PCA9685 servo controller: resistor needed in control line?

[earckens]

Attached schematic by Adafruit shows a 16 channel pwm servo motor controller. The control lines all show a 220R resistor in series. Is that necessary? If so, why?

 

[rjenkinsgb]

I'd guess it's fault protection if a servo is wrongly connected or the servo / controller power are not linked, to avoid excess current in to an unpowered servo.

 

[earckens]

I'd guess it's fault protection if a servo is wrongly connected or the servo / controller power are not linked, to avoid excess current in to an unpowered servo.

Is a servo control line not high impedance?

 

[rjenkinsgb]

It's likely a direct connection to an IC input pin, so high impedance as long as the input voltage stays within the supply voltage range.

If the servo is not well designed, an unpowered one may try to pass current through the IC body diodes to other parts of the circuitry - no different to input limiting on any setup where different parts may be separately powered.

 

[earckens]

Thank you!
Erik

 

[dr pepper]

You often see such a resistance inline with an op amp when its likely to have long leads attached to it, to protect it from capacitive cables, if your app has long leads this might be why.

 

[earckens]

Hi dr. pepper, yes indeed the outputs to the servo control line will be a line several meters long. Thnks for that info!