Hi all,
I'm fairly new to the world of electronics. I'm trying to design a coarse and fine pot set up between two step up transformers. This is for a test fixture to repair cards for pulse modulated track circuits in the railroad industry.
Historically, this is how people have built these fixtures. The point is to be able to take the signal from an oscillator card and have it stepped up for a stronger signal to a receiver card. We have to be able to adjust the signal for testing purposes so the receiver card cannot interpret the signal below 8vpp. It is normally around 20vpp.
My problem is with a 25k and 2k pot wired like the other people's I'm having wild jumps with the 25k pot. It adjust nicely to a certain point that just jumps up to max vpp. Any advice?
Below is how it's wired.
Thank you

 

You can buy a high precision potentiometers, they usually have like 10 turns so the change of value is very small in according to the movement, you can really fine tune values with those.

 

What is the current or power requirements of the load?

A few things come to mind.
I suspect that the current draw is significant and the carbon track has been damaged at the maximum output of the pot's range.
I would suggest using a higher power wire-wound potentiometer. A 10-turn pot is a good idea.

Also, I would insert a resistor in series to limit the maximum current and power output when at the top of the range.

 

10-turn pots like these.
To determine the setting position and accurately repeat a setting, you can add a scale-dial.

 

A ten-turn pot eliminates the need for the 2k pot.