It is often desirable or essential to isolate one circuit
electrically from another, while still allowing the first circuit to
control the second. For example, if you wanted to control a
high-voltage circuit from a control desk, you would not want
to connect it directly to control panel in case something went wrong
and the high voltage became connected to the control desk.
One simple method of providing electrical isolation between two
circuits is to place a relay between them, as shown in the circuit
diagram of figure 1. A relay consists of a coil which may be
energised by the low-voltage circuit and one or more sets of switch
contacts which may be connected to the high-voltage circuit.
How Relays Work
In figure 2a the relay is off. The metal arm is at its rest position
and so there is contact between the Normally Closed (N.C.) switch
contact and the 'common' switch contact.
If a current is passed through the coil, the resulting magnetic
field attracts the metal arm and there is now contact between the
Normally Open (N.O.) switch contact and the common switch contact,
as shown in figure 2b.
Advantages of Relays
The complete electrical isolation improves safety by ensuring that
high voltages and currents cannot appear where they should not be.
Relays come in all shapes and sizes for different applications and
they have various switch contact configurations. Double Pole Double
Throw (DPDT) relays are common and even 4-pole types are available.
You can therefore control several circuits with one relay or use one
relay to control the direction of a motor.
It is easy to tell when a relay is operating - you can hear a click
as the relay switches on and off and you can sometimes see the
Disadvantages of Relays
Being mechanical though, relays do have some disadvantages over
other methods of electrical isolation:
Their parts can wear out as the switch contacts become dirty - high
voltages and currents cause sparks between the contacts.
They cannot be switched on and off at high speeds because they have
a slow response and the switch contacts will rapidly wear out due to
Their coils need a fairly high current to energise, which means some
micro-electronic circuits can't drive them directly without
The back-emf created when the relay coil switches off can damage the
components that are driving the coil. To avoid this, a diode can be
placed across the relay coil.