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Old 10-24-2010, 08:03 PM
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Default Understanding an Electric Blanket Heat Controller

I'm trying to understand just how an electric blanket heat controller works. Any insight would be helpfull.

The one I have is soley a mechanical device. A set screw determins the ability of a contact is made or broken. So how does it work? Is is based on the thermal properties of the metal used? Does it detect current flow?

All I know is that they have so much slop in the temperature of the blanket.
You turn it till the heat comes on and it gets too hot, so you turn it till the heat goes off and it gets too cold. So somewhere between the contact being turned on or off is the temperature I would be interested in and seems to be a big guessing game!

enlighten me please.

Thanks

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Old 10-24-2010, 08:05 PM
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There could be a bimetallic strip in there, they bend in relation to temperature.
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Old 10-24-2010, 08:15 PM
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The one I took apart many years ago had a bi-metal that also carried the current to the blanket. A combination of ambient temp and heat generated by the current opened and closed the contact.
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Old 10-24-2010, 08:58 PM
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Here is the schematic for one:

Patent 4034185 by Northern Electric, applied for Sept 2, 1975
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Old 10-24-2010, 09:05 PM
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That is most likely what it is. It is 15+ years old. But since the ambient temperature is relatively stable and the metal strip is isolated (5ft +) from the actual heated blanket I would think that the current primarily is heating the metal strip. That could certainly explain the slop in temperature sensitivity of the device. Although the scale is set from 1 to 10, normal operating conditions in home keep is between 3 and 4, 3 being sometimes too cold and 4 being sometimes being too hot! What I need are some levels between 3 and 4. I guess I'm asking for too much from 15 year old technology. Need a new one based on PWM, thus settings will produce sensitive and predictable levles of heat. You could say that the "hysteresis" of the metal plate version is very wide, +/- 10 degrees F or more.
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Old 10-25-2010, 01:41 AM
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It's just timed, the higher you set the control the longer the on cycle is. Timing is provided by that bimetal strip heating and cooling.

There are protection devices inside the blanket to limit the maximum temperature only, they're rarely any sort of temp sensing device.
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Old 10-25-2010, 02:07 AM
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Quote:
Originally Posted by marshallf3 View Post
It's just timed, the higher you set the control the longer the on cycle is. Timing is provided by that bimetal strip heating and cooling.

There are protection devices inside the blanket to limit the maximum temperature only, they're rarely any sort of temp sensing device.

That doesn't sound as good as packing it all into once sentence, say
"VLF PWM open loop temperature control, the load having integral heat and current overload protection"

I suppose more could be abbreviated and some extra buzzword bingo type stuff could be added for extra fluffiness since we are discussing blankies.
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Old 10-25-2010, 03:52 AM
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Sad but true, I probably took one apart 50 years ago out of curiosity and when my aunt's quit working a few years ago I said I'd look at it - control system was essentially the same setup.

I suppose there is a small amount of feedback introduced from the room temperature's effect on the bimetal switch.
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Old 01-19-2011, 12:46 PM
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The current going through it. Or, in some cases, it is wrapped in an electrically insulating material and then wrapped with a nichrome wire which carries the current and acts as the heating element.
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Old 01-19-2011, 12:52 PM
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Hello,

Here is the ehow article:
http://www.ehow.com/how-does_5641870...ket-works.html

Bertus
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