Strange potentiometer behaviour

Hello there everyone,

I am in the process of making a small variable voltage power supply using one of these common little LM2596 reference circuit boards, and a 16v DC power supply. It's a DC/DC step down circuit, that can take in anything up to 42v and output anywhere from 1 - 40v, with the upper limit being about a volt less than the input voltage. I've been using these boards for various purposes for a while, mostly bench supplies, but over time the cheap multiturn potentiometers on the board wear out.

I've had good results by replacing that pot with a larger single turn pot from sparkfun, but the tradeoff with that is that now the whole adjustment threshold for 0 - 40v is squashed into the 270 degrees of the single turn pot. It makes getting fine tuned voltages kind of difficult, and I thought I would be able to get around this by springing for one of these more expensive wirewound Bourns potentiometers. However, I'm getting some really weird behavior out of these pot's when I wire them to the board.

If I use my ohmmeter to check the pots beforand, I can verify that they range from 0 - 10K ohm over ten turns. The same ohmmeter shows the single turn pots go from 0 - 10K over one turn. However, when it's hooked up to the board, I can adjust it like normal about halfway through it's range (going from 0 - 10v), but once I get about halfway through it's range, it starts declining as I turn the dial forward.

If this description makes no sense I can try to make a video, but I'd really like to figure out why these potentiometers are doing this.



It's a switching converter IC. Is there anything in the app note about what type of pot you should be using?

Switching regulators are more complicated than linear.

How much current is going through the pot?

You've noticed that the pins on your 3590 potentiometer are in a different order than the original 3296-style potentiometer? The pinout on the 3590 is 3-1-2 from front (knob side) to rear.

Hi, I'd say that you have not got the 10turn pot wired properly, on the side of the pot they usually have a diagram of what the pin functions are.
It sounds like you have not got the wiper of the pot identified.

Tom..... :slight_smile:

Those pots are wire-wound internally. If it's sending a high frequency pulsed signal through it you might have an inductor.

You cant use wirewound in a feedback network, far too much phase shift, probably send the feedback amplifier
into oscillation. Wirewound for DC only.