your kind help!
Ian.M:
That circuit sucks, and so does your photo!

You can treat a LM317 as a thermally protected power transistor and use it in the classic OPAMP + transisistor current sink circuit, where the current negative  feedback is taken from the top end of a low value emitter resistor.

However its far from ideal for low voltages, as even with a negative bias supply for he OPAMP, its dropout voltage can be up to 2.5V.
VEGETA:

--- Quote from: Ian.M on April 14, 2018, 03:26:55 am ---That circuit sucks, and so does your photo!

You can treat a LM317 as a thermally protected power transistor and use it in the classic OPAMP + transisistor current sink circuit, where the current negative  feedback is taken from the top end of a low value emitter resistor.

However its far from ideal for low voltages, as even with a negative bias supply for he OPAMP, its dropout voltage can be up to 2.5V.

--- End quote ---

I took the photo at 6:00 AM.

So is there any solution to the current circuit? any modification which still uses lm317?

Is my negative voltage generation method any good? what is your method if you want negative rails from a single DC supply?

My shunt resistor is about 0.5R or even less... so I need millivolts to make amps which is, as you said, not ideal for lm317. So what if I made the shunt resistor 1 ohms? this way we can use 1v for 1a which is kinda better.

I currently have small transistors like 2n2222 or so... Dave mentioned that he used a logic level mosfet for the job and it is important to be like that, Why? is it because of low gate voltage form op-amp?
Cliff Matthews:
..what is your method if you want negative rails from a single DC supply? This works (maybe add a third diode for your cct.)
VEGETA:

--- Quote from: Cliff Matthews on April 14, 2018, 03:35:06 pm ---..what is your method if you want negative rails from a single DC supply? This works (maybe add a third diode for your cct.)

--- End quote ---
Thanks cliff, but I used this method in my circuit.

Anyway, I'd like to know if this circuit has any hope of success or I should just revert back to traditional opamp+mosfet. In that case, there are lots of options.
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