How to make LM317 output voltages less than 1.25 V

Thread Starter

Evan Moore

Joined Dec 18, 2021
8
Hello!
I am trying to build a variable dual power supply, and I think I have most of the circuit planned out. however the only thing I do not have planned is how to get the power supply to supply voltages less than 1.25V. I have read online that in order to do this instead of starting the LM317 at 0v to whatever you max out at, instead you start from -1.25V, is this correct? If it is how do I do this? The only way I know how to produce a negative voltage off of the top of my head is to reverse the power entirely, but then I would only be able to give the control pins negative voltage only.
 

dl324

Joined Mar 30, 2015
14,318
Welcome to AAC!
I have read online that in order to do this instead of starting the LM317 at 0v to whatever you max out at, instead you start from -1.25V, is this correct?
Yes, you connect the resistor on the adjust pin to a negative voltage. It needs to be at least -1.25V, but can be more negative.
If it is how do I do this?
Show us what we have to work with. Are you using a transformer for the unregulated voltage? If not, you can use an inverting switching regulator to generate a negative voltage.
 

Thread Starter

Evan Moore

Joined Dec 18, 2021
8
Here is the LTSpice file, I removed the LM317 and LM337 but I put text where it goes, I only started using LTSpice a week ago so I am not sure how sharing third party subcircuits works, so better safe than sorry. Anyways I am currently using a potentiometer (two resistors) as a voltage divider to vary the adjustment pin of the LM317 and LM337, so currently It only works for positive voltages. I have not actually hooked it up because I wanted to be able to have the output start at zero, and I am not sure if you have to put a negative voltage in the adjustment pin of the LM337. Anyways I hope this helps.
 

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dl324

Joined Mar 30, 2015
14,318
Using LTspice to simulate simple voltage regulator circuits seems like a waste. It's akin to using a calculator to add 123 +234...

The National Semiconductor datasheet showed how to do it for LM317:
1640017521314.png
 

Thread Starter

Evan Moore

Joined Dec 18, 2021
8
Ok thank you! I was honestly just using a simulation to help wrap my head around it to make sure I have a schematic for when I breadboard it, and just as a sanity check. Also is R5 a mistake? because that would just be a resistor
 

dl324

Joined Mar 30, 2015
14,318
I was honestly just using a simulation to help wrap my head around it to make sure I have a schematic for when I breadboard it, and just as a sanity check.
You'll learn more if you do it yourself, even if you burn up some components. An inexperienced person using a simulator before understanding the basics could find themselves in a position where they can't do much without one because they never took the time and effort to learn the basics. That's why you weren't allowed to use a calculator in school until after you should have mastered arithmetic by rote.
Also is R5 a mistake? because that would just be a resistor
It's not a mistake. It's an omission. I didn't connect it because I didn't want to think about whether you wanted clockwise to be increasing negative voltage or decreasing.
 
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