Cal Resistor Using Stacked Trimmers

Multi-Turn Multi-Pot Clip-In Cal Resistor Thing

A low budget substitute for a decade resistance box.

Kind of a variable resistor with course, fine, fine-fine, fine-fine-fine,... controls.

a photo that's too big for something this simple

Make it:

Find some multi-turn "trimmer" resistors with different values. In this case the values are 100, 1000, 10k, 100k, and 1M ohms.
Glue them to something or just to each other.
Connect them in series. (Use a ohmmeter to pick the pins that make the R go the right way when you turn it the right way.)
Attach wires and clips of your choice.

Use it:

Start with them all set to somewhere in the middle of their range.
Connect to the "circuit under calibration".
Adjust the highest value pot fairly close or until you run out of turns.
Adjust the next lower one fairly close or until you run out of turns.
Adjust the next lower one fairly close or until you run out of turns.
Keep going until it's close enough or starts getting ridiculous.
Pull it out of the circuit and measure with an ohmmeter.

Why it works better than a decade resistance box:

Why it works worse than a decade resistance box:


The values listed worked good for what I used it for, but you don't get very fine control action below a few hundred ohms. You could use lower values or attach a higher value pot in parallel with the lowest one. Maybe 1k or 10k across just the 100 ohm. Or something like that.

There is no reason the part values have to be decade steps.

Watch out for self heating changing the values between in-circuit and the ohmmeter check. Maybe even heating from your fingers.

A little insulation might be nice. Maybe a blob of RTV.

Maybe a switch to switch it to the ohmmeter and back.


Suggestions, corrections, comments, and general abuse are more than welcome.

Go back to Music Gear

Go Home

 know, the time you wasted here is gone. You will never get it back.

© 1999 Toby Paddock