Lessons · Engineering · Using a transistor as a switch
A small current controlling a large one
Used as a switch, a transistor is either fully off or fully on. A small current or voltage at the control terminal decides which. That is how a logic output that can supply milliamps operates a relay, a motor or a lamp that needs amps.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the bridge between the part of a circuit that thinks and the part that does work. Almost every microcontroller project needs it, and driving a load directly from a logic pin is the commonest way to destroy the chip.
How to think about it
Design for fully on, not partly on: give the control terminal enough drive to saturate the device, because a half-on transistor dissipates heat and gets hot. Then check what the load does when the transistor turns off -- an inductive load needs somewhere for its current to go.
Worked example
Off or fully on, never in between.In between means heat.
Small control current, large load current.That is the whole purpose.
Drive it hard enough to saturate.Otherwise it dissipates.
An inductive load needs a flyback diode.Or the turn-off spike destroys the transistor.
Your turn
What must be fitted across an inductive load switched by a transistor?
An inductive load needs a flyback across it
Solve one, graded on the server
The trap
Driving a relay or motor straight from a logic pin. The pin cannot supply the current, and the turn-off spike from a coil will destroy whatever is switching it.