I got a servo connected to a wemos/arduino with a capacitor(470uf). Before I didn`t use a cap.

Now with the cap the device use to much power while sleeping(capacitor on the servo). I want to switch off the power to the servo with a MOSFET. I tried this with a BS170 N channel MOSFET. There is a youtube movie with the same settings except for the mainboard. They use an Arduino uno instead a wemos.

When I test the circuit with a led everything is working. When I use a servo instead a led, nothing happen. There is a little sound from the servo(I think not enough voltage).

When I check the voltage without anything hooked on the drain of the mosfet the output is perfect(5V). If I connect a led or a servo the voltage will drop to 3v.

Wemos mini pro has 3.3v output on the gpio pins.

Can someone explain me what I doing wrong? I saw and read some articles about Mosfet. I think it`s not clear enough for me.

Wemos with servo and mosfet

  • 2
    Please show your schematic. – Majenko Mar 22 '18 at 19:36
  • Schematic looks OK. However, I'd choose a MOSFET with a lower R(DSON) and a lower V(GS) threshold. – Majenko Mar 22 '18 at 22:10

My supposition:

The MOSFET's R(DSON) is too high and causing a large voltage drop.

A servo uses more current than a little LED. That current passing through the MOSFET, with an R(DSON) of potentially up to 5Ω, reduces the voltage available to the servo, causing it to not work. Further more, ground for the servo is raised up above the real ground level adding an offset into the logic levels that could cause communication issues.

Suggestion: Pick a MOSFET with a lower R(DSON) (ideally measured in the milli-ohms), and a lower V(GS) threshold (something around 1-2V max would be good).

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