Electronics > Electronics

Resistor selection while modifying a servo

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Inigo Montoya:
Hi guys!  I have read the servo modification tutorial here:  http://www.societyofrobots.com/actuators_modifyservo.shtml
I have also read a lot on the internet about modifying servos.  I've chosen the method of replacing the pot with a resistor y bridge.  Generally, 1k resistors are recommended, and they say up to 3k is ok.  However, I am working with micro servos, so I'm not sure if that value is appropriate.

How is the appropriate resistance value calculated?  I have not been able to figure that out from any of the tutorials here or elsewhere on the internet.  Would someone here wise in the ways of electronics please help me with this bit of theory?

FYI the specific servo I am working on is here:  https://solarbotics.com/product/25500/

waltr:
One starting point is to measure the total resistance of the pot used in the servo. Measure across the pot ends. If the pot measures 3k Ohm then chose two resistors that sum around 3k Ohm. The sum does not need to be exact so anywhere between 2k and 5k would work.
To better understand what the pot does and what you are doing by replacing the pot with two fixed value resistor look up and study "resistor divider" circuit.

MrWizard:
Which ever resistors you use, measure them and choose the best results which are as near as possible/the same. Resistors can vary in resistance. Otherwise you could end up with servo's driving with different speed.

Inigo Montoya:
Success!  I followed your advice and it worked great the second time.  These servos are extremely small and their innards are very delicate.  Good thing they're cheap.  The only thing I noticed was the servo wasn't centred at 90 degrees being stop, even though the resistors were almost a perfect match.  But no matter, that's something that can be ironed out with software calibration.  I'm so glad it works!  Thanks for your advice guys!

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