I'm a beginner, so please be gentle!

I'm trying to dim the linked LED light fixture driver using a Raspberry Pi (RPi). I want to dim using PWM in conjunction with a FET. Before dimming I am trying to simply turn the fitting on/off. I was hoping it would work on the basis of the top answer to this post.

I have gotten some BS170 FETs and tested them for turning on/off a single LED using an RPI GPIO, they work fine.

I then connected the LED driver to the FET, connecting the +DIM of the driver to the drain, the -DIM to the source, and the gate to the RPi GPIO pin as with the single LED. I attempted to set the GPIO high, then low, as worked with the single LED but I get no change, only a dimmed fixture. Fixture goes to full brightness when I disconnect the +/-DIM wires from the FET, opening the circuit.

I perhaps have the FET oversized for the task? I'm guessing that it is allowing current to flow (with low resistance) from +DIM to -DIM regardless of voltage on the gate??

Any help, tips or advice appreciated.

Fig 1. Circuit Arrangement

Fig 2. LED Driver Dimming Data

  • 1
    \$\begingroup\$ Connect your Pi's ground to the FET's source. \$\endgroup\$
    – brhans
    Commented Jul 26, 2022 at 18:49
  • \$\begingroup\$ Thanks. Would you happen to know if there is any risk to the RPi by applying the 10V (200uA) signal from the converter to the GND pin? \$\endgroup\$
    – D_Don
    Commented Jul 26, 2022 at 19:17
  • \$\begingroup\$ You wouldn't be applying 10V to the Pi's GND pin. There's only 10V on the FET's drain. You have to connect the FET's source to the Pi's GND. The FET urns on when you apply voltage across its gate & source - since voltage is a measurement between 2 points you can't just connect 1 wire between the Pi and the FET. \$\endgroup\$
    – brhans
    Commented Jul 26, 2022 at 19:32
  • \$\begingroup\$ That worked. Thank you! \$\endgroup\$
    – D_Don
    Commented Jul 26, 2022 at 20:33


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