New HM 4.2.4 - Fan running constantly


 
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Dave Fisher

New member
First HM, and first post, although a long-time lurker. I just put three HMs together, and the first one I set up is working perfectly, except for the fact that the fan runs wide open regardless of the command from the microcontroller. I checked the output to the blower, and it stays at 12v regardless of the % output. I swapped out Q3, thinking it may be bad, but no change. Any thoughts? No solder bridges that I can find, and I tried to be very diligent in my soldering technique.

Thanks in advance for any input...
 
Sounds to me like you have 12V shorted to the blower output somewhere, but that's just a long distance guess. Best bet would be to take a clear close up pic or the front and back of the board and post it here, maybe somebody can spot your problem for you.....
 
I will try to get some pictures posted tonight after work (you know - the pesky thing that keeps me from spending all my time at the BBQ pit...)
 
You can check Q1 (the BS170) near the fan. The center pin should read 0V, the pin closest to the output should read 0V and the remaining pin should be 12V. The power MOSFET should be 12V,floating,12V. If something's off there, it could point you in a direction.

You could also try removing the power MOSFET to see if the blower still runs. If it does then you know you've got a bridged connection somewhere. If it does not then there's a possibility the MOSFET is bad?
 
Thanks for the info Bryan.

At 0% fan, Q1 (BS170) reads 3mV on the center pin, 0V on the pin closest to the output jack, and 40.2mV on the remaining pin. The power MOSFET reads (from the output jack side) 40.2mV, 12V and 12V.

At 100% fan, the readings are the same.

With the power MOSFET Q3 removed @ 0% fan, Q1 (BS170) reads 3mV on the center pin, 0V on the pin closest to the output jack, and 13mV on the remaining pin. Q3's pads read (from the output jack side) 13mV, 30mV and 12V.

With the power MOSFET Q3 removed @ 100% fan, Q1 (BS170) reads 3.27V on the center pin, 0V on the pin closest to the output jack, and 40.8mV on the remaining pin. Q3's pads read (from the output jack side) 40.8mV, 30mV and 12V.

The voltage from the drain of Q1 is tracking with the gate of Q3, so it appears that the signal trace is intact. Since the gate of Q1 is seeing a change in voltage and the drain connected to the gate on Q3 isn't, wouldn't that indicate a problem with BS170?
 
Yeah It seems like the gate on Q1 is always conducting. Try removing it and Q3's output jack side pin should jump up to 12V. Also make sure you didn't install a 3.3V regulator in there by mistake, which some others have done.

If the voltage doesn't jump up to 12V when you remove Q1, the pullup resistor (that 2.2k one right there) should have 12V on both sides, check there.

If the voltage DOES jump up, replace Q1.
 
Update to the issue: As it turns out, on all three HM 4.2.4 boards that I put together, the BS170s that drive Q3 were all bad. I only had two spares, but after checking everything possible, I went ahead and replaced them. Two of the three boards are now functioning perfectly!

Thanks for all the help - this has been a great and fun project. Hats off to you Bryan for a fantastic design!!
 
Dave Fisher said:
Update to the issue: As it turns out, on all three HM 4.2.4 boards that I put together, the BS170s that drive Q3 were all bad. I only had two spares, but after checking everything possible, I went ahead and replaced them. Two of the three boards are now functioning perfectly!

Thanks for all the help - this has been a great and fun project. Hats off to you Bryan for a fantastic design!!
Click to expand...

That's really odd that would happen. I can understand maybe one, but more then one in the same order. You might want to play the Lottery lol
 
I've been trying to figure out what I could have done to ruin all three components the same way on three different boards. The two spare BS170s were part of the same order, so I'm pretty sure that even with my outstanding 'luck' I wouldn't have been able to randomly pick three out of five bad components...
 
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