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Thermistor for a drinks machine

Discussion in 'Troubleshooting and Repair' started by Quovadis, Aug 17, 2017.

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  1. Quovadis


    Jul 26, 2017
    Hi All , looking for a replacement thermistor that will allow the water to boil in a stainless steel tank then switch off at boiling point and then maintain temperature whilst its on ....

    The thermistor is housed in a stainless steel tube which in turn sits in a stainless steel water tank with element

    we have one good one left ... and you cannot order replacements as they are no longer available

    The thermistor reads 8.5 k ohm approx at room temperature, and if you hold it , the value falls , if you put it in a cup of hot water it falls further so we now assume its normally closed , and as the temperature rises then resistance breaks down to a pint that it opens and the power is stopped to the heating element , we also assume this thermistor is variable

    we have taken one apart showing the tube and the thermistor inside ...

    Does anyone know where I can get some of these thermistors of similar value to replace the broken ones we have ? any help appreciated

    Attached Files:

    Last edited by a moderator: Aug 3, 2018
  2. kellys_eye


    Jun 25, 2010
    The thermistor does not go 'normally closed' - it will vary its resistance with temperature but you need a small circuit to convert this resistance change into a 'switching' signal to turn the heating element on and off.

    To this end you can use 'any' thermistor - its value will be irrelevant f you simply design the circuit to take its hot and cold resistances into account.

    If you can't be bothered with the electronics side of it then use the KISS principle and fit a thermal switch like this:

    Thermal Switch.png
    This device will be 'on' if the temperature is below the stated setting (you can get them in different temperatures) and thereby apply power to your heating element (maximum 10A so around 2.5kW at 240V) until the container it is bolted/riveted/welded to hits the temperature and the power to the heater goes off.

    It will continue to keep the temperature at the set value until you remove power (switch off).
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    You assume too much. Since the resistance decreases with increasing temperature, this is a negative temperature coefficient thermistor. It does not "switch" anything, much less the current of the heater element. There are also positive temperature coefficient thermistors. Omega Engineering is the "go to" source for things of this nature. There are others, so make sure Google is your friend to find them!

    An electronic circuit "measures" the thermistor resistance to control power to the heating element. If the thermistor resistance is too high (thermistor too cold), the control circuit applies power to the heating element. After the temperature rises, and the resistance decreases to less than the set-point value, the control circuit removes power to the heating element. Depending on the nature of the power control, power to the heater element may be proportional to the temperature error or it may be a bang-bang on/off type of control. In either case, the thermistor is just a temperature sensor. Actual control of the heating element depends on what type electronic circuit is implemented.
    Quovadis likes this.
  4. Quovadis


    Jul 26, 2017
    Thank you for your valued knowledge on this it is much appreciated . what started off as a general repair issue , has seen us getting dragged deeper into areas that we know very little about when trying to understand how this part actually works ... Obviously we have much more work to do I have been to the link you refer and note all the probes listed , but is it possible to build our own probes, or does this requires specialist equipment or components ?

    Thank you again for sharing your knowledge and replying ...
  5. Minder


    Apr 24, 2015
    Sounds and looks like you may have a thermocouple junction, but these are generally used for set point temperature settings, the alternative thermal bi metal switch (as already shown) is often used for one particular on/off set point in your application..
  6. kellys_eye


    Jun 25, 2010
    Can you get the temperature to exactly 25C and do the measurement again?

    Most thermistors (as replacements) are quoted at this 25C rating and, as a starting point, you need to establish what this is.

    Ideally you would do the same for 0C too.

    The thermistor itself is the tiny 'bead' at the end of the wires and should be readily available - there aren't too many different values and/or shapes around. Have a look at the link to see a selection of typical device and maybe spot one that matches yours (I can't see it properly due to the poor focus).

    If/when we get a match it's a simple matter to solder a new one in place.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    @kellys_eye provided a link to a large selection of "bare" thermistors. Your problem is to select one that works for your application and then mount it in the stainless steel tube you showed in your photos, replacing the (apparently) defective one in the photographs.

    Sure, but you need to know two things: the resistance at 25 C and the beta factor to allow you to select a thermistor compatible with your existing control electronics. Ordinary soldering skills are required to attach the thermistor leads to the external wiring before it is inserted (along with the thermistor) inside the stainless steel tube. You can purchase closed-end stainless steel tubes or make your own from tubing stock, maybe even get fancy and heli-arc a small bead on the end to close the tubing. Or just epoxy the thermistor in place.

    Thermistors are highly non-linear, but extremely sensitive devices. Knowing their zero-power resistance at 25 C provides just one point on their characteristic curve. Knowing the "beta" factor allows you to determine the resistance at a second, different temperature, although not with very good accuracy (because the curve relating temperature to resistance is, well... curved.) The formula below provides an approximation to the actual curve and is probably "gud enuf" for heating hot water in a drink dispenser:

    R(T) = R(To) exp [B (1/T - 1/To)]

    Find the above formula at this Vishay link.

    Note the temperature is expressed in degrees Kelvin, so be sure to make the appropriate conversion from Celsius. You should "calibrate" your only working thermistor in boiling water (100 C = 373 K) by measuring its zero-power resistance there and at "room temperature" (25 C = 298 K) and then calculating the value of "B" or beta in the above formula to find a replacement thermistor. It is important to actually measure the zero-power ambient temperature and the zero-power elevated temperature independently with an accurate thermometer, such as a laboratory grade mercury thermometer, because small errors in defining these two zero-power temperatures will show up as errors in the value of B or beta that is calculated using these resistance measurements.

    Most modern multimeters will use a constant-current source, typically on the order of one milliampere or less, to measure resistance. Unless your thermistor bead is truly tiny, this small current will not cause any appreciable self-heating of the thermistor and the resistance you measure will be the "zero power" resistance to within a sufficient level of accuracy. You can test this hypothesis by observing the resistance reading after connecting the meter probes to the thermistor: if the resistance starts to noticeably decrease, then self-heating is occurring from the multimeter measuring current. I won't go into any detail about how to avoid that, but basically you would need to reduce the constant-current excitation, which will decrease the voltage measured across the thermistor, and increase the sensitivity of the voltmeter measurement. Be careful not to add your own body heat to the ambient temperature measurement. A glass of water, well stirred, should be used as the ambient temperature reference.

    If an accurate thermometer is not available, an ice-water bath (well stirred) can be used as a 0 C reference and boiling water (roiling boil) as a 100 C reference. You can use the resistance measurements at these two temperatures to calculate what the thermistor resistance should be at 25 C, which will then allow you make a selection from a table of available thermistors.

    Once you have determined the B or beta value and the 25 C resistance value, find a thermistor from the list of available, off-the-shelf, devices and purchase a half dozen or so. If desired, you can purchase closed-end stainless steel tubes and suitable wire and connectors to pre-assemble the replacement devices.

    It would be extremely helpful if you could photograph and describe the electronics the thermistor is connected to. Better yet, part numbers and manufacturer could be helpful. Are you preparing to repair just one drink dispenser, or are there are several that are potentially failing? If the latter, then it is important that you have a robust and inexpensive solution at hand to get them up and running with a minimum of down time.
    kellys_eye likes this.
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