Heatermeter fan constant low voltage

I just put together my heatermeter last week and everything is functional except the fan. The output voltage I get is stuck at 0.75-0.77V whether fan output is 0-100% or inverted. Servo is working fine. When I checked voltages against the diagram under hardware, the 12V and 5V are all correct. The 0/3.3V cycling doesn't appear to behave that way for me. I thought it may have been a bad atmega chip but replaced it and it exhibits the same behavior. Any thoughts on where I should look next to see what's causing this?

You can try maybe setting it to pulse mode and 100% manual output and see if you're getting 12V out at the blower. If not, work your way back to the 47uF capacitor, then both sides of the inductor, the 1N5819 diode, and the MOSFET output then the input. If the MOSFET feels warm at all, then you've got a short somewhere in this.

It sounds like the 1N5819 might be installed backward? That would short it straight to ground and leave only the diode drop at the blower output, which would be ~0.6V.

Bryan Mayland said:

You can try maybe setting it to pulse mode and 100% manual output and see if you're getting 12V out at the blower. If not, work your way back to the 47uF capacitor, then both sides of the inductor, the 1N5819 diode, and the MOSFET output then the input. If the MOSFET feels warm at all, then you've got a short somewhere in this.

It sounds like the 1N5819 might be installed backward? That would short it straight to ground and leave only the diode drop at the blower output, which would be ~0.6V.

Click to expand...

1N5819 is installed with the silver band towards the edge of the PCB (same as the silkscreen). With the positive probe on the ground side, the diode registers 139ohms while in the other way it is ~44k. The MOSFET middle pin is registering 1.13V and both outer pins are 12V. The inductor has 1.13V on the side connected to the MOSFET and 0.75V for the blower trace. Checking for shorts (still checking other parts of the board referencing the diagram), I noticed the middle pin of the MOSFET is connected to the metal backing. Is that the problem?

Try resoldering R14 (2.2K) on the top and bottom. (it should be over there by the large MOSFET if I recall correctly) I had a similar problem with a HM once and after extensive troubleshooting I found there is a trace on the top and bottom of that resistor and the trace on the top wasn't connecting even after soldering several times from the bottom. This was puzzling because when I tested the solder side of that resistor the voltage was sweeping the way it should, then I noticed following the trace on the top of the board it wasn't. I put the iron on the top and made sure it was soldered and the problem went away.

Eureka! Turns out it was a poor solder joint on the BS170 right above the big MOSFET. While probing more voltages, I got lucky and it triggered the fan when I applied pressure to the joint. Thanks Bryan and Ralph for the help!

Now to get smoking...