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#1
10-24-2010, 08:03 PM
 iONic Senior Member Join Date: Nov 2007 Location: Vermont, USA Posts: 1,403
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!

Thanks

iONic
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#2
10-24-2010, 08:05 PM
 Markd77 Senior Member Join Date: Sep 2009 Location: Birmingham Posts: 2,782 Blog Entries: 1

There could be a bimetallic strip in there, they bend in relation to temperature.
#3
10-24-2010, 08:15 PM
 BillB3857 Senior Member Join Date: Feb 2009 Location: St.Louis, MO area (GMT-6)_ Posts: 2,088

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.
#4
10-24-2010, 08:58 PM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

Here is the schematic for one:

Patent 4034185 by Northern Electric, applied for Sept 2, 1975
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#5
10-24-2010, 09:05 PM
 iONic Senior Member Join Date: Nov 2007 Location: Vermont, USA Posts: 1,403

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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#6
10-25-2010, 01:41 AM
 marshallf3 Senior Member Join Date: Jul 2010 Location: Oklahoma City, OK. Large enough to be modern but plenty of country left around it. Posts: 2,358

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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#7
10-25-2010, 02:07 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

Quote:
 Originally Posted by marshallf3 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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Also-Impedance Graph paper.
#8
10-25-2010, 03:52 AM
 marshallf3 Senior Member Join Date: Jul 2010 Location: Oklahoma City, OK. Large enough to be modern but plenty of country left around it. Posts: 2,358

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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The very first course in engineering school should be how to use Google and Wikipedia
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I very often misspell or miss things while making posts so come back and double check a few times.
I also have a full time job and often 10 projects going on at the same time so I'm not always online.
#9
01-19-2011, 12:46 PM
 BillB3857 Senior Member Join Date: Feb 2009 Location: St.Louis, MO area (GMT-6)_ Posts: 2,088

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.
#10
01-19-2011, 12:52 PM
 bertus Administrator Join Date: Apr 2008 Location: Amsterdam,Holland (GMT + 1) Posts: 12,035

Hello,

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

Bertus
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 Tags blanket, controller, electric, heat, understanding

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