Thermistor? Wrong temperature detected


I am trying to repair an incubator BB-100 that is not reading the right temperature. It says 37 Celcius, when it is in reality 26 Celcius. I believe the defective part is the one I am pointing on the picture, but I do not know how it is called or how to choose one properly. I think it might be called a Thermistor.

All the numbers around the part points to the shrink tube or the wires and not the part itself.
9mm wide
5.4mm thick

Could someone give me some guidance?
Thank you so much,


I am sorry to say that I am not seeing data on the sensor to be able to offer an item.

I forgot to add that the voltage at the connector is 6mV (10mV on another incubator similar model). I do not know if it can help.

I tried to look on Digi-Key and at large for a temperature sensor that works at that voltage, but without success. That’s why I was more thinking about a thermistor then a sensor, but I have very little information on that part…

Hello CVL,

This is a difficult question to answer.

It’s complicated because the temperature sensor could be a thermocouple, RTD, thermistor, a 1-wire device, or even something else.

While I’m 90% confident that DigiKey has a solution, the challenge remains How can we find a suitable substitution?


  1. Please reply with the manufacturer and model number of the incubator. Hopefully a community member can bring us closer to a solution.

  2. I’m hesitant to say this as this test could damage the sensor. However, if the first suggestion does not obtain results, we may be able to determine a clue by performing a resistance test.

First disconnect the probe from the PCB then measure the resistance:

  • A to B
  • B to C
  • C to A

Reverse the meter’s probes and measure resistance again, this time as:

  • B to A
  • C to B
  • A to C

Welcome to the DigiKey community. We hope to hear back from you soon.

Best Wishes,




As this is a medical device designed to save the lives of infants, I would strongly recommend you contact the manufacturer, Zhengzhou Dison Instrument and Meter Co., ltd or certified repair facility for their products for any malfunction/repair issues to either have them do the repair, or at the least, get a recommendation from them as to what part should be used. Using anything other than the specified parts could cause serious injury or death to any patients.


Thank you all for your help!

@APDahlen I will post again with the results of those suggested tests, but it might take a few days.

@David_1528 I totally agree, but our facility is not one of human medicine and, if I manage to repair that unit, it will not be used in human medicine. I am already in contact with the manufacturer, but the language barrier is a bit of a challenge since they are in China.

Thank you again!

OK, I feel a little better about this, then.

So, looking at your images, it looks there are 3 wires coming from the sensor, is that correct? If so, that reduces the likelihood that it is a thermistor or RTD-based sensor, as those are typically 2-wire devices (though that’s no guarantee).

One question about its current behavior: You mention that it is showing 37°C when it’s actually 26°C – does the displayed value change if you change the actual temperature, or does it remain fixed?

I see that CN6 labels the first three pins. Are there any labels on the last three pins which appear to be where the sensor wires enter? I would think that most likely two of those leads are power and ground, and the third lead would be the sensor output signal.

It sounds like you may have access to another incubator with similar connections. In addition to the tests @ APDahlen discusses, would it be possible for you to get an oscilloscope probe on the sensor leads of that working unit? It’s possible that the signal could be either analog (voltage or current) or digital (PWM, pulse frequency, pulse count, etc), and an o’scope would help with either of those scenarios to try to figure out what type of output the sensor provides…

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Agree, @David_1528, it’s very hard to tell from a picture. That third wire could be a shield, or it could be a ground return.

With regards to the resistance test,here is my thinking:

  • low resistance implies a thermocouple
  • 100 Ω or 1000 Ω likely an RTD
  • 5 to 15 kΩ likely a thermistor
  • no identifiable pattern is something else

Does this look reasonable?



I agree. Measuring those values would likely indicate those conclusions.

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Here are all the answers except the one about the oscilloscope. I have to admit that I am quite rusty with mine, so I’ll need to re-familiarize myself with it before giving a good information.


