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  Thermistors

An example of a fridge or freezer thermistor used for temperature control

On modern electronically controlled fridge freezers you will find that, instead of an old analogue thermostat that we now have small digital temperature sensors known as thermistors.

These electronic sensors are also widely being used in other appliances as well now notably in washing machines but we are seeing them increasingly used in tumble dryers and cookers as well. 

Thermistors come in two basic flavours, NTC and PTC. 

If the resistance rises with increasing temperature then the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. 

Resistors that are not thermistors are designed to have a resistance as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range. 

Where appliances are concerned the most commonly used type appears to be an NTC and they commonly referred to being such in the industry. 

The reason for using these devices is very simple, they are cheaper and more accurate than old analogue thermostats and allow far greater control as they read a range of temperatures as opposed to specific preset ones found on older thermostats.

  Thermistor Reliability & Testing

It is worth noting that while on some appliances that these fail rather regularly, such as in the Hotpoint Mistral fridge freezer range, they are for the most part a pretty reliable device in our experience. 

We are often asked to supply these when we pretty much know that they won't be faulty. 

Thankfully they are easy to check to see if they are faulty. 

Normally thermistors used in appliances are measuring temperature and reporting that back to the electronic controller as a value kΩ (kilohms). The value of resistance can be easily measured using a common multimeter set to the correct range and that will display the current resistance. 

It should be noted that the resistance values can vary widely from appliance to appliance and it is therefore not possible to tell you what value you should be looking for generally. In fact, in many cases even in the "service" information we get as engineers we don’t get the values either. 

However, that doesn't really matter in many ways when proving if a thermistor is faulty or not. 

With the multimeter connected and showing a resistance value you can then change the temperature of the thermistor by cupping it in your hand or placing it near a cool or hot source, which is another reason to give the engineer a nice cup of tea as it's ideal to hold the tip of the thermistor against the outside (safely of course) and watch on the multimeter for any changes in resistance. 

If the resistance changes, chances are that the thermistor is good. This obviously isn't always the case but 99% of the time this will be true. 

If the resistance does not change, chances are the thermistor is faulty. Again, obviously not always the case but it will be almost all the time. 

Although this is a quick and dirty way of testing a thermistor, it works and it's fast.

  Electronic Fridge and Freezer Controllers

If most refrigeration that uses thermistors, some notable exceptions excluded, it is much, much more likely that the electronic control card has failed rather than the thermistor but there are a few things to check before you swap that out. 

Make sure that the connections are good and clean on the board as, if they are not, this can cause all sorts of strange errors. Remember that as the controller read resistances anything that can alter that reading by the electronics will cause a problem. 

If you can, reset the card totally. 

Make sure that there is nothing touching the thermistor that could cause a false reading or that could be blocking a proper reading. 

Defrost the fridge freezer completely and restart it, even a frost free one as ice build ups can cause the sensors to read incorrect values, especially in a freezer. 

If all that's okay and the thermistor tests out okay, chances are the board is faulty. 

Control boards and Thermistors for the most popular applications are available in our online shop and, if you can't find what you're after then please just ask us. 

As ever, if you cannot use a multimeter of if you're not sure get a professional to look at the machine for

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