Understanding an Electric Blanket Heat Controller

Discussion in 'The Projects Forum' started by iONic, Oct 24, 2010.

  1. iONic

    iONic Thread Starter Senior Member

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    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

    iONic
  2. Markd77

    Markd77 Senior Member

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    There could be a bimetallic strip in there, they bend in relation to temperature.
  3. BillB3857

    BillB3857 Senior Member

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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.
  4. thatoneguy

    thatoneguy AAC Fanatic!

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    Here is the schematic for one:

    Patent 4034185 by Northern Electric, applied for Sept 2, 1975
  5. iONic

    iONic Thread Starter Senior Member

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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.
  6. marshallf3

    marshallf3 Well-Known Member

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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.
  7. thatoneguy

    thatoneguy AAC Fanatic!

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    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. :D
  8. marshallf3

    marshallf3 Well-Known Member

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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.
  9. BillB3857

    BillB3857 Senior Member

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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.
  10. bertus

    bertus Administrator Staff Member

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  11. someonesdad

    someonesdad Senior Member

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    I have enormous respect for the bimetallic thermostat -- because it works, it's reliable, and it's cheap and easy to make. Like Marshall, I took one apart in the 50's because I was curious how it worked and I'll bet there's little difference today.

    The basic principle upon which they depend is that metals expand when they are heated. Different metals expand at different rates (look up thermal coefficient of expansion, which is usually on the order of 10e-6 per K if memory serves. Then two dissimilar metal strips are mechanically joined together (riveted, welded, etc.) and temperature changes cause the assembly to move, usually to open or close some electrical contacts.

    The principle is used in a variety of places. For example, fluorescent bulb starters depend on the principle. Many thermometers use the principle. There are a gazillion thermostats in the world that use it. You can even calculate the curvature of a bimetallic beam if you know the elastic properties and thermal coefficients of expansion.
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