breadmaker thermistor malfunction: any ideas ?

[chris witt]

a friend asked me to look at her countertop bread machine. she claims it
functions 'normally' *except* it won't warm up enuf to incubate the
yeast at the beginning of the incubation cycle.
so i ran the beginning of the cycle and sure enuf, it doesn't warm up.
(my own machine makes audible clicking noises during this part of the
cycle, which probably corresponds to the heater relay as it energises
the heater element on & off)
i resisted the temptation to tell her to just put the machine in a warm
closet while operating it.

so i tore into it and found what appears to be a thermistor which is
held in contact with the tub while the machine is operational. i don't
know much about thermistors but i suspect it's not normal for one to
read as an open circuit at room temp, which this one does. (and even if
it were a tiny thermostat, it shouldn't be open at 65 F).
so i tried shorting the leads while running the machine. right away the
display said 'hot', all three indicator LEDs on the panel flashed, and
the cooling fan started to hum, so apparently i am on the right track.
then i tried substituting in fixed resistances from very small to as
high as 100k, but even at that level i am still getting the "hot"
response. at this point i gave up subbing in higher resistances. it
seems intuitively odd that the control circuit would be looking for a
higher resistance than 100k, but less than infinity, but as i said, i
don't know much about thermistors. (remember, at infinity, the machine
runs normally, except it doesn't warm up to incubation temp)

anyway i looked at the manufacturer's website which wasn't much help and
couldn't find anything about tech specs or even how to order parts,
except that they say if you want parts you need to contact one of our
Service Centers. so i looked one up in their listing (in SF) and called
them. they said they don't sell anything for breadmakers but tubs and
lids so i asked for the tech specs of a thermistor. the guy said he
couldn't help me there.
i took a quick look online at generic thermistors and the first one i
saw said it had an upper temp limit of 135 C. i suspect the tub of this
thing gets above 300 F and i don't know if 'upper temp limit' means the
temp at which the resistance stops changing, or the temp at which it dies.

anybody got any observations on this? maybe we're back to the Warm
Closet solution....

[Kent Leech]

On the thermistor, are there any identifying marks or numbers? We can search them to find a match, partial or whole. Or at least some parameters.
-----

Kent Leech Illustration
http://www.kentleech.com
ke...@kentleech.com
+1 (510) 504-0634
--

---

From: chris witt <ctw...@sonic.net>
Date: Tue, 07 May 2013 12:12:43 -0700
To: <fixi...@googlegroups.com>
Subject: [FixitClinic] breadmaker thermistor malfunction: any ideas ?

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[chris witt]

On 5/7/2013 12:21 PM, Kent Leech wrote:
> On the thermistor, are there any identifying marks or numbers? We can
> search them to find a match, partial or whole. Or at least some parameters.
> -----
>

uh, good idea, it's encased in a little metal pod with no visible
markings but it looks like i may be able to pry the crimptabs open to
look at something inside. i will do so and report any interesting results.

[Michael Shiloh]

a picture might help us as well

[Michael Shiloh]

are you sure it's a thermistor? could be a much more simple bimetal
switch, which is temperature sensitive.

[****]

thanks michael, i will look into that a bit more thoroughly.

[****]

wow, you've given me a lot to think about, peter. i will pursue your
suggestions and report back.


On 2013-05-07 17:08, A. Peter Blicher wrote:
> In the first link, the guy has the exact same problem. He reports
> that the resistance of the device ranges from 100K to 200K. He also
> says that if he sprays it with cold spray, the bread machine does send
> power to the heating element. He also says it's 100Kohm when hot, and
> 200Kohm at room temp. That implies to me that first of all it's a
> thermistor with a negative thermal coefficient, and in addition that
> 200K is not high enough to make it believe it is cold enough to turn
> on the element. That is all consistent with what you have seen. So,
> it seems you need to use some bigger resistors in your experiments
> (like see what happens with say 1 meg). Maybe you can narrow down the
> crossover points enough to be able to spec out a replacement
> thermistor. At that point, you can probably get it from Digikey or a
> similar outfit.
>
> BTW, when you say your thermistor reads as open circuit, what
> resistance does that mean? If it's supposed to be say 500K, but it's
> only 300K, will your tester still say open circuit?
>
> Note that the following, from the ebay link, has parts for dozens of
> different breadmaker models. You did not specify what model you are
> working on, but the chances are good that it's in the list:
>
> http://stores.ebay.com/Bread-Machine-Parts-Finder?_trksid=p2047675.l2563
>
> Also, the guy says he will work with you to find the part you need.
>
> --peter
>
> On 5/7/2013 12:31 PM, A. Peter Blicher wrote:
>> You may find the following links useful:
>>
>> http://www.ebay.com/gds/find-bread-maker-machine-thermistor-temperature-sensor/10000000175778223/g.html
>>
>>
>> http://www.doityourself.com/forum/electric-kitchen-small-home-appliances/66579-bread-machine.html
>>
>>
>> The 2nd link says that guy's problem is not a thermistor, but a
>> thermal fuse.
>> (But the thermal fuse is a specialized thermistor.)
>>
>> --peter

>>
>> On 5/7/2013 12:21 PM, Kent Leech wrote:
>>> On the thermistor, are there any identifying marks or numbers? We
>>> can search
>>> them to find a match, partial or whole. Or at least some parameters.
>>> -----
>>>
>>> Kent Leech Illustration
>>> http://www.kentleech.com
>>> ke...@kentleech.com
>>> +1 (510) 504-0634
>>> --
>>>
>>>
>>>

>>> -----------------------------------------------------------------------------
>>> *From: *chris witt <ctw...@sonic.net>
>>> *Date: *Tue, 07 May 2013 12:12:43 -0700
>>> *To: *<fixi...@googlegroups.com>
>>> *Subject: *[FixitClinic] breadmaker thermistor malfunction: any