# Need help selecting a replacement thermistor

I have a used piece of equipment (basically a specialized laminator) that uses a thermistor to measure the temperature in operation. When I got it, I found that it was heating, but the system never saw the temperature rise.

Upon taking it apart, I found the thermistor (a tiny 4mm diameter x 2mm high metal disc with no obvious markings) completely detached from the tiny wires that used to keep it in circuit.

In order to find a replacement, I used some normal resistors to see what result I got on the temperature readout on the machine. These are regular cheap (+/- 10% tolerance) resistors.

From this data, how can I select a replacement? I see lots and lots of NTC thermistors on Digi-Key, but I'm not sure how to derive the values like B25/85 to select one.

The data I gathered is as follows:

 R (Ω)  | T (°C)
---------------
10k    | 25
5.6k   | 42
3.3k   | 57
1.5k   | 84
820    | 107
470    | 129
330    | 146
220    | 167
150    | 186


The laminator's setting ranges from 100°C to 180°C. It doesn't need to be SUPER accurate, but within 5°C +/- would be preferable.

I know this is a specific question, but I can't really find any documentation on how to derive these values. I found a thermistor calculator and was able to get the various coefficients, but I don't know how to select from a supplier based on that.

Thanks!

Take a look at this one:

http://www.ussensor.com/do-35-standard-glass-encapsulated-thermistors-103e1g1k-table

IT appears to have the same temp/resistance curve as yours, but is in a DO package. This device has an "E1" R-T curve, and assuming this is a standardized measure, you should be able to find alternatives to it. Good luck with your repair!

• This is very helpful, thank you! I'm definitely coming from the software side of things, and even just simple little repairs like this give me so much respect for the actual EE experts. – csyria Apr 8 '16 at 16:53
• No problem! I love being able to help sometimes :) – KilowattLaser Apr 9 '16 at 14:43

Digikey has a nice search engine. I picked one with 10 k ohm at 25C.

The murata data sheet was OK.
http://www.murata.com/~/media/webrenewal/support/library/catalog/products/thermistor/ntc/r44e.ashx

I copied this from there.

1. Zero-power Resistance of Thermistor: R
2. B-Constant R=R0 expB (1/T-1/T0) ..............(1) R: Resistance in ambient temperature T (K) (K: absolute temperature) R0: Resistance in ambient temperature T0 (K) B: B-Constant of Thermistor as (1) formula

B=Rn (R/R0) / (1/T-1/T0) ............(2)

I'll let you plug numbers.... remember to convert to K.