Toaster
Toaster
A toaster is an electric appliances using radiant heat to cook bread, converting electricity into heat, a color caused by Maillard reaction.
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Inventor: Crompton & Company of Chelmsford, Essex
Inception: 1893
inception: establishment
Resistance
Resistance
A toaster converts electric energy into heat, with resistance as a key factor.
Resistivity formula (by physical properties):
R = resistance
ρ (rho) = resistivity
L = length
A = area
Note: Not confused by resistance formula of Ohm's Law
Material wires of the highest resistance must be used.
Nichrome (1100 Ω) is used more often than copper (170 Ω) and won't oxdize easily, stopping rusting, and has a high melting point that won't melt in toasters.
nichrome: alloys used as resistive elements
Nichrome wires are often wrapped in mica to protect hands.
mica: mineral with great insulation properties and heat resistance
Mica is a natural mineral with great thermal and electrical insulator, stable and high temperature.
Fig 1
Infrared radiation
Infrared radiation
[2] All toaster uses infrared radiation to heat bread. The red glowing coils make infrared radiation and chars the surface of the bread.
char:
The most common way a toaster to make infrared radiation is to use nichrome wire wrapped back and forth across a mica sheet (Fig 1).
[2] Nichrome wire is an alloy of nickel and chromium. It has 2 features making it a good producer of heat:
alloy
Nichrome wire has fairly high electrical resistance unlike materials like copper wire--even a short length of it has enough resistance to get hot.
The nichrome alloy won't oxidize when heated. Iron wire rust fast at temperatures in a toaster.
The simplest toaster cab have 2 mica sheets wrapped in nichrome wire, spaced to make a slot ~1 in (2.5 cm) wide, and connect directly to a plug.
Toasters often has 2 other features:
A spring-loaded tray pops the toast out from having to turn the toaster upside down.
A timer turns the toaster off automatically and simultaneously releases the tray so the toast pops up.
Fig 2. A normal toast slot above
Mechanism
Mechanism
Spring-loaded tray
Spring-loaded tray
[3], [4], 2 nichrome sheets line either side of the slot. A metal holder rides up and down in the slot to raise and lower a bread.
Springs are stretched to an electron magnet to keep it in and heat place til the time is up and the magnet releases
Toasters have a grates pair on either side of the slot. The grates press against the bread and center it. 2 metal springs get pushed when the holder nears the slots bottom and they pull the grates inward.
The holders per slot connect to a handle that lowers a bread.
grate: a metal frame closing a fireplace/furnace
Plastic plate attached to toast-lowering lever, plastic wedge (on left) switching on the power.
Popping a toast up
Popping a toast up
Pushing a handle down, 3 things occur:
A mechanism must hold the handle down to keep the toast inside.
Power must be applied to nichrome wires.
A timer must release the holder at a certain time so the toast pops up.
This toaster has both a hold-down mechanism and a power switch part of its handle.
If the bar lowers, the metal tab contacts the electromagnet.
Above you can see a plastic bar and a metal piece 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:
If the bar lowers, the metal tab contacts the electromagnet.
toaster circuit board
Timer
Timer
3 usual ways to set toasting level:
Costly toasters use digital timer with a microchip
Most use a capacitor as a timer, like a battery that activates the shoot-off and eject switch if charged. Select the value on a dial changing the rate filling the capacitor. The longer it fills, longer the toaster switches off.
Cheap toasters use biometallic strips as a timer, using 2 metals, often steel and copper on 1 side. Cpper expands faster than steel if heated and bend. The biometallic strip disconnects the circuit, ceasing the heat and eject the toast.
Circuit board
Circuit board
These 2 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 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 V power runs directly into 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 metal piece on the handle, holding the bread in.
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.
This toaster's 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.
Toaster circuit card showing electrical contacts
Toaster circuit board with contacts engaged
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 makes more consistent toast.
For more information, check out the links on the next page.
History
History
Heating bread starts in ancient Egypt where we put bread on fire to stop molding, and idea the Romans used in 500 BC. For thousands of years, til the first electric toaster scostman Alan Mcasters in 1980s, but it didn't get commerical success til nichrome's invention by Albert Marsh.
Quizzes
Quizzes
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