How to Check a Thermistor Using a Multimeter

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Many modern HVAC systems depend on a thermistor for the correct sending of liquid and air temperatures to be utilized by system control boards for adjustments to operational speeds. As temperature conditions adjust, the thermistor reports such temperatures to the control boards, which then change the blower and compressor speeds to compensate for such changing conditions. 

Experimental Drawing Techniques x
Experimental Drawing Techniques

Thermistor type selection differs based on the use required or application needed. HVAC thermistors’ resistance ranges utilized by CBP are the 10K ohm and 50k ohm selections. Those are referred to as NTC thermistors, as they react inversely to the temperature change. When the measured temperature increases, the temperature resistance value lowers. Similarly, the thermistor resistance value increases if the measured temperature lowers. 

What is a Thermistor?

A thermistor is a temperature-sensitive resistor. The term thermistor is a merge of two words: thermal and resistor. In case you didn’t know, thermal is derived from the Greek word heat. On the other hand, a resistor is a passive device utilized to stop the electrical current. Normally, these resistors are utilized in electronics, and thermistors are a special group of resistors. 

Electrical current will flow to a thermistor, and a limited amount of it will flow out due to the properties inside. Apart from discussing what a thermistor is and how it works, we will also dig deeper into what a real thermistor looks like on the inside. 

You will find basic parts of this component:

  • two metal leads carrying a signal in and out 
  • a metal oxide wafer
  • a protective outer sheath which is made out of resin or glass

Sometimes, a solder is utilized to help fuse the core and the leads together. However, the outer cover compresses the leads to the core in other manufacturing methods. Those are enough to keep them in place. At the middle of a thermistor is the core—a metal oxide disc.

That’s a combination or compound of metal and oxygen elements, such as manganese, copper, or nickel. It’s that compound that changes the flow of electrons by an amount depending on the temperature in the environment around the thermistor. 

That change of electron flow or resistance could be measured. You can determine how much current is flowing into one lead of a thermistor and how much less is flowing out the other lead. Keep in mind that a thermistor will only act the similar way every time, opposing only based on the temperature around.

Hence, the quantifiable difference it is causing to the current could be associated with a particular temperature. 

What Causes Them to Fail?

Rarely, the thermistor fails totally, even though we often see it fail because of an open circuit leading to a break in wires between the main control board and the thermistor. That typically happens when the wires are spliced wrongly, letting moisture penetrate.

The most typical reason why these electrical components fail is because of aging. In the end, sintered non-oxides in thermistors lose their efficacy and offer signals, which aren’t longer relevant and accurate. 

What Are the Steps to Checking a Thermistor? 

Keep in mind that thermistors are normally utilized to control heat and cold. However, they could also be utilized for measuring circuit-protecting, volume, and voltage. Most types of products depend on such resistors to keep proper functionality and efficiency.

The most typical way to check if your thermistor is bad if it begins showing wrong temperature readings. That could be caused by incorrect handling, too much heat, a thermal mismatch, or a dip in resistance accuracy because of age and regular use. Further, an open circuit could also result in thermistor problems. 

So, how can you check a thermistor using a multimeter?

A thermistor is a sensitive component. This is divided into negative temperature coefficient thermistor and positive temperature coefficient thermistor. 

The alleged resistance of a thermistor is calculated with a special device at a temperature of 25 degrees Celsius. Under standard conditions, it could also be measured with a multimeter. Nonetheless, using that tool means there’s a formed thermal effect because of the massive working current. That often causes the calculated value to be not consistent with the assumed resistance value. 

If the thermistor’s resistance is only needed to be checked to identify its type and whether it could function properly, a multimeter can be utilized to check it. To check your thermistor’s overall accuracy, you will need a multimeter, a heating device of any type like a space heater or blow dryer, and of course, a thermistor. 

After you have all the materials ready, you can start checking your thermistor in just a few simple steps. Here are the steps you need to follow:

  1. Set your digital multimeter to the ohms setting and allow the two test leads to touch the two pins of the thermistor. The reading is the thermistor’s resistance value at room temperature. If the reading is infinity or zero under the premise of correct selection of ohmmeter, it means your thermistor has been damaged.
  1. Place an electric soldering iron close to your thermistor. Has the resistance shown by the meter changed from a normal temperature resistance value and reverted to the normal temperature resistance value after the electric soldering iron is moved away? Then it means your thermistor is still efficient and useful. 
  1. Connect the two pins of the thermistor with a meter clamp and place it into the refrigerator. The resistance value shown by the meter for the negative temperature coefficient thermistor is bigger than the resistance value at room temperature.

The resistance value shown by the meter for the positive temperature coefficient thermistor is lower than the resistance value at room temperature. 

Final Thoughts

There you have it! We hope the above steps are helpful to you. We recommend that you pick regular manufacturers when buying thermistors, so the product quality is always 100% guaranteed. 

So, what about you? Are you ready to test your thermistor? Share your thoughts with us by leaving your comments down below! Enjoy your testing!