Identification of NTC Thermistor for replacement

Thread Starter

Mambo8

Joined Jan 9, 2024
4
Hi there!
I am trying to repair a coffee machine, Francis Francis X1. The temperature sensor is broken and needs replacement. The component rectangular or rhombic was placed in a well at the top of the boiler (5mm×24mm) filled with thermal paste. I anticipated a NTC thermistor, and thus tried to identify the component from the measured R-T characteristic.
francis-francis_thermistor.jpg
Unfortunately, it did not fit my expectations and has apparently been linearized by inclusion of a resistor parallel to the thermistor, I suspect, after fitting the corresponding equation (R_P = 1.673e+04 (1.668e+04, 1.679e+04), R_R = 3.783e+04 (3.753e+04, 3.814e+04),B = 4133 (4116, 4150)). What are such circuits/devices called and can they be bought pre-assembled? Maybe, I am completely off, and you can suggest a better route of identifying the component. In any case, I am grateful for your thoughts and comments.
 

MisterBill2

Joined Jan 23, 2018
18,388
How broken is the temperature sensor? Is that what you are measuring??The one critical value is the resistance at the system temperature setpoint. The other needed value is the resistance at the normal ambient temperature. The rest of the curve is really not very important.
 

Thread Starter

Mambo8

Joined Jan 9, 2024
4
How broken is the temperature sensor? Is that what you are measuring??The one critical value is the resistance at the system temperature setpoint. The other needed value is the resistance at the normal ambient temperature. The rest of the curve is really not very important.
Thank you, MisterBill2. The one that I am trying to replace is completely destroyed. Yes, the curve shown (red dots) is what I measured at a functioning machine. So \( R_{25} = 11601 \Omega \). As for the set point, I assume that it is prescribed in terms of a resistance value. At the new machine it appears to be set at a value corresponding to \( T = 90°C\), i.e. \( R_{90} = 2661 \Omega \). The measured resistance (i.e. output voltage) appears to be adjustable by a potentiometer. But if anyone could with actual insight into the board logic could explain it, I would certainly revise my guess.

francis-francis_board_notes.jpg

Check through this PDF for information that may help with linearizing your Thermistor.
What is the resistance at 25Cdeg?
Thank you, ericgibbs. Looking through the PDF I found the configuration I had intuited and plotted above, i.e. the parallel configuration as shown in AN685, p. 5 Fig. 5b. So I measured \( R_{25} = 11601 \Omega, R_{90} = 2661 \Omega \).
 

MisterBill2

Joined Jan 23, 2018
18,388
Fortunately the temperature range required for a coffee maker is fairly narrow, and very fortunately two points on the curve are known. So now the shopping for a replacement thermistors can begin. But tracing out a bit of the circuit is probably a good idea .
 

Alec_t

Joined Sep 17, 2013
14,310
it did not fit my expectations and has apparently been linearized by inclusion of a resistor parallel to the thermistor
Are you sure it's in parallel, rather than in series? A table I checked suggests a 12k (at 25C) thermistor would be about 1.5k at 90C., i.e. about 1.2k less than you are wanting.
 

Thread Starter

Mambo8

Joined Jan 9, 2024
4
Are you sure it's in parallel, rather than in series? A table I checked suggests a 12k (at 25C) thermistor would be about 1.5k at 90C., i.e. about 1.2k less than you are wanting.
Thank you, Alec_t. Actually, the fit to my data (blue line, first figure in thread) suggested that the parallel resistor \(R_P = 16730 \Omega \) and the NTC thermistor at 25°C \( R_{25} = 37830\Omega\). Wherefrom is this table you referenced?
 

MisterBill2

Joined Jan 23, 2018
18,388
Consider that there are two pots, the one appears to be sealed, that might be the second resistor. My impression is that the circuit surrounding the thermistor has not yet been traced out.
 

Thread Starter

Mambo8

Joined Jan 9, 2024
4
Check the link.
Yes, I wanted from which book or data sheet the page had come, but I guess it does not matter to make your point.

Consider that there are two pots, the one appears to be sealed, that might be the second resistor. My impression is that the circuit surrounding the thermistor has not yet been traced out.
The temperature probe was analysed after de-soldering the two black wires through which it is connected to the board, thus no potentiometer interfered.
 

MisterBill2

Joined Jan 23, 2018
18,388
Yes, I wanted from which book or data sheet the page had come, but I guess it does not matter to make your point.


The temperature probe was analysed after de-soldering the two black wires through which it is connected to the board, thus no potentiometer interfered.
My point was that there may be a second calibration adjustment of some kind. It may not be directly associated with the temperature control, or maybe it is.
 
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