1-wire temperature sensor

Hey,

While fixing my about-15-years-old washing machine, I encountered a device I'm not familiar with inside it I was hoping someone could help me understand.

The thermostat I found was a small box that had a ribbon connecting it to the control box, two wires going out to the heater, and a single wire connected to something I can only assume is the water's temperature sensor, as it sits partially inside the drum.

It is a little metal cylinder (about an inch in length and half an inch in diameter) with two leads - One connected to the thermostat, and the other isn't connected at all. Instead, it is wrapped with an insulator, and the wire going to the thermostat is wound around it about two or three times.

I was wondering how this device is used to measure temperature with only one wire. I'm familiar with NTCs, but they require current. I was thinking along the line of AC going in the wire (like an antenna) and the device is inducing interference into the signal in proportion to its temperature (perhaps with a phase lag). This could allow the thermostat to filter out the original signal and measure the interference - but I'm not too sure about the details...

Can someone shed some light? Thanks!

Any reason to assume, that it's an electronical sensor? It may be a liquid filled capillary thermostat as well.

FvM said:

Any reason to assume, that it's an electronical sensor? It may be a liquid filled capillary thermostat as well.

Click to expand...

Hmmm. no, but how would I know? The wire connecting the sensor to the thermostat seems like an electrical wire, but that's all the indication I have.

but how would I know?

Click to expand...

Is the "wire" connecting to any kind of electronical circuit?

A schematic view of a mechanical bellows-type thermostat:

Could that be a Peltier thermo junction temp sensor with one lead to the water (floating) the hot junction wire connected to thermostat and the junction fitted to
the heaters?
I have used the same with application of bias currents so that one face is hot(heatsinking with forced convection) and other gets ice deposit.

FvM said:

Is the "wire" connecting to any kind of electronical circuit?

A schematic view of a mechanical bellows-type thermostat:

Click to expand...

If I understand correctly - you're suggesting the "wire" is actually a tube?

It's solid metal, as much as I can tell (I had a look at the cross-section - I broke it :smile

vimalkhanna said:

Could that be a Peltier thermo junction temp sensor with one lead to the water (floating) the hot junction wire connected to thermostat and the junction fitted to
the heaters?
I have used the same with application of bias currents so that one face is hot(heatsinking with forced convection) and other gets ice deposit.

Click to expand...

Sound like it would measure temp. current (i.e. first derivative), and not constant temp... Can it be used as a temp. sensor as well?

Thermocouple thermostats are mostly used for higher temperatures e.g. to check the presence of a pilot flame. A washing machine can be expected to have either a capillary thermostat or a simple electronical thermostat with a NTC sensor.

If I understand correctly - you're suggesting the "wire" is actually a tube?
It's solid metal, as much as I can tell (I had a look at the cross-section - I broke it

Click to expand...

Yes a capillary is a tube, but with a rather small bore. You may overlook it at first sight.

FvM said:

Thermocouple thermostats are mostly used for higher temperatures e.g. to check the presence of a pilot flame. A washing machine can be expected to have either a capillary thermostat or a simple electronical thermostat with a NTC sensor.


Yes a capillary is a tube, but with a rather small bore. You may overlook it at first sight.

Click to expand...

Well what do you know... it does have a little bore in it!

Thanks!