Arduino not Switching Transistor

Ok, so I have an Arduino (Redboard) with a Digital Out (LEDPIN) attempting to switch a transistor in order to drive a relay to switch on a circuit.

The Transistor (FDP5680) is attached to a 5v supply with a shared ground to the Arduino, thus we have a regulated 5 v supply to the board and an unregulated 5 v supply to the transistor. Transistor has a 10k resistor between source and gate pins.

I seem to be having trouble providing enough current to the transistor in order to switch it on/off. However, when I take the gate pin and connect it to the regulated 5v pin on the Arduino, it is able to switch on/off.

Anyone have any suggestions as to how I can revise this in order to get it to work? do I need a larger Gate Source Resistor?

That doesn't appear to be a "logic level" MOSFET, so it requires more than 5V to turn it fully-on. But, you say it works when you connect the gate to 5V so maybe it's "close" but I think you need a different MOSFET.

Is there anything else connected to the Arduino output besides the 10K resistor? i.e. If you have an LED without a proper current limiting resistor, that will hold-down the voltage.

And, please show a schematic rather than trying to describe the circuit in words. A hand-drawn and scanned schematic is OK.

I seem to be having trouble providing enough current to the transistor in order to switch it on/off.

MOSFETs are voltage-controlled devices. Almost no current flows into the gate except there is a small capacitance so some current flows while switching.

The Transistor (FDP5680)

FYI - When we say "transistor" we are usually referring to a "regular" bipolar NPN or PNP transistor. Technically, FETs and MOSFETs are classified as transistors but in the electronics world we like to be more specific, and it's even better if you say "N-channel MOSFET".

The Transistor (FDP5680) is attached to a 5v supply with a shared ground to the Arduino, thus we have a regulated 5 v supply to the board and an unregulated 5 v supply to the transistor. Transistor has a 10k resistor between source and gate pins.

Well you need the drain of the FET to one of the relay coils, and the other end of the relay coil to 5V.

However that FET needs 10V to fully turn it on. The data sheet says it will partially turn on at 6V,. so basically you are not using the right device.

What is the relay coil current? If less than 300mA you may be able to use a common 2N2222 BJT transistor with a 330 ~ 470Ω base resistor.
NPNMot-2.png
Replace the motor (M1) with your relay coil, don't forget the flyback diode, (D1).

NPNMot-2.png

llessuryort:
Ok, so I have an Arduino (Redboard) with a Digital Out (LEDPIN) attempting to switch a transistor in order to drive a relay to switch on a circuit.

The Transistor (FDP5680) is attached to a 5v supply with a shared ground to the Arduino, thus we have a regulated 5 v supply to the board and an unregulated 5 v supply to the transistor. Transistor has a 10k resistor between source and gate pins.

That device is rated for a minimum gate drive of 6V, not 5. However its likely to function OK, but not absolutely guaranteed.

I seem to be having trouble providing enough current to the transistor in order to switch it on/off. However, when I take the gate pin and connect it to the regulated 5v pin on the Arduino, it is able to switch on/off.

MOSFETs don't take any current, except very brief pulses as they switch on or off. I suspect either:

  1. Arduino pin bust - check it with resistor+LED.
  2. Code wrong - post your code.
  3. MOSFET damaged - did you observe anti-static precautions when handling it - this is vital.
  4. Circuit wrong - post it.
  5. pinout wrong - double check

Anyone have any suggestions as to how I can revise this in order to get it to work? do I need a larger Gate Source Resistor?

10k is fine, its not critical. Are you sure its not 10 ohms though, that would explain it!