If both inputs are the same voltage, the outputs to the motor will be the same voltage, and the motor won't be able to spin. (Note: there's usually quite a bit more to a well-designed H-bridge including flyback diodes, base resistors and Schmidt triggers.) The value of the resistor, and voltage across it, will set the current.Ĭan you guess why it's called an H bridge? The base-emitter node can get its happy voltage drop of 0.6V, and the resistor can drop the remaining voltage. The series resistor between our control source and the base limits current into the base. If you supply a current over the maximum rating, the transistor might blow up. Some transistors may only be rated for a maximum of 10-100mA of current to flow through them. The diode only needs 0.6V to turn on, more voltage than that means more current. We're forward-biasing the base-emitter diode to turn the load on. Recall that, in a way, a transistor is just a pair of interconnected diodes. Don't forget to add this resistor! A transistor without a resistor on the base is like an LED with no current-limiting resistor. You'll notice that each of those circuits uses a series resistor between the control input and the base of the transistor. In that case, it'd be impossible to turn the switch off because V B (connecting to the control pin) would always be less than V E.

For example, this circuit wouldn't work if you were trying to use a 5V-operating Arduino to switch off a 12V motor. This can cause complications, especially if the load's high voltage (V CC being 12V connecting to the emitter V E in this picture) is higher than our control input's high voltage. This circuit works just as well as the NPN-based switch, but there's one huge difference: to turn the load 'on', the base must be low. This time however, the emitter is tied high, and the load is connected to the transistor on the ground side. Similar to the NPN circuit, the base is our input, and the emitter is tied to a constant voltage.

Transistor as a switch download