HM 4.3 new build - fan always on


New member
Long time reader and recently decided to try my hand at building a HeaterMeter.

Fan runs nonstop in manual mode.

  • HeaterMeter 4.3 build with thermocouple support and Raspberry Pi Zero W
  • Parts kit and 12VDC PS purchased from
  • Button functions,thermocouple and thermistor jacks, servo, and web UI tested with no issues

I followed troubleshooting steps from another thread that was related to HM 4.2 and looking for advice on where to go from here.

Fan troubleshooting (so far):
  • Cleaned solder flux and rechecked solder joints
  • Rechecked the MOSFETs are in the correct locations
  • Removed ATmega328P to test if fan would stop
  • Set fan to pulse mode
  • With power connector on left...
  • Q1 left to right voltage
  • 0% 2.5, 0, 0
  • 100% 0, 3.3, 0
  • Q3 left to right voltage
  • 0% 12, 12, 2.5
  • 100% 12, 12, 0

Thanks in advance for the help.

Main board images front and back.
Last edited:

Bryan Mayland

TVWBB Honor Circle
Oh wow really good post with all the information I'd ask for, and nice clean work on the soldering too!

I'm lookin at the Q1 voltages there as being not right. The center pin is the gate, so when it has 0V (0%) the left pin should be 12V. And when the center pin is 3.3V (100%) the left pin should be close to 0V-- which it is, CHECK! So for some reason the gate of Q3 isn't being pulled up properly, or it is being pulled down by something else?

First, check the 2.2K resistor right next to Q3. One side should be 12V, and the other side which connects to Q3 should also be 12V although I think you're going to see that it is 2.5V. Measure it at the resistor, not at Q3. If you see 12V and 2.5V on that resistor, I'm not sure where to look next apart from removing Q1 entirely. That would eliminate it as a possible issue (maybe it is partially conducting somehow?). If you still see 2.5V on the hole where its left pin was after that, there's something on that line that's not letting the voltage be 12V. The blower is only off when that has 12V on it.

EDIT: But I should say that thanks to the information you've given, we know for sure the problem is somewhere between Q1 (either itself or the left pin's trace) and Q3 (either itself or the right pin's trace or the 2.2K resistor).


New member
Bryan, thanks for the reply and additional troubleshooting tips. I did check the 2.2K resister and as you expected it's 12V on one side; 2.5V on the other. I will remove Q1 and go from there.

When I find the issue I will follow up here.


New member
Turns out Q1 was the culprit. I removed it, checked the voltage on the left pin hole to verify, dropped in a new BS170 and the fan is working correctly now. I must have damaged the component when I did the initial soldering.

Thanks again! Now it's time to fire up the pit!

Bryan Mayland

TVWBB Honor Circle
Excellent! That one seems to be more susceptible to damage on build, possibly due to its position near the edge of the board. Good work with the troubleshooting and fix.