Thermistor Check

[Husky250]

Hi Everyone,

What procedure do you guys use for checking thermistors? I recently checked resistance on one which according to the resistance chart, the thermistor checked good. Later, I determined the thermistor was bad when ambient temperature changed around the thermistor.

What would you all recommend?

Thanks to all!

[scuba]

There are two types of thermistors:

• Positive Temperature Coefficient (PTC) types have a resistance that increases with increasing temperature.
• Negative Temperature Coefficient (NTC) types have a resistance that decreases with increasing temperature.

For a small thermistor, put an ohmmeter on it and the heat it up with a blow dryer, heat gun, or the tip of a soldering iron - the resistance should change smoothly (up or down depending on whether it is PTC or NTC type). If the resistance changes erratically, or goes to infinity or zero, the device is bad. However, you will need specifications, temperature measuring sensors, etc. to really determine if it is operating correctly.

[Tesla]

HI Husky250
Do you have a reading or display of the thermistor temp? If so then do a callibration test - compare it with a known accurate thermometer over a period of five minutes and allow time for it to settle first. If it is for chilled water then calibrate in water and ice using an accurate thermo as referance. If there is no display of temp for this thirmister then you must find the resistance comparance chart in the manual or get the numbers off the thermistor and search google for them. Whan compareing against another thermo ensure it is the same type to match the response of sensor. If you need more help with details let me know.

[Magoo]

Generally the controller fails in my experience

[Husky250]

There are two types of thermistors:

• Positive Temperature Coefficient (PTC) types have a resistance that increases with increasing temperature.
• Negative Temperature Coefficient (NTC) types have a resistance that decreases with increasing temperature.

For a small thermistor, put an ohmmeter on it and the heat it up with a blow dryer, heat gun, or the tip of a soldering iron - the resistance should change smoothly (up or down depending on whether it is PTC or NTC type). If the resistance changes erratically, or goes to infinity or zero, the device is bad. However, you will need specifications, temperature measuring sensors, etc. to really determine if it is operating correctly.

Thanks to everyone, this is really good information.

[nike123]

http://www.sensorsmag.com/sensors/el...mperature-1087

[goshen]

HI maggo :
not necserally we had some ntc sensores that just died,turns out they were water sensitive.
water got in to pocket which was not filled with silcone grease.

[cadillackid]

I find thermistors do go bad.. thermistors are not linear so you cannot assume that a certain difference in resistance for a degree measurement.. meaning it is not .1 ohms for every .1 degree.. they are usually logarithmic so you must use the chart...

they are constantly heated and cooled.. and semiconductors over a period of time can and do change their properties so this will cause thermistors to become out of calibration....

if you have a known good part of the same type you can dump both into the same conditions.. amibent air, a glass of water (for ones that are waterproof), etc and see how the 2 resistance values agree...

often when im working on mini spliuts the whole bank of thermistors for the indoor unit (or outdoor) will plug into one connector on the board.. pull them off the pipes and subject all to same conditions and then compare.. they will differ slightly but all should be close...

the only time a unit will issue an error code related to a thermistor is if it is open, shorted, or hits an out-of bounds value that the controller is designed to recognize... ie reads 250 degrees F on an A/C unit....

with the newer units and EEV's these thermistors are the only feedback as to the refrigerant temperatures and flow inside a coil.. in effect the units measure their own superheat and subcooling.. some units now even take humidity measurements of interior and exterior units to make a better decision on what to do based on wet bulb / dry bulb temperatures...

your best Kit will even have a mode to calibrate thermistors....

in effect those little guys have a pretty big role in the performance of a system
-Christopher

[Robert Nimni]

The best way to test sensor is when the sensor connected to the controller
We need to check with multi meter In VDC mode.
Put the probe on the sensor connection.
At room temperature (25degrees) you will receive 2.5VDC If you receive 5VDC this means that there is a disconnect If you receive 0 VDC means that there is a short
There is controller that working at 3.3V. In this controller when the sensor is disconnect we get 3.3 VDC. at room temperature (25degrees) we get 1.7 VDC.
Of course, the result varies according to temperature.

Robert

[sachin230]

Remove the thermistor from the circuit in which it is used. Most ohmmeters have automatic range finders, but if you need to select a range, start with the highest range and then adjust the range downward after making your resistance measurement.

Connect the ohmmeter probes to the thermistor. If you get a resistance reading, the thermistor has an extremely high probability that it is not defective. Most thermistors that are defective will be open and show infinite resistance.

Turn on the blow dryer and direct the air flow at the thermistor. The resistance measurement on the ohmmeter display should decrease. A typical thermistor will decrease from about 10,000 ohms to about 1,000 ohms, although these values can vary with the thermistor model.

[Robert Nimni]

For example.
Note:
that the sensor is the same but ZC207 works at 3.3V and 138 works on 5 V.

[cadillackid]

all thermistors are different.. not all are 10k thermistors.. some of the chinese splits are using 5k thermistors.. some are using a mixture depending on the range they expect it to be in.....

and as stated above some send out a reference voltage of 3.3 volts and some send out a reference of 5 volts....

most often if the Micro is an 8051 or an ATMEL of some type they are using standard 5 volt reference voltages....

the ultimate concept here is that a thermistor is nothing more than a resistor that varies with temperature...

in fact using digital pots you can progmatically make a controller "think" its getting whatever temperature you want....

the micro simply looks at the return voltage it gets back and correlates that to a temperature... some as simple as a lookup table in the controller (for micros that dont have good math capabilities) and some actually calculate the typically logarithmic formula for resistance to temperature conversion....
-Christopher

-Christopher