Electronics > Repair

Trimmer pot repair and selection

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chipwitch:
I have a blown current probe pcb.  The coil (not a hall effect probe) is intact but the trimmers blew.  One is completely unidentifiable.  The other has the following marks: 82MR50 9534F.  My guess is this is a 50 ohm pot due to reading 0 to 44 ohms and then infinite for about 10 degrees of rotation.  The "MR50" seems to suggest the same.  It's a single turn pot.  I can repair the pcb, but I need to determine the pots' size. 

The second one is more difficult.  It sustained significant damage and basically disintegrated.  Both pots are parallel to respective power resistors.  The 50 ohm pot is parallel to a 2 ohm resistor.  The other is parallel to a 200 ohm resistor.  These are obviously used to limit current  from the clamp's coils, one resistor/pot pair for each of the two scales.  I realize I could put just about any pot in their place and that as long as the pot is larger than its respective resistor, the total resistance value will be anywhere from half the resistor value up to the value of the resistor, asymptotically.   

I am new to electronics, and through analyzing this simple circuit, I learned a couple things.  One, don't burn up your current probe.  But, aside from that, this arrangement of pot/resistor is rather elegant in that the resistor is carrying the brunt of the current and the trimmer is simply fine tuning it.  Plus, using this arrangement means you could get very high precision resistance values by using a resistor and a much higher value of pot.  Beautiful! 

So, with a very high value pot one gets high precision, but not much change in Rtotal.  As one decreases the value in trimmer pot, down to the value of the parallel resistor, one increases the change in Rtotal at the expense of precision.  Another way of saying it is, precision is directly proportional and Radjust is inversely proportional to Rdelta between resistor and pot.  Of course, current capacity of the pot will trump too low value of pot .

My question is this: is there some rule of thumb resistor/pot ratio that is often used for general selection of trimmer that is a compromise between precision and R adjust range (pot power limits aside)?  I'm inclined to use a 5k pot for the 200 ohm resistor in keeping with the ration of the 2 ohm resistor.  I'm sure that's fine, but I'm still curious if there is general rule of thumb.

Conrad Hoffman:
The main resistor plus high value trimmer is the classic way to do it, and almost any precision can be achieved. The only rule of thumb is Ohm's Law, and I find it handiest to make a spreadsheet. You start with the resistance limits you need (or voltages if working with dividers or similar) and then choose parts to match the desired window. You can also include power in the sheet so you know if the parts are going to go up in smoke!

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