I recently aquired a somewhat old server with loud non-PWM fans hooked to a straight up 12V power distribution board ("molex" plug and 6 3 pin fan ports, one side 12V, the other ground). Since the motherboard in it (and the one that I'm swapping in) supports PWM, I decided to make my own replacement distribution board with two PWM channels; while controllers can be bought, each of the 4 fans is rated at 1.5A... so a cheaply made 2A max unit of ebay wasn't going to cut it.
The PCBs came in today (ordered from allPCB, they turned out nice), but unfortunately it's not throttling connected fans.
Attached are the schematic for the board, gerber files for the PCB, and the intel PWM spec I designed it to meet.
The topology is based on a schematic I found here:
https://electronics.stackexchange.com/questions/275561/use-motherboard-pwm-to-control-non-pwm-fans-with-mosfet...and I verified the circuit with some rough back of the envelope calculations.
Here's what I've done for diganostics
- No PWM signal: Fans run at full blast
- grounded PWM signal: Fan shuts off
- connected to computer with known working PWM fan connector (tested before and after with a PWM fan): No throttling, fans at full blast
- Tach signal: present on output
- grounding the ground connector on the PWM in ports: No change
- Swapping D1 for two more 2.7K resistor's in parallel: no change
- testing a PWM fan with a jumper wire on the PWM port: grounding slows fan, connecting to 12V does nothing, confirming that PWM fan signals are NPN, not PNP in nature
Ideas on what to check next. Sadly, unlike my college days, I don't have good scope access; I do have a couple of good mutimeters though.