Educate me about this part?

[whitesco]

Last night we noticed our chest freezer seemed to be getting warmer... which of course prompted my wife to say "oh yeah, it's been making this clicking noise for a day or two"...

Long story short I'm pretty sure that the start relay / PTC has gone belly up and I ordered a new one. Compressor checks out the best I know how with a multimeter - nothing shorted to ground and resistance across all three of the terminals. The PTC is reading a few ohms higher than the 4.7 rating.

Curiosity is getting the best of me though; I get that (in the part below) the all black part is the PTC relay. The half white/half black part is just called an overload protector, but what IS it actually? Capacitor? Circuit breaker? Kryptonite? It seems very simple and solid state but I just can't figure out what it is or how to test it.

On a side note, I'm curious how the PTC works if anyone knows that. There's a disk inside the housing that I assume expands or contracts (or otherwise changes it's conductivity based on heat) but I don't see how that works either. So like I said... I have the curiosities.

Here's the part(s), I'm sure this gang is familiar with them...

 

[Meursault74]

What is Overload Protection?

Learn about overload protection, including its importance, different types, and applications.

www.c3controls.com

I've replaced a few of those. They usually sell them in pairs because even if the thermal relay tests ok, it has been "exercised" by the faulty start relay for some time. It's also, you're already down there replace them both anyway.

The white relay can be tested with a heat gun. Should have continuity at room temperature. Heat it up too much and it'll click open circuit.
cool back down and it'll close the circuit.
The black piece is usually crusty and rattling with particles falling out if you shake it enough when it's gone bad.

 

[LXCam]

Since I’m way to lazy to one finger the answer about the thermistor

bimetallic thermistor - Google Search

 

[whitesco]

Thanks gents! I'll test that relay just to satisfy myself... the PTC/thermister LOOKS fine visually, no crumbling, discoloration, burned smells or loose bits inside which I expected to find when I took it out. I went ahead and ordered the new parts last night anyway - I'm out $10 if I'm wrong and I'm sure I'll use them later.

I did try to find some info about the thing though - its' not the bimetallic strip/spring that I've seen in other stuff. It's literally a disk of some material that must be heat sensitive. There was resistance touching either side of it (room temp) so it's not just metal. Looks like resistance should be 4.7 ohms and I was getting over 6, so I'm hoping that's the problem and not the compressor or something more expensive. I just couldn't figure out how the disk works, and wasn't sure what type of component the other piece was.

Figured I'd ask here rather than admit to my 18-year-old that I didn't know

 

[Lightning rod]

it's still bimetallic in a disk form instead of a strip form
Two different metals disk sandwiched together

 

[alfredeneuman]

It's also called a Klixon

 

[RPH]

If it’s bi-metallic then it’s not ptc (positive thermal coefficient) device. Thermistor’s read temperature by changing resistance values as temperatures change. The resulting change in voltage or current can be measured. Ntc and ptc are solid state devices.


“Fabrication of Switching PTC Thermistors
The material composition is blended, milled, dried and then crushed into a powder. Following this, the material is calcined and then milled again with suitable binders. Next, binders are blended into the mix and the material is spray dried in towers. A mixture of particles with an outer coating of binders is thus formed into a fine powder with carefully controlled particle sizes. This powder is then compacted to form the desired geometry in a die using very high pressures.
The PTC thermistors must then be sintered to form the ceramic body. Metallized contacts are applied to the surface of the device by painting, dipping, sputtering or flame spraying.
The processing of PTC thermistors requires very careful control of materials and particle size in order to produce a device with the proper switching characteristics and voltage ratings. Contaminations on the order of a few parts per million will cause major changes in the thermal and electrical properties of the PTC thermistor.
Most PTC thermistors are designed to operate with a transition temperature somewhere between 60°C and 120°C, however, devices can be manufactured that can switch as low as 0°C or as high as 200°C.
The majority of switching PTC thermistors are intended to operate without an insulation coating. The devices are available, however, with insu- lation coatings when required. Also, small PTC disks / pellets can be sealed into glass envelopes for operation as fluid level detectors.”
Hope this helps to clear the fog.
There is a bi- metallic device that can sense temperature. They use heating effect from the current flow to the end device.

https://www.cdiweb.com/datasheets/ge_thermometrics/ptcnotes.pdf