Onboard Resistors for PCA9685 PWM Expansion Board

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dagnabbitt
 
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Onboard Resistors for PCA9685 PWM Expansion Board

Post by dagnabbitt »

Good morning

So this may be an obvious question, but anytime I make assumptions, I let the smoke out of things.

The board come populated with 220 ohm series resistors on all of the output lines, but if I'm trying to drive a blue led with a 3.4V Vf, then I max out at something like 8mA.

Would I have to remove those resistors and populate with something closer to 85 ohm to get the 20mA @ 3.4Vf I'm looking for?

Thanks!

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adafruit_support_mike
 
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Re: Onboard Resistors for PCA9685 PWM Expansion Board

Post by adafruit_support_mike »

dagnabbitt wrote:So this may be an obvious question, but anytime I make assumptions, I let the smoke out of things.
Congrats.. learning to ask *before* the smoke comes out is one of the hardest skills to learn. ;-)

I can't think of any reason why changing the onboard resistors would cause any trouble electronically. The big questions are, "how comfortable are you reworking SMT?" and "are you dedicating this board to a single purpose, or would you like to reuse it later?"

If you're comfortable soldering grains of rice, you have a couple of options. You can pull the existing resistors and replace them with 82 ohm resistors (that's the standard value closest to 80 ohms), or you can piggyback a 120 ohm resistor on top of each one that's already there. The piggybacked version will probably be a little easier, and should get you almost exactly 20mA with a 5v supply and 1.6v of headroom.

If you aren't happy soldering through a magnifying glass, or just want to keep the board in original condition so you can use it for other things later, I'd suggest using the PWM signal to drive transistors, and letting the transistors drive the LEDs.

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