Help me understand this variable power supply circuit.

Thread Starter


Joined Jan 18, 2016
Hi guys,
I was hoping that you could explain the following circuit used in a variable power supply. It consists of an LM317.
1. I don't understand what the two diodes V5, V34 and zener diode V8 is used for? What kind of a 'magic' does that bit of circuit do?
2. The JFet is used as a variable resistor for the Adjust pin, but why use it when there is R4 a potentiometer.
3. R7 I assume sets a kind of a lower limit, because R4 is the main potentiometer used to adjust the output voltage. R7 is one of those small blue potentiomter and the adjustment screw is glued. The power supply could be varied from 0-30V.

I hope you guys could help me understand this circuit a bit more. Any more tips would be greatly appreciated.

LM317 circuit.jpg

Thanks in advance.
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Joined Aug 1, 2013
1. V5 prevents the node labeled -2V from going more positive than 0.7 V. V8 looks like a 1.4 V zener. Combined with V34, this makes a 2 V diode, which makes sense if you think htat V8 is drawn in backwards. By itself, an LM317 cannot regulate down to 0 V. But if you reference the adjust pin circuit to -2 V, then the circuit can regulate down to 0 V.

2. The FET is not operating as a voltage-variable resistor. V12, R5, and R6 form a constant-current circuit. This *might* have the effect of linearizing the effect of pot R4, but I have not run the numbers.

3. R7 sets the max output voltage, not the max output power. It is there to trim out the combined tolerance errors of R4, R5, R90, R91, V8, and V34.


Thread Starter


Joined Jan 18, 2016
@ericgibbs - value of VSS is -15V. The voltage at the input pin is around 45V, comes from a step down transformer and is rectified and smoothed.

@AnalogKid -
1. yep V8 is a 1.4 Zener diode, just wanted to clarify I didn't understand when you said "which makes sense if you think htat V8 is drawn in backwards." Should it be the other way round? I can confirm that the schematic matches with how it is in circuit.

2. Constant current sources - this is something I have found a bit hard to get my head around. Could you briefly clarify this for me.

3. That makes sense with the R7, how do you think that initially it was adjusted and glued - Turned the main pot to max (R4) and adjust R7 until the output reads 30V ?

Thanks so much for the help :)


Joined Mar 14, 2008
A simpler way to provide a regulated -1.25V to allow the output to be adjusted to 0V is to use an LM337 negative regulator.
Example circuit below.
The LM337 may be seem to be overkill for this purpose, but they are less than a dollar (US) in the small package.