How come my transistor doesn't work?

Question

I have a TIP 120 to 229 NON 60V 5A transistor

I'm trying to use it to switch an electro magnet. Using the blink example to test it out:

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin 13 as an output.
  pinMode(13, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(500);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(500);              // wait for a second
}

pin 13 is connected to the base and I'm trying to powering the magnet with a 12V DC adapter. I'm not sure if the transistor is before or after the magnet, but I'm not sure how that would make a difference.

I'm not great with diagrams but something like this (the power source might be the other way around:

^{simulate this circuit – Schematic created using CircuitLab}

I've tried switching between the collector and the emitter. In one instance the magnet is always active and in one instance it's always passive. What could be wrong?

The cords running of the table in the bottom is just a bit of extra wiring.

Answer 1

Is there the only wire from Arduino to the Base of Transistor?

Basically you need something like this:

If you reverse the transistor (E and C), builtin protection diode will be in forward direction, so electro magnet is on.

If you have only one wire to the base, it cannot work as you need current to the base.

If you don't have R1 into the base (about 500R-1K), it'll burn your output eventually. (not if you don't have common ground)

If you have electro magnet between ground and Emitter, you'll have negative current feedback and about 4.2V on L1 tops as there must be some current into the base.

Answer 2

The program is fine.

Looking at the first photograph - The Ground of the Arduino needs to be connected to the ground of your circuit.

I do not understand the two-transistor part of your circuit, so I can't say if this is OK or not. You could also switch the electro-magnet using a relay.

Answer 3

Assuming the - of the power supply is connected to ground your circuit becomes an emitter follower utilizing a Darlington transistor. In that case the emitter will have the same voltage as the base - the VBE which is about 5 Volts. If you reverse it, you will need about 5 volts VBE to turn it on. Note since it is a transistor there is no protection diode. Best suggestion is to use a MOSFET, that will give you a lot more speed then a power transistor. Substitute the MOSFET in the circuit shown above. Source connects to the Emitter point, the Drain connects to the Collector point and the Gate connects to the base. Use something in the 50 Ohm range for the resistor. Your 12V - becomes ground connected to the Arduino ground. Use a logic level avalanche protected MOSFET and the diode is not needed.