I have a 12 VDC pump, 8.5A, connected to a MOSFET transistor PSMN5R6-100PS,127.

The connection from the transistor to transformer, pump and a ESP32:

1 Gate --> port to ESP32

2 Drain --> (-) pump

3 Source --> Gnd ESP32, (-) transformer

(+) transformer to (+) pump

This is, as I understand it, how it ought to work. But with this configuration it doesn't... But changing 3 Source --> (-) transformer, removing the Gnd connection to the ESP32, then it works. It, however, seems to be some kind of leakage in terms of current because the pump has a "clucking" sound, not fully activating, just making a sound. Sometimes it doesn't work at all, where a reset makes it work again. Any ideas how I can fix it reliably?

  • That transistor has Vgs(th) typical 3V if I read the datasheet correctly. ESP is at 3.3V? Might not be enough
    – Mat
    Nov 9 '20 at 11:02
  • 1) This is not a question about Arduino. 2) Removing the common GND connection from the transformer (-) to the ESP32 will almost certainly lead to problems. 3) My guess is your MOSFET does not fully conduct at the ESP32 port output voltage (3.3V). You need to increase the gate voltage. Did you add a flyback diode across the pump to protect the MOSFET?
    – StarCat
    Nov 9 '20 at 12:36
  • (2) Yes, I suspected this...What kinds of problems could occur? (3) Yes, this has been my hypothesis previously, especially when considering that when connecting this same transistor to LEDs the light intensity is significantly lower that if the LEDs are directly connected to the transformer. No flyback diod has been added. Would it be best to just look for another transistor with a lower requirement in terms of gate voltage? Any ideas?
    – Lennart
    Nov 9 '20 at 15:22

As can be seen from the datasheet for the PSNB5R6 (Fig 7) at 3.3V the MOSFET is barely even beginning to switch on.

You need to select a more suitable MOSFET that switches on fully at a voltage well below 3.3V, such as the STP40NF04L.

  • Will purchase this transistor and will have a look if it works!
    – Lennart
    Nov 10 '20 at 14:05
  • When you're looking at them, you may want to look at the "safe operating area." I don't know if I was reading it correctly, but in the datasheet for your PSMN5R6-100PS, the graph seems to suggest that at continuous (DC) operation at 12V about 6 AMP would be maximum. So even if it had been able to turn on fully, it's not clear that you'd be able to use it unless you were pulsing 8.5 amps to the pump.
    – timemage
    Nov 10 '20 at 14:44
  • Looks more like 9A to me on that graph.
    – Majenko
    Nov 10 '20 at 15:10
  • I struck lines over it to double check just now. Though, I don't see a way to insert an image into a comment. As long as they're aware of this aspect to picking one out, I'm content.
    – timemage
    Nov 11 '20 at 0:32

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