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.
Nichrome wire is an alloy of nickel and chromium. It
has two features that make it a good producer of
heat:
1.
Nichrome wire has a fairly high electrical
resistance compared to something like copper wire,
so even a short length of it has enough resistance
to get quite hot.
2.
The
nichrome alloy does not oxidize when heated. Iron
wire would rust very quickly at the temperatures
seen in a toaster.
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:
1.
You
would drop a piece of bread into the slot.
2.
You would then plug in the toaster and watch the
bread.
3.
When the bread became dark enough, you would
unplug the toaster.
4.
Then you would tip the toaster upside down to get
the toast out!
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:
1.
A spring-loaded tray pops the toast out. This
keeps you from having to turn the toaster upside
down.
2.
A timer turns the toaster off automatically and
at the same time releases the tray so the toast pops
up.
Many toasters include a pair of grates on either
side of the slot. The grates press against the bread
and centre 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.
When you push the handle down, three things have to
happen:
1.
Some sort of mechanism needs to hold the handle down
to keep the toast inside the toaster for a period of
time.
2.
Power needs to be applied to the nichrome wires.
3.
Some sort of timer needs to release the holder at the
proper time so the toast pops up.
In this particular toaster, both the hold-down mechanism
and the power switch are part of the handle:
Plastic plate attached to toast-lowering lever, plastic
wedge that switches on the power.
To the right 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. |
Toaster heating element,
nichrome wire on mica sheet
When
the bar is lowered,
the metal tab contacts the
electromagnet. |
