Reverse Voltage fried my HeaterMeter



New member
Howdy Everyone-

A few years ago, I decided to build a HeaterMeter and have enjoyed building and using it. Well, a few weeks ago, I (partially) fried my HeaterMeter through a major mistake on my part.

A few notes:
Heatermeter (v4.3) was connected to a 12v battery (but reverse polarity - BIG oops)
I had the HeaterMeter connected to RalphTrimble's RD3 with the Aux Thermocouple (and necessary modification for use documented in forum)

Upon correcting the polarity, I found the Unit will boot up, but lost all blower and servo response.

During diagnosis, one of the first things I did was to replace Q1

I then eventually found that the trace on the circuit board between D3 and the +12 power of Q3 had been fried/melted/broken

Upon bridging/repair of the trace, the blower now seems to be operating correctly. However the servo only gave a few "grunts" and stopped working. Subsequently, I replaced the servo with a new one from Vorpal.

Everything appears to by working now. After the electrical stress that subjected my poor HeaterMeter to, are there any other components that I should check and/or replace?

I appreciate any insight or help.



TVWBB Member
Well, if it works, it's probably good. Perhaps the polarized capacitor C12. In theory, D3 should have protected the other components. Perhaps check Q2 if you care about the buzzer.

Wonder why D3 is not before the 12v feed to rest of board? Perhaps did not want the voltage drop?


TVWBB Diamond Member
I've fixed one HM that was hit with reverse polarity, I believe the inductor in the blower feedback circuit was one of the things that fried. After reverse polarity was applied (and the smell of heat/burn detected and power quickly removed), I applied proper polarity power and the HM seemed to work. Quickly I noticed the blower seemed to go from off to max speed rather abruptly instead of sweeping smoothly, long story short, the inductor in the blower feedback circuit was damaged. You can compare pulse mode to voltage mode to get an idea if the feedback circuit is damaged.

Darren L

TVWBB Member
Sounds like adding a diode for protection might be a good idea. I think the 1.2 drop shouldn't affect anything but the fan. I'll add one externally in my supply lead to see what happens.

Bryan Mayland

TVWBB Hall of Fame
Yeah the diode D3 is intended to provide some reverse polarity protection, but the fan circuit is outside of that. I didn't tap off the far side of the diode because of the voltage drop. The diode is also limited to 1A max current and I was concerned folks might run beefier blowers, although I supposed that's more of their problem than mine haha. I'm surprised that Q3 wasn't destroyed as well since it should conduct through its own body diode if hooked up to power backward.

I'm not really clear on what exactly would be affected though, and it is possible that D3 doesn't protect anything at all if it damaged the servo and Q1 (which should be on the safe side of D3).

I'd definitely add external protection if connecting a battery, and perhaps a 2A fuse as well, but it wouldn't be needed if using a regular power supply which has its polarity fixed and its own overcurrent protection.