The Transistor as a Switch

Driving LEDs and relays from a logic signal.

Used as a switch, a transistor operates in two states: cutoff, where no base current flows and the load is off, and saturation, where plenty of base current flows and the load is fully on. A base resistor limits the base current to a safe value.

A common rule of thumb is to supply a base current of about one tenth of the collector current so the transistor saturates firmly. When the load is a coil, such as a relay, add a diode across it to absorb the spike produced when the current is cut.

• Cutoff = open switch; saturation = closed switch.
• Always use a base resistor.
• Inductive loads need a flyback diode.

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