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I have the following circuit wired up:

schematic

simulate this circuit – Schematic created using CircuitLab

Where 4 STP55NF06L MOSFETS are wired to 12V LED stripes, and one STP55NF06L MOSFET drain is wired to an ATX PSU green (PSU ON) wire.


System just plugged in. Arduino pulling "PSU" MOSFET gate LOW. System OFF. System just plugged in. Arduino pulling "PSU" MOSFET gate LOW. PSU OFF.



Arduino pulling "PSU" MOSFET gate and "RED LED" MOSFET gate HIGH. Both system and RED LED turning ON accordingly. Arduino pulling "PSU" MOSFET gate and "RED LED" MOSFET gate HIGH. Both PSU and RED LED turning ON accordingly.



Arduino pulling "PSU" MOSFET gate and "RED LED" MOSFET gate LOW (0.02V with FLUKE 106 multimeter). PSU and LEDs failing to turn off. Arduino pulling "PSU" MOSFET gate and "RED LED" MOSFET gate LOW (0.02V with FLUKE 106 multimeter). PSU and LEDs failing to turn off.


When the whole system is unplugged from mains:

  • MOSFETs Drain-Source resistance = 9Mohms.
  • MOSFETs Gate-Drain resistance = 9Kohms.

When the whole system is plugged to mains but no MOSFET has been turned on yet:

  • MOSFETs Drain-Source resistance = fluctuates every second between 3.5Kohms and Open Line.

When the whole system is plugged to mains, MOSFETs have been turned on before and now all the Gates are being pulled LOW:

  • MOSFETs Drain-Source resistance = Open Line. (But LEDs shining and PSU still ON in spite of "PSU" MOSFET gate pulled LOW) Gate-Source voltage is 0.02V when LOW and 5.2V when HIGH.

I haven't managed to fix this myself in the last few days. There are many posts in the internet about MOSFETs not working properly but I don't understand why do MOSFETs work OK when the system is first plugged in and cease working once their gates have been turned HIGH at least once. I'd appreciate any suggestions. Please don't hesitate to ask me for any information I might have forgotten to post ! Thank you in advance.

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    \$\begingroup\$ Please share a complete circuit showing the pin connections on the Arduino. Also please share the code running on the Arduino. \$\endgroup\$ – Eric Urban Sep 12 '17 at 2:55
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    \$\begingroup\$ Also I'd like to point out the way you have your MOSFETs sitting there is crazy. It's far too easy for the aluminum heatsink to contact multiple points of voltage. Some of those are fatal, like the HVDC rail inside almost every ATX PSU \$\endgroup\$ – Eric Urban Sep 12 '17 at 3:00
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    \$\begingroup\$ show your earth grounds please connected to all supply grounds. Too many chances for CM noise. \$\endgroup\$ – Sunnyskyguy EE75 Sep 12 '17 at 3:15
  • \$\begingroup\$ Gate-Drain resistance of just 9kOhm? I'd expected that it was gate-source resistance that low, because there is R1 and R2... Could you please check if you accidentally swapped drain and source? \$\endgroup\$ – next-hack Sep 12 '17 at 5:43
  • \$\begingroup\$ Please see Pwr on circuit for any of chips used in ATX power supplies, for example siti.com.tw/product/spec/Power/SP-PS113-A.007.pdf page 2, pin named PSON/ From here you can see, that after your MOSFET gate signal goes low, the PSON/ signal voltage should rise. Can you check your M2 drain voltage level? How does it change? \$\endgroup\$ – smajli Sep 12 '17 at 7:21

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