PCA9685 not using external power

Projects General Guidance
21

Hi,

Sounds like a nice work around for allowing a larger current than via the small pin connectors on the side of the board.

Do you think it would be possible to replace the transistor, was is surface mount or normal package? Some of the moving parts carry too much momentum on our designs and I think it’s knocking out the the power to some of the servos which then need resetting to get going again.

Cheers
Matt

22

I'm sure an appropriate P-channel enhancement mosfet could be substituted. The boards I have use a small SMT package-- SOT-23-3 I believe. The Adafruit schematic specifies a AOD417, which is a much larger package. I suspect the design change was to reduce cost but is perhaps flawed.
I notice a couple of things on the Hiletgo version on Amazon:

  1. The MOSFET is entirely missing, so they ditched the polarity protection (probably because the design w/the tiny MOSFET doesn't work, and the AOD417 increased cost too much).
  2. The LED resistor is 470 ohms, instead of 220, probably because using the 220 makes it so the LED is so bright it burns holes in your retinas :smiley:

I'd go with just jumpering the MOSFET. Replacement is probably not worth it. Don't hook up your power backwards :wink:

23

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