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LM317 Based Digitally Controlled Power Supply

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EthannCraftt:
Hello. I'm looking to construct a digitally controlled power supply. I have a circuit drawn up with a PWM voltage control circuit of my own design, and the current control design that Dave drew on his PSU series hooked up to the input of the second LM317.

After being plugged into a circuit simulator, it seems to have really weird behavior even though both circuits work great on their own. Any help??? Schematic below.

(The +5v inputs are where the PWM will be inputted. An RC filter will be added in the final design. The 1ohm resistor to the far right will also be removed.)

TerrySt:
The voltage regulator using the pwm to the lm317 won’t work as drawn.  You have eliminated the feedback for the lm317 by connecting the opamp output directly to the reference pin of the lm317.   You can sum in a voltage to the ref pin or use the pwm voltage to raise the ground but you still need the output of the lm317 to affect the ref voltage.  But the way you have it, the connection from the output of the lm317 to ref is basically eliminated. 

TerrySt:
Sorry.  I spoke too soon. I was going by memory and it obviously isn’t too sharp.   That should work, although the resistor from the output to the ref pin isn’t doing anything other than raising the dissipation of the opamp. 

Zero999:

--- Quote from: TerrySt on April 21, 2024, 01:06:34 am ---Sorry.  I spoke too soon. I was going by memory and it obviously isn’t too sharp.   That should work, although the resistor from the output to the ref pin isn’t doing anything other than raising the dissipation of the opamp. 

--- End quote ---
The 220R resistor provides a minimum load for the LM317. It won't work without it. A lower value resistor might even be required, because the minimum load current for the LM317 is 10mA.


--- Quote from: EthannCraftt on April 20, 2024, 02:57:43 am ---Hello. I'm looking to construct a digitally controlled power supply. I have a circuit drawn up with a PWM voltage control circuit of my own design, and the current control design that Dave drew on his PSU series hooked up to the input of the second LM317.

After being plugged into a circuit simulator, it seems to have really weird behavior even though both circuits work great on their own. Any help??? Schematic below.

(The +5v inputs are where the PWM will be inputted. An RC filter will be added in the final design. The 1ohm resistor to the far right will also be removed.)


--- End quote ---
What a strange circuit.

What does the Darlington pair and all of the op-amps do before the LM317? It appears to be a current limiter, but the LM317 already has built-in current limiting.

All this circuit will do is output 30V.

Why not skip the LM317 and just use a Darlington pair? All it does in that circuit is increase the drop-out voltage.

TerrySt:
Yes, I see that the 220R is a good way to ensure a minimum load current for the 317.

The first stage is a programmable current limit using a pwm controlled voltage in place of the +5V input to the opamp.  Either circuit (the voltage control or the current control) should work on its own, but combining them might be tricky.  The current control won't take effect until the voltage control goes into drop out, which might make for some erratic behavior.  Whenever I've tried something similar, I've run into issues making it work predictably under all conditions.
I would try to sum the current control into the the opamp for the voltage control by eliminating the darlingtons and integrating the current error.  That might make things behave a little more predictably.

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