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I'm trying to power my servo from an external power source and control it with my Pi and I've found following circuit: http://razzpisampler.oreilly.com/images/rpck_1001.png

Why is there a resistance of 1k Ohm? Is it to limit the Pi's amperage to 3mA?

Ohm's law: V = I * R
3.3v = I * 1000ohm | / 1000ohm
3.3v / 1000ohm = 3mA

If yes, why 3mA?

nullexception
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1 Answers1

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Without knowing the interal circuit of this servo it is hard to tell. But let's assume this does not want to drive the servo directly from GPIO pin ;)

The series resistor is a measure of precaution to limit the current the GPIO pin will sink/source depending on its setting (in/out) and the load (in this case - whatever input circuit there is in the servo). GPIO current should be below 16 mA per pin (50 mA in total, all GPIO pins), so 3 mA is a reasonable value. Electrical engineers just pick a standard value, in this case 1 kOhm and that is that.

See also:

Community
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Ghanima
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