Manufacturer and model number: Zhengzhou Dison Instrument and Meter, BB-100


First disconnect the probe from the PCB then measure the resistance:

A to B: 17.1 kΩ, but slowly increasing

B to C: 17.6 kΩ, but slowly increasing

C to A: 35.9 kΩ, but slowly increasing

Reverse the meter’s probes and measure resistance again, this time as:

B to A: 18.2 kΩ, but slowly increasing

C to B: 18.4 kΩ, but slowly increasing

A to C: 36.9 kΩ, but slowly increasing




Since the incubator was warm when I shut it down and started the measure, the fact that the value kept changing in the same direction is most likely because the sensor was slowly changing temperature.


Yes, there are 3 wires coming out of the sensor


The displayed value changes. When I start the incubator, it shows around 21°C. If I set the unit for 37°C, it will progressively get there. If I change it for 31°C, it will gradually get there.


CN6 identifies the whole 7 pin connector, there is another label on the other side of the connector, (CN5) but it identifies the other white connector of 4 pins.

Hello CVL,

Thank you for the measurements.

Also, I understand this device is not used for human medicine.

I was so hoping that the resistance readings would yield a short A to C or even 100 Ω. At this point I’m not sure what we are dealing with.

One quick question, when you were taking the resistance measurements were you careful to keep your fingers out of the equation? Which is to say, are we confident that we are not reading your skin’s resistance? It’s an easy mistake I’ve made without thinking as we all need to hold the circuit as we manipulate the probe.

@David_1528 makes a good point about the oscilloscope. It’s possible that this is a semiconductor device.

One word of caution - be careful with the working unit as a mistake could result in two out of commission units.

The only thing I have left is to peel back the sensors cover to expose what is inside. Unfortunately, that would be a destructive exploration and may not return any results.

Sorry I couldn’t help.

I hold hope that a more knowledgeable community member will respond. I’m always amazed at what people can come up with. Also, I hold hope that you are able to contact the OEM to obtain a replacement part.

Best Wishes,


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Yes, I was careful not to put my skin in contact with the probes when I measured the resistance. And yes, it’s such an easy mistake :slight_smile: Just like when someone tells you the printer is broken, but it’s just not plugged in or is empty of paper :wink:

Thank you very much for your concern for not damaging the second unit. It is always something on my mind when I do those tests and I am very careful with every steps and review the techniques if I’m rusty.

If I do not get any news from the manufacturer and we are unsuccessful, I agree that dissecting the piece just to understand what it is could provide some additional informations.

Thank you very much for your suggestion and advices. I really appreciate it.

You are very welcome, CVL.

Yes, that’s the first rule to troubleshooting: did I do something wrong!

This is certainly a challenging problem.

Please let us know if we can assist in the future.

Also, please let us know if you find a solution as it may help others.



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Your measurements would suggest that there are two identical NTC resistors inside the shrink tube. The “B” wire being the common.

As you have another similar incubator, you could try to swap the sensors to see which one fails. If the problems persists even with the another sensor, then the main board has the problem (which one should anticipate, as the measured resistance values seem sane).



Hello all,

@David_1528 Here is the graph on the oscilloscope:

I apologize, I’m still a beginner with it.

@APDahlen I will totally keep this trend updated if a resolution is found.

@ heke It would be difficult to remove the sensor from the working unit. Altough, during my troubleshooting, I’ve installed a new board I had in my inventory in the defective unit and the problem was still present. It then confirmed that the sensor was really the problem.

I will want to retake the reading of A,B,C when the unit is cool, but it might take a few days because it is in use right now (26 Celsius is better then room temperature)


Whenever there is a chance, I wonder, could you perform the A, B, C -measurement also for the sensor of the working unit? That would help to determine where the delta is.

Cheers, heke


Hello @heke,

In the original instructions you gave me, you mentionned the test could damage the sensor. I didn’t mind doing it on the sensor I new was defective, but I am not sure I want to risk disabling our working unit.

I agree with the first answer made by heke. There can be a problem with the board itself. I suggest you test the PCB. Perform some basic tests like the short test, continuity test etc. By the way, if you want to make a thermostat with NTC thermistor, here is a design that you may want to see.

@bidrohini during my troubleshooting steps, I replaced the PCB for a brand new one and the problem was the same, so the issue is probably the part itself.

Thank you for sharing that link

I managed to have an answer for the manufacturer and I should be able to order the part. I’ll let you all know if it resolves the issue.


I see. Have you received a replacement yet?