How Toasters Work

You can purchase a toaster at any discount store for less than $20 (U.S.). The particular model shown on the right allows you to set the darkness of the toast it produces and also has a defrost mode.

The basic idea behind any toaster is simple. A toaster uses infrared radiation to heat a piece of bread (see How Thermoses Work for information on infrared radiation). When you put your bread in and see the coils glow red, the coils are producing infrared radiation. The radiation gently dries and chars the surface of the bread.

The most common way for a toaster to create the infrared radiation is to use nichrome wire wrapped back and forth across a mica sheet, like this:

Nichrome wire is an alloy of nickel and chromium. It has two features that make it a good producer of heat:

The very simplest toaster would have two mica sheets wrapped in nichrome wire, and they would be spaced to form a slot about an inch (2.5 cm) wide. The nichrome wires would connect directly to a plug. To make toast:

Most people don't have this sort of patience, nor do they like crumbs all over the counter. So a toaster normally has two other features:

­The photo below shows you the view down one of the slots of a typical toaster. Two mica/nichrome sheets line either side of the slot. A metal holder rides up and down in the slot to raise and lower the bread.

Many toasters include a pair of grates on either side of the slot. The grates press against the bread and center it. Two metal springs get pushed when the holder nears the bottom of the slot, and they pull the grates inward.

The holders in each slot are connected to the handle that you depress to lower the bread into the toaster, as shown below:

When you push the handle down, three things have to happen:

In this particular toaster, both the hold-down mechanism and the power switch are part of the handle:

Above you can see a plastic bar and a piece of metal attached to the handle. The plastic bar presses into a pair of contacts on the circuit board to apply power to the nichrome wires, and the piece of metal gets attracted to an electromagnet to hold the toast down. You can see both the contacts (copper strips on the right) and the electromagnet (green block on the left) below:

The following two photos show how the plastic bar applies power to the toaster. In the first photo, the plastic bar is being simulated by a pencil, and you can see how it pushes the contacts apart:

In this particular toaster, here is how the whole mechanism works:

In this toaster, the darkness control is simply a variable resistor. Changing the resistance changes the rate at which the capacitor charges, and this controls how long the timer waits before releasing the electromagnet.

Less sophisticated toasters use a bi-metallic strip (see How Thermometers Work for details on bi-metallic strips) to turn off the electromagnet. As the strip heats up (due to rising temperatures inside the toaster), the strip bends and eventually trips a switch that kills the power to the electromagnet. The bi-metallic strip approach has two problems:

The electronic circuit in this toaster provides much more consistent toast!

For more information, check out the links on the next page.