Switching & protectionBench electronicsTest and identify

NTC thermistor

Also called temperature sensor, inrush thermistor, 10k NTC.

Resistance falls as it gets hotter. Two different parts share the name: a small sensor bead, and a big inrush limiter on a power supply that is meant to run hot.

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.

The three legs of a snap switch

Common is the moving contact. NO means normally open: that leg connects only while you press. NC means normally closed: that leg is connected until you press. Click the button while the meter is on continuity and write down which beep matches which leg.

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
Sensor	Often 10 kΩ at 25 °C in consumer gear, but 5 k, 47 k, and 100 k are all common. The beta value is rarely printed on a salvaged bead.
Inrush	A disc in series with the mains input of a supply, a few ohms when hot, higher when cold. Not a sensor.
Washer / dryer	Often a 12 kΩ or similar plastic-sleeved probe. Identify by the connector and the service sheet if you still have it, else calibrate it yourself.

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

Temperature sense after you record a few calibration points
Inrush limiting left in the original supply

Leave this one when

You need 1 °C accuracy and you have no way to calibrate. Buy a digital sensor.

Buying a new one is the better idea when

A 10 k NTC with a published beta, or a DS18B20 / TMP36, when the project is a measurement rather than a switch.

How to tell if this one still works

Measure at room temperature, then warm it in your hand or in water in a bag. Sensor resistance should fall. Write the number down; that number is now your datasheet.

Tools you will want: Meter, A known temperature.

Words on this page

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

mains: Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.
series: Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.
datasheet: The manufacturer's document for a part: voltages, pin order, limits. A salvage part with no readable number does not have one you can trust.

Where people actually find it

5 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
Washing machine	Bonus	1	usually	On the tub or the heater pocket.
Clothes dryer	Bonus	1	sometimes	Moisture or temperature probe on electronic models.
Dishwasher	Bonus	1	usually	Near the heater.
Refrigerator	Bonus	1	sometimes	Electronic control models.
Window air conditioner	Bonus	1	sometimes	Room sensor.

Bonus, if you are already in thereWashing machine1 · usually
In this particular machine
A probe. Calibrate it; do not assume 10 k.
Where to look
On the tub or the heater pocket.
How to get it out
Unplug.
The condition to expect
Good.
Bonus, if you are already in thereClothes dryer1 · sometimes
In this particular machine
A sensor, not a fuse.
Where to look
Moisture or temperature probe on electronic models.
How to get it out
Calibrate.
The condition to expect
Good.
Bonus, if you are already in thereDishwasher1 · usually
In this particular machine
Temperature probe.
Where to look
Near the heater.
How to get it out
Calibrate.
The condition to expect
Good.
Bonus, if you are already in thereRefrigerator1 · sometimes
In this particular machine
A thermistor on the evaporator.
Where to look
Electronic control models.
How to get it out
Calibrate.
The condition to expect
Good.
Bonus, if you are already in thereWindow air conditioner1 · sometimes
In this particular machine
A bead on a lead.
Where to look
Room sensor.
How to get it out
Calibrate.
The condition to expect
Good.