Circuit Board
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:
 Toaster circuit card showing electrical contacts |
 Toaster circuit board with contacts engaged |
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.
For more information, check out the links on the next page.
