Fan on all the time

Hey all. I finished my build earlier this summer and ran into this fan issue. I've just gotten some time to sit down and troubleshoot it.

I verified that there was no shorts at the CAT5 jack between the pins. Using a post that someone else had with the same issue I also verified 12 VDC between blower and ground when the fan was at 0/100.

I switched the mode to Pulse for the Fan output and measured voltages for ATMega Pin 5, Q1 and Q3. This was taken without the blower connected.

Pulse Mode - Fan 100%
AT Mega 3.3 VDC

Q1 Right 0.0135 VDC
Q1 Center 3.3 VDC
Q1 Left 0.0003 VDC

Q3 Right 11.9 VDC
Q3 Center 11.9 VDC
Q3 Left 0.0136 VDC

Pulse Mode - Fan 0%
AT Mega 0.0039 VDC

Q1 Right 4.6 VDC
Q1 Center 0.0039 VDC
Q1 Left 0.0002 VDC

Q3 Right 11.9 VDC
Q3 Center 11.9 VDC
Q3 Left 0.0136 VDC

I also switched the Fan Output back to Voltage Mode and saw the same behavior. It seems to me that the issue is with Q3 since the Control leg is not getting 12 VDC when the Fan Output changes. Do you guys have any other thoughts on what it could be?

Solder side of Board


Component Side

Mathew - not sure if you were referencing this thread, but I encountered a bad BS170 that caused my blower to run non-stop. If you haven't already, I would suggest reading through this post to see how it was diagnosed.

http://tvwbb.com/showthread.php?62777-Fan-amp-Servo-Questions-for-New-Build-with-RD3

Just realized the values I posted earlier had a mistake. See below for the correct values.

Pulse Mode - Fan 100%
AT Mega 3.3 VDC

Q1 Right 0.0135 VDC
Q1 Center 3.3 VDC
Q1 Left 0.0003 VDC

Q3 Right 11.9 VDC
Q3 Center 11.9 VDC
Q3 Left 0.0136 VDC

Pulse Mode - Fan 0%
AT Mega 0.0039 VDC

Q1 Right 4.6 VDC
Q1 Center 0.0039 VDC
Q1 Left 0.0002 VDC

Q3 Right 11.9 VDC
Q3 Center 11.9 VDC
Q3 Left 4.6 VDC

Dominick - Yours was the thread I was referring to. The ATMEGA Pin 5 on my unit seems to be working properly since it switches from 0 to 3.3 VDC when the fan comes on.

My concern is the 4.6 VDC I'm seeing on the right leg on Q1. Should I be seeing that?

It looks like the voltages are right on the line between the ATMega and the BS170, when the blower is OFF there should be no volts on Pin5 ATMega and the top (center) leg of the BS170, when the blower is 100% there should be 3.3v present there. Check.

When the Blower is at 100% the top leg of the BS170 should be 3.3v, the right (output) leg of the BS170 should have no volts, which is connected to the left (input) leg of the large MOSFET which should also have no volts. The center (output) leg of the large MOSFET should be full blast 12V at that point.

When the Blower is at 0% the top leg of the BS170 should have no volts, the right (output) leg of the BS170 should have 12v, and the left (input) leg of the large MOSFET should have 12V as well. The center (output) leg of the large MOSFET should have no volts at that piont.

So the conditions for your blower to turn OFF would be 12V on the two outer legs of the large MOSFET.

You are not getting a full 12V there on the input leg to make the blower turn off, for whatever reason you are only getting 4.6v even though the input on the BS170 is going down to 0V.

You would want to check that you have a BS170 installed in the Q1 position (read the label on it). Check that R14 is 2.2K and has 12V on one end. I can't tell the diodes apart in the pic, but D2 is part of the blower circuit so you dont want to have those mixed up. Maybe try another BS170 there if perhaps you have over heated that one when soldering or shorted it out or something.