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How to test Bias thermostat


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7 replies to this topic

#1 appliancejack

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Posted 25 November 2010 - 09:05 PM

Trying to learn how to test a Bias thermostat Part #307249 5 wire connections. Any help would be appreciated.

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#2 BrntToast

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Posted 25 November 2010 - 09:20 PM

the "bias" part of that thermostat ar the 2 center wires, they energise a heater within the therm to make it cycle at a lower heat
without the wiring diagram for the unit it comes from i will say the one single wire on one side would be a common the 2 on the other side will be one normally open, other normally closed

meter for continuity on the common to one of the 2 on the other side will show continuity(this is the normally closed contact), heat the therm to its temp rating and it should click, opening that contact, at the same time you should now find continuity to the other contact from common

to test the bias heater(2 center wires) test for continuity on the meters high resistance scale(sorry i forget what they ohm out at)
common use for a therm like this was auto cycle in older dryers, when the therm was cold it energised the heater and killed power to the timer, every time the therm cycled the heater off the timer was energised and advanced
with cold wet clothes the heater was on alot longer, as the clothes dry and get hotter the heater cycles off sooner and the timer advances faster

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#3 RegUS_PatOff

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Posted 25 November 2010 - 10:35 PM

Posted Image



BrntToast is correct.
the 2 (smaller) wires are for the interal bias heater.

It may be about 3k OHMs

The thermal switch part of it is a Single Pole Double Throw Switch,
normally connects the circuit for the Dryer Heating Element.

Once the Operating Thermostat (thinks) the air temperature is warm enough,
it will disconnect the Dryer Heating Element, and then also allow the Timer Motor to run,
so that the clothes are at the "operating temperature" for the selected amount of time.

The Temperature Switch and/or the Timer Dial selections, allows for the use of the internal bias heater on the Delicate Cycle.
On the "Regular" Switch setting, the Bias Heater isn't used, so that the air temperature must "really" reach the Operating temperature.
On the Medium Temperature setting, there's a 3k Resistor in series with the Bias Heater, so that the Operating Thermostat doesn't reach it's tripping point as fast.
.

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#4 appliancejack

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Posted 25 November 2010 - 10:42 PM

the "bias" part of that thermostat ar the 2 center wires, they energise a heater within the therm to make it cycle at a lower heat
without the wiring diagram for the unit it comes from i will say the one single wire on one side would be a common the 2 on the other side will be one normally open, other normally closed

meter for continuity on the common to one of the 2 on the other side will show continuity(this is the normally closed contact), heat the therm to its temp rating and it should click, opening that contact, at the same time you should now find continuity to the other contact from common

to test the bias heater(2 center wires) test for continuity on the meters high resistance scale(sorry i forget what they ohm out at)
common use for a therm like this was auto cycle in older dryers, when the therm was cold it energised the heater and killed power to the timer, every time the therm cycled the heater off the timer was energised and advanced
with cold wet clothes the heater was on alot longer, as the clothes dry and get hotter the heater cycles off sooner and the timer advances faster




I have been trying a test. First, when I place my meter leads on outside terminals (far left and far right) I get zero ohms. 2nd, When I place my meter leads on the inside terminal of the two on one side and the other meter lead on the terminal by its self on the other side, I get zero ohms. 3rd, when I place meter leads on the two center terminals I read 28 K ohms. Now, when I apply heat to the thermostat all the outside readings read open line to indicate thermostat has open. As for the two center terminals with the themostat still open, the reading remains the same at 28 K ohms. What does that tell you?

#5 RegUS_PatOff

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Posted 25 November 2010 - 10:53 PM

the center terminals (bias heater) may actually be 28k OHMs
That's an internal resiator that won't change with temperature.

The Switch contacts are the (3) "big" terminals.

When cold, (2) of those terminals shound show 0 OHMs.

When warm, those (2) should show "open",
and another combination of (2) big terminals should now show 0 OHMs.

Edited by RegUS_PatOff, 25 November 2010 - 10:54 PM.

.

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#6 appliancejack

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Posted 25 November 2010 - 10:54 PM

I have been trying a test. First, when I place my meter leads on outside terminals (far left and far right) I get zero ohms. 2nd, When I place my meter leads on the inside terminal of the two on one side and the other meter lead on the terminal by its self on the other side, I get zero ohms. 3rd, when I place meter leads on the two center terminals I read 28 K ohms. Now, when I apply heat to the thermostat all the outside readings read open line to indicate thermostat has open. As for the two center terminals with the themostat still open, the reading remains the same at 28 K ohms. What does that tell you?

#7 appliancejack

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Posted 25 November 2010 - 10:56 PM

Thanks guys for all your help. I really appreciate it. I hope you guys had a great Thanksgiving. Thanks again!
Jack Lowe

#8 appliancejack

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Posted 25 November 2010 - 11:01 PM

Hey fellows, Sorry about adding the last reply twice, I'm still getting the hang of this new site.




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