I want to use my Arduino Nano to control a 12V peristaltic pump via a BDX33c N-Channel MOSFET. The pump has a maximum current draw of 1,4 A. The Arduino (via VIN) and the pump are powered by a 12V supply. I connected the parts like shown in the schematic diagram.

Schematic Diagram

When I plugged the supply in, the Arduino voltage regulator and the Mosfet got really hot (I could not touch it longer than 1-2 seconds) and the RX-LED was on continously, even after turning it off and on again. The Arduino isn't recognized by my Computer anymore, so I assume it is dead. What did I do wrong with my wiring?

  • 2
    I think this is not a MOSFET, but a NPN bipolar Darlington transistor
    – chrisl
    Aug 20, 2018 at 18:45
  • 1
    The 'schematic' shows both sides of the motor connected to ground. The 5V output from the Arduino's regulator goes nowhere. Have you connected it up like this in real life? There's a chance you have pumped 12V to one of the Arduino inputs. Aug 20, 2018 at 18:49
  • The maximum base current is specified as 250mA, which is really much for an Arduino
    – chrisl
    Aug 20, 2018 at 18:49

2 Answers 2


That little blue wire on your breadboard is shorting out the motor. You have most likely now blown the transistor and maybe your power supply. The 12V may also now have made its way into the Arduino through the dead transistor and nuked the rest of it.

Alternatively, the massive base currents imposed by the shorted ground connection may have burned out parts of the Arduino.


That little blue wire on your breadboard is shorting out the motor.

That is not the problem. That blue wire is actually correct. It connects the source to ground. (-ve)

Your drawing shows the Drain of the Mosfet connected to +12 volts and the Source connected to ground. (the -ve of the supply) If you didn't have the short (blue wire from Source to ground) it still would not have worked.

You can't put an N channel mosfet in the positive side of the supply and control it from a voltage lower than the positive supply. (something the previous answer didn't even mention)

You must have the source connected to ground (as you have) but the drain has to then connect directly to the motor, and the other side of the motor goes to the +12 connection. This would be the correct way, but there still is an important omission. You need to have a diode connected across the motor to control the flyback pulse when the mosfet turns off. Otherwise the voltage spike generated by the motor's inductance will destroy the mosfet switch.

Your blue wire on the breadboard is correct. What is wrong is the red wire to the center terminal of the mosfet. The red wire should be going to the motor. The Blue/yellow connection. The green wire should be going to the mosfet's center pin.And as I said before, there needs to be a diode across the motor, with the cathode (line on the body of the diode) connected to the +ve side of the motor connection.

This is a correct drawing as far as the mosfet and motor connections are concerned. This drawing shows 5 volts being brought to the breadboard to power the motor. In your case this should be the 12 volts from the power supply.

enter image description here

While the beadboard may be adequate for verifying the circuit, it may cause connection problems due to the significant current that the motor will draw. Even if you wired it up properly you may still have issues due to the poor connections.


I looked up the specs for the BDX33B and it is a darlington transistor and not a mosfet. But in the end, all that I said also applies whether it was a bipolar transistor or a mosfet. But you need to include a series resistor as shown in the drawing I provided. You can't drive it direct from a port pin as if it were a mosfet.

Another schematic.

enter image description here

This shows a darlington transistor like the one you listed, and a 12 volt power source.

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