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:
- When you push down on the handle, the plastic bar presses against the contacts and applies power to the circuit board.
- 120-volt power runs directly through the contacts to the nichrome wires to start toasting the bread.
- A simple circuit made up of transistors, resistors and capacitors turns on and supplies power to the electromagnet.
- The electromagnet attracts the piece of metal on the handle, holding the bread in the toaster.
- The simple circuit acts as a timer. A capacitor charges through a resistor, and when it reaches a certain voltage it cuts off the power to the electromagnet. The spring immediately pulls the two slices of bread up.
- In the process, the plastic bar rises and cuts off power to the toaster.
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:
- If the kitchen is cold, the first piece of toast will be darker than usual.
- If you try to make a second batch of toast, it will be too light because the toaster is already hot.
The electronic circuit in this toaster provides much more consistent toast!
For more information, check out the links on the next page.