Hotheaded? Check Motorhead's thermals.

There's nothing much going on in the Motorhead, dissipation-wise. The main victim of dissipation will be the LV1117 linear regulator that drops the supply down from 12V to 5V to power the relay. Beyond that, the A4988 stepper driver might become slightly warm but it's a PWM controller, so should be bothered by what we are asking of it, not least as it's running from 12V here.

The linear regulator on the Motorhead gets hot, not surprisingly. It's not easy to predict the thermal performance of an SMT device with any precision when it's almost entirely reliant on PCB area for cooling.

When running the unit yesterday, I was able to confirm that it's certainly running above ambient and body temperature. However, that doesn't tell me much, not least as the pain threshold for most of us is around 50C at most and I'd expect this thing to be running closer to 100C, against a max junction temp of 150C.

Let's check, using the thermal camera - but first some calculations:

• The relay is a "3V" version, with 64.3R nominal resistance. 
• The RP2040 GPIO are 3.3V outputs and there's a 5V-to-3.3V LDO on board the Waveshare RP2040-Zero 
• The 12V-to-5V LDO I have used to supply the RP2040 Zero is the 5V version of the LD1117 in the SOT_223 package.

This should suffer ~ 110C/W. What is the dissipation?

The worst case duty cycle is 100% ie when the relay is permanently enabled. At this point, the current in the relay could should be ~50mA. 

Yes, that is (just) within max ratings for the GPIO outputs:

...and the dissipation in the LV1117 regulator will be approximately Ptot = (12V-5V) * 0.05A = ~350mW. With an Rth.j-a of 110C/W, that might result in a rise of ~38C. This would be above the pain threshold but shouldn't be a problem for the regulator.

What does the thermal camera tell us? Looks like 78C ie a 53C rise. I guess the assumed PCB tracking isn't present. However, a case temperature of ~80C is fine.

The A4988 stepper driver is just about visible under the crosshair (you can just about make out the fins). It's not even breaking a sweat.

All looks well - but it's good to check.