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Posted by David Farber on May 31, 2006, 7:37 pm
>
> >I have a Kenmore 110.76912692 gas clothes dryer. The 186ºF non-resettable
> > thermal fuse opened which caused the burner to cut out. There was a
> > considerable amount of lint built up so I cleaned it all out. I'm about
to
> > replace the fuse but I also had some other questions regarding the
> > operation
> > of the thermostat.
> >
> > When I disassembled the dryer, there was a nice schematic (more of a
block
> > diagram) hiding under the control panel. According to the diagram, the
> > series of devices in line with the heating circuit is as follows:
> >
> > 186ºF thermal fuse.
> > Operating thermostat, 150ºF or 155ºF.
> > High limit thermostat 205ºF.
> >
> > Then from there, it goes to the gas burner assembly.
> >
> > If the thermal fuse is set at 186º what's the point of having a high
limit
> > switch at 205º?
> >
> > The operating thermostat is a 4 terminal device which sits right next to
> > the
> > thermal fuse. Two of the terminals make up a single pole switch. The
other
> > two terminals have a resistance of 7k ohms and from what I can figure
out
> > at
> > the company's website (see link below) that makes this device, it uses
> > some
> > sort of biasing to alter the cutoff point. Is this correct?
> >
> > http://www.thermodisc.com/productdetail.asp?ProductID=8
> >
> > Thanks for your reply.
> > --
> > David Farber
> > David Farber's Service Center
> > L.A., CA
> >
> >
>
>
> The physical location of the high limit and thermal fuse might answer the
> question. Not really sure about that. The thermal fuse usually dosen't
> blow until the airflow is restricted by lint build up anywhere in the
stream
> as you found out. I think the fuse is further from the burner, again not
> too sure.
>
> The other two terminals connect to a small strip heater beside the bimetal
> switch inside the thermostat. This can be biased with AC voltage to bias
> the temperature at the switch (making it think it is warmer than it
actually
> is). I believe they do this during the gentle cycle (or delicate -
> whatever) to get a lower temp without needing to have a different
> thermostatic switch.
>
>
I guess it makes sense that the sensor nearest the burner would have a
higher cut off temperature.
Using that AC voltage bias is a neat trick. The schematic shows a thermostat
heater resistor with a resistance of 5600 to 8400 ohms. It's not shown
linked to the thermostat. Perhaps a dotted line between the thermostat and
the resistor would have made things a bit more clear.
Thanks for your reply.
--
David Farber
David Farber's Service Center
L.A., CA
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