How Clothes Dryers Work

Cycle switch, front view
Cycle switch, front view

This dryer has no electronics in it at all. Instead, a system of gears, cams, electrical contacts and motors forms a sort of mechanical computer. Let's start by looking at the cycle control knob.

Cycle Switch

By turning this knob to various positions, you can control both the type of cycle and the length of time it runs. Let's take a look at what is inside this switch.

Cycle switch, back view

Here is a view of the back of the cycle switch. Attached to the back is a little motor. The picture below shows the motor unscrewed from the switch.

Cycle switch motor

The tiny gear on the motor turns very slowly; and it engages a bigger gear inside the switch that makes the switch turn even slower.

Inside the cycle switch

The motor turns the gear on the dial, which is connected to a set of four cams stacked on top of each other. Each of the cams engages one of the four contacts in the switch.

Inside the cycle switch box

Each of the four contacts has a bend in it, and each bend is located at a different height inside the box. Starting with the bottom left contact, the heights increase in a counterclockwise manner; the bottom left contact is the lowest, the top left contact is the highest. A different cam engages each of these contacts. In the pictures below, you can see the four cam layers; each of these layers corresponds in height to one of the contacts.

Cycle switch cam

The cycle switch determines how long the elements stay on. In conjunction with the heat setting buttons, it also controls which heating elements are on at a given time. If none of the heating elements are on, only cool air blows through the clothes; if one is on, the air is warm; and if both are on, the air is hot.