Switches & relays

You switch electricity dozens of times a day: light switches, the doorbell, the keys on your keyboard. A switch always does the same thing: it interrupts the circuit or closes it. Sounds simple, but behind it is a whole family of components, and with the relay there is even a switch operated by current instead of by your finger.

Push button or switch?

A switch stays in its position until you flip it again: the light stays on until you switch it off. A push button, on the other hand, springs back by itself as soon as you let go: the doorbell only rings while you press it. Both come as normally open contacts, which close the circuit when pressed, and normally closed contacts, which interrupt it when pressed. Which kind you need depends entirely on the job.

The two way circuit in the hallway

In the hallway you can switch the light on downstairs and off again upstairs. Two two way switches make this possible: each one routes the current onto one of two wires. Only when both switches point to the same connecting wire is the circuit closed and the lamp lit. Flipping either of the two switches flips the state of the lamp: off becomes on, on becomes off. Two switches with two positions each give four combinations, and two of them make the lamp light up.

The relay: an electromagnet does the switching

A relay is a switch operated by an electromagnet. When a small control current flows through the coil, it becomes magnetic and attracts a springy metal lever, the armature. The armature pushes a contact closed, and a second, completely separate circuit is completed. When the control current stops, the spring pulls the contact open again. How large the control current turns out to be depends on the resistance of the coil: current is voltage divided by resistance. The coil usually sees only 5 to 24 volts and a few hundredths of an ampere, which keeps the control side below one watt, since power is voltage times current. This way a tiny current from an electronic circuit can switch a large current for a motor, a heater or headlights, without the two circuits ever touching.

Where relays are hiding

In a car, relays switch the starter, the horn and the headlights, and the familiar clicking of the indicator used to come straight from a relay. In a heating system a relay switches the burner, in a washing machine the heating elements. The big advantage is the separation: the delicate control electronics work with a small, harmless voltage, while the load circuit at 230 V stays cleanly separated from it. The first automatic telephone exchanges and even early computers such as Konrad Zuse's Z3 from 1941 were built entirely from relays.