Current rating of PWM output pins

I want to connect an opto- isolator that draws 20mA to a relay channel on the Navigator. Do you know what the current carrying capacity is for a single channel?

Hi @gcelec,

The Navigator’s PWM outputs have a Max Signal Current Draw of 15 mA, as specified in the technical details. If you want to drive a higher current device you’ll need to use an electronic switch (like a relay or MOSFET).

That said, is 20mA the minimum current for the opto-isolator, or just the typical/recommended? LEDs can generally operate at significantly lower currents than their typical, just at lower brightness, which might still be sufficient for your opto-isolator to work.

For some context, the PCA9685 chip is technically capable of driving up to 25mA per PWM output pin, but the Navigator board has 220Ω resistors at the outputs, so at the 3.3V supply voltage provided to the chip the output current is limited to 15mA.

The reasoning there is that the 3.3V LDO on the Navigator is only capable of supplying 300mA, and it’s expected that everything else will use less than 60mA, which leaves the remaining 240mA for the PWM pin outputs, which split into 16 gives 15mA per output. Technically that’s an average limit, but design-wise we’ve gone for the “consistent and always safe” approach, rather than allowing higher currents on some pins iff others are lower, with the possibility of frying all the outputs if the average current gets too high.

Hey Eliot,

I must have spent 10 minutes trying to find that information on the web page and of course I went back there now and found it straight away. I actually went to the data sheet for the chip that is used for the pwms and got the information from there. It says 25 milliamps per channel but I think I will err on the side of caution and assume it is 15mA

Thanks for the info.

There’s a blurred out “spoiler” section in my original comment explaining the context around this. Without modifying the physical components, the Navigator PWM outputs are limited to 15mA.

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