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Hacking a Resistor to 100.00% accuracy

Tunes a resistor to the value that "You" want

Which 10KΩ Resistor would you choose :-

Well to tell the truth its the same one ..... just the one below has been Tweaked !

The Trick is to file the body of the resistor (beneath the outer shell _ into the carbon/metal) using a needle file (or cosmetic/nail file).

Most common resistors are made from Carbon/Metal film or metal oxide, by file_ing the Carbon/Metal away the resistance can be tuned to whatever value you want.


Minimal damage....

Basic rule :- The resistance increases with file_ing.

Resistance = Length of resistor / cross section Area x (substance conductivity)


The Bare facts ........ the shell removed reveals how the manufactures also mill/file the surface to trim (did i dig out an industrial hush-hush)

Caution :-

Resister values are are quoted @20°C ..... and fluctuations of the resistance can occur due to external environment.

If you make excessive deep cuts then the power rating of the resistor will also suffer.....

Do not adjust the Resistor on live equipment....... (risk of shock)


Uses :-

How many times have you hacked a servo to continuous servo and added the two resistors to replace the potentiometer...... only to find that when you send a center (1500) command to the servo ....it creeps....creeps...creeps albeit slowly but it nerves the heck out of you.... and you have to put software jigs to stop the motor from creeping......well look no further just get the file out and tune away........(leaving your code perfect)

Or.... your trim pot is just to coarse to balance a circuit ..... place a cheaper resistor in and file it too suit......

....enjoy ...... comments and likes are appreciated........... other ideas/uses even more soo...

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The centre point value made by the two resistors will probably be unaffected my temperature, but how about the other half of the system, the servo's internal circuitry. In there, you will find some sort of circuit which "calculate" the difference between the analog input from your two resistors and the digital input from the servo control signal. Most likely it's made with some sort of sawtooth oscillator.

Two comments:

  • When damaging the lacquer of the resistor, you are opening it to humidity, which now might creep in there and change the value of the resistor far more that your handmade adjustment did.
  • The hacked servo creeping at 1500uS is a problem that you will find it hard to work around, even with extreme precision resistors. The set point varies a little with temperature too. My best suggestion is to handle the problem in software, simply stop sending the servo signal when you want it to stand still. On the Arduino it can be done with Servo.detach(). I have a nifty little library doing this somewhere. I will see if I can find it if anybody wants it.

Thanks for feedback.......

Yes an arduino automatic Servo.detach() would be neat - that's one thing i like about the picaxe servo command.....ie if no servo position change after xxx seconds then shutdown....

Switching the servo power off when not needed is a bonus for battery powered projects (provided the attached equipment does not require the tension)

Wouldn't an acrylic finish seal it adequately once the adjustments had been made?  Like an MSA or even nail polish?

Sealing should do the trick..... i will monitor the resistor and if it drifts then it not the first time my "Better Halfs" nail varnish gets painted into one of my projects :0)

Nail polish. It works great for many things. I use the "outdated" colours from the girls here for sealing and as a soft lock tight. A little dap on a screw and the nut will stay where you left it, but you will still be able to get it off if you want.

Once again Gareth digs out a gem! Thanks! You are the Jedi Master of the workbench.

Very useful to know :)

Nice and simple hack!