Switching & protectionRebuild shelfTest and identify

Bimetal thermostat

Also called Klixon, steam stat, cycling thermostat, limiter.

A disc that snaps open at a temperature and, on the cycling types, snaps shut again as it cools. Kettle steam-stats, dryer cycling thermostats, coffee makers, water-heater thermostats.

If you are new to this category

A switch opens or closes a circuit. Some switches are for your finger. Some open when they get too hot and stay open. Those do not reset. A relay is a switch that a small current turns on so it can control a larger current. Use the rating printed on the part.

Click it and listen. Then use a meter on continuity, which beeps when the contacts touch. Do not guess that a small switch can run a motor.

A heater is a chain

Wall power goes through a switch, a one-time thermal fuse, and often a resettable thermostat, then the element. If you remove the fuse because it is open, the element can stay on after something else has already failed. That is how the donor caught fire the first time.

Heaters without their cutout. Nichrome and tubular elements are happy to start a fire if the thermal fuse or tip-over switch is left out of the new circuit.
Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Kettle	Steam-stat trips near 100 °C when steam reaches it. Often paired with a higher one-shot thermal fuse.
Dryer	Cycling thermostat regulates; a separate high-limit and a thermal fuse back it up. They are not interchangeable.
Current	They often carry the full element current. The rating is stamped.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

A crude temperature control
A resettable limiter above a thermal fuse

Leave this one when

The disc is warped or the rating is illegible and the load is a heater.

Buying a new one is the better idea when

A known control, once temperature matters to better than 'hot' and 'not'.

How to tell if this one still works

Continuity when cold, for a normally-closed stat. A careful heat test (hot air, not a flame on the leads) should open it. It should reclose when cool if it is a cycling type.

Step-by-step on the bench: Tell a thermal fuse from a thermostat

Tools you will want: Meter, Heat gun used carefully.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

thermal fuse: A one-time safety link that opens when it gets too hot and stays open. It is not a switch you can reset. Replacing it with wire removes the protection.
continuity: A meter mode that beeps when two points are connected. It is how you map a switch: beep, then click, then see if the beep changes.

Where people actually find it

6 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Clothes dryer	Worth it	1–2	usually	Heater housing.
Electric kettle	Worth it	1	usually	Steam path in the handle or base.
Hair dryer	Worth it	1	usually	Beside the fuse.
Toaster and toaster oven	Bonus	1	sometimes	The timer on older pop-ups.
Window air conditioner	Bonus	1	sometimes	Older mechanical thermostats.
Space heater	Bonus	1	usually	The thermostat knob.

Worth pullingClothes dryer1–2 · usually
In this particular machine
Cycling thermostat and a high-limit. They are different parts.
Where to look
Heater housing.
How to get it out
Read the temperatures.
The condition to expect
Good if it still clicks.
Worth pullingElectric kettle1 · usually
In this particular machine
Trips near boiling when steam reaches it.
Where to look
Steam path in the handle or base.
How to get it out
The failed one is often this. Test the click.
The condition to expect
Fatigued is common.
Worth pullingHair dryer1 · usually
In this particular machine
Resettable limiter.
Where to look
Beside the fuse.
How to get it out
Test the click.
The condition to expect
Good.
Bonus, if you are already in thereToaster and toaster oven1 · sometimes
In this particular machine
A heater-warmed bimetal releases the carriage.
Where to look
The timer on older pop-ups.
How to get it out
Amusing, not precise.
The condition to expect
Fatigued.
Bonus, if you are already in thereWindow air conditioner1 · sometimes
In this particular machine
A knob and a switch.
Where to look
Older mechanical thermostats.
How to get it out
Crude.
The condition to expect
Good.
Bonus, if you are already in thereSpace heater1 · usually
In this particular machine
Crude regulation.
Where to look
The thermostat knob.
How to get it out
Test.
The condition to expect
Good.