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Switching Basics and More 79

Experiment 10: Transistor Switching

essentIAls

Transistors and relays

One limitation of NPN and PNP transistors is that they are naturally “off” until
you turn them “on.” They behave like a normally open pushbutton, which
conducts electricity only for as long as you hold it down. They don’t normally
behave like a normal on switch, which stays on until you apply a signal to turn
it off.

A relay offers more switching options. It can be normally open, normally closed,
or it can contain a double-throw switch, which gives you a choice of two “on”
positions. It can also contain a double-pole switch, which makes (or breaks) two
entirely separate connections when you energize it. Single-transistor devices
cannot provide the double-throw or double-pole features, although you can
design more complex circuits that emulate this behavior.

Here’s a list of transistor and relay characteristics.

Transistor Relay

Long-term reliability Excellent Limited

Configurable for DP and DT switching No Yes

Ability to switch large currents Limited Good

Able to switch alternating current (AC) Usually not Yes

Can be triggered by alternating current (AC) Usually not Optional

Suitability for miniaturization Excellent Very limited

Sensitive to heat High Moderate

Ability to switch at high speed Excellent Limited

Price advantage for low-voltage low-current Yes No

Price advantage for high-voltage high-current No Yes

Current leakage when “off” Yes No

The choice between relays or transistors will depend on each particular
application.