Bi- metal thermostat, heaters, timer

Hi,

I don't know the fridge and cannot find any parts information for it, but it would be unusual for both the heater and the defrost thermostat to be connected together unless the thermostat’s contacts are used to disconnect the heater and maybe the control board detects the change in resistance on the heater circuit and stops the defrost cycle and starts the compressor and evap fan again. This is only conjecture on my part as I said, I don't know.

Are you sure that it is a Bi-Metal thermostat and not a thermistor? Are there any component markings on the case that may help to identify what it actually is? Does the resistance change if the “Bi-Metal thermostat” is allowed to get to room ambient temperature and is not at the temperature of the evaporator unit which given how much ice was on it would be close to <0-10 deg.C? If possible, maybe hold your fingers on it for a while to warm it up and then check the resistance. Watch out that you don’t get ice burns and also do this with the power disconnected from the refrigerator.

Bi-Metal defrost thermostats are a simple device. Just two dissimilar metals that depending on the type of thermostat either move away or move towards each other as the temperature changes thereby either making or breaking a circuit connection, i.e. either open circuit or short circuit. Their design specification dictates at what temperature the contacts actually make or break. There is no external electrical power input required to make the thermostat function. It is solely due to the temperature of the environment where it is located.

Post some images of the heater/defrost thermostat arrangement and perhaps it can be worked out as to how and why they're wired as you say.

Hi,

OK

It’s a bit hard to see exactly as I can’t see where the wire at the other end of the heater is connected to.

This is what the heater should look like.

Here’s where I think the heater is and also one wire from the heater

(click on image to enlarge for better viewing)

You need to find the wire at the other end of the end of the heater (see the link above and see where it is as I can’t see it in the images) and then disconnect the plug connector where the heater wires connect (the one shown in the first image you posted and also in the image I posted with the yellow insulation? on it) and then place the probes directly where the brown wire is and also where the wire from the other end of the heater is in the plug and check the reading.

As best that I can find out is that a P-5M1N is a THERMO BIMETAL-PROTECTOR 125/250V,10/5A,100. I’m not sure what the “100” refers to but it is designed to carry at least 5A if the voltage is 250V AC where you are so maybe the heater does go through the thermostat, but it will not have any resistance. It is just a switch and should have no resistance or at best <0.1 Ohms.

Depending on what the voltage supply is where you are if the thermostat is in series with the heater and its’ rating is as shown then if the maximum current was flowing through the thermostat to power the heater then 125V with 10A = 1250W of power and the heater’s resistance would be 12.5 Ohms. If it were 250V with 5A = 1250W of power and the heater’s resistance = 50 Ohms. Even if the heater’s wattage was only rated at 625W the resistance would then be 25-100 Ohms.

Either way the heater’s resistance is between 12.5 Ohms and 50 Ohms and not 433 Ohms. To get 433 Ohms @ 240V AC= 130W which is little better than a light bulb as far as a heater goes and also it would mean only 0.5A current flow. Manufacturers wouldn’t put in a thermostat with a rating of 5A for a 0.5A current flow just on the cost difference alone..

So it could be the thermostat (high resistance contacts?) or the heater. You would have to isolate them from each other and then test them individually to know.

I suspect that the thermostat should test open circuit at temperatures above 0 deg. C as most thermostats operate to signal the control board when this temperature is reached. In your fridge perhaps if it is wired in series with the heater then it is used to cut off the power to the heater.

The thermostat should measure short circuit i.e. 0.00 (zero) Ohms at temperatures below 0 deg. C so that when the auto defrost cycle is commenced the next time the heater circuit has been reestablished by the fact that the thermostat has closed again when the temp was below 0 deg C as the freezer would be at -18 deg. C at the start of the defrost cycle