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Help with thermal switches

Discussion in 'General Electronics Discussion' started by jeremyz, Oct 18, 2012.

  1. jeremyz

    jeremyz

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    Oct 18, 2012
    I've been working on building and Android based thermostat lately and would really like to add a purely mechanical safety net to ensure no matter what may happen in the app, the inside temperature will not exceed a safe range. I would like to add a couple of thermal switches to turn the heater off and AC on if the temperature gets over 100F and turn the heater on if the temperature gets below 45F.

    I've spent days searching for bimetal switches and thermal fuses and haven't been able to find suitable components. The major US manufactures do not offer a means of buying just one or two switches, you have to get a quote for bulk orders. I found one decent switch from a Chinese manufacturer, but it doesn't carry any US certifications. The only UL certified components I could find are overkill for what I need and too large to reasonably fit within a thermostat. Could anyone suggest specific components I could use?
     
  2. CocaCola

    CocaCola

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    Apr 7, 2012
    Look for older style household mechanical thermostats that use mercury switches... They can be had for about $10...
     
  3. jeremyz

    jeremyz

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    Oct 18, 2012
    Thanks CocaCola. The device already has a temperature sensor and code that will trigger relays to turn the heater and AC on or off for normal operation. What I'm looking for is a backup device that will force the circuit open or close at certain temperatures (40F and 100F), to provide a bit of a mechanical sandbox for the code to operate in.

    Unless you're referring to something different, these mercury switches are designed to be the primary control mechanism, not a safety cutoff. If I was to try to use them as a safety cutoff, they would need to be mounted to the board at a precise angle and then would be dependent of the user mounting the thermostat on the wall perfectly level in order for them to function properly. I'm hoping to find something along the lines of a bimetal switch or thermal fuse that are designed and tested to open/close at specific temperatures, with very little room for the user to mess something up.
     
  4. duke37

    duke37

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    Jan 9, 2011
    My coal fired central heating has a thermostat to turn on the pump if the fire goes wild in high winds and there is danger of boiling. The stat is a large unit which is strapped to the boiler output pipe.

    Standard room thermostats may also be suitable.
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
  6. jeremyz

    jeremyz

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    Oct 18, 2012
    Hi Cdrive. I've spent countless hours Googling this too. :) I've been through each of those links very thoroughly and it keeps coming down the same three problems.

    #1 The disc action devices are WAY too large. They say 1/2 and 3/4" but that's the radius of the disc. The full devices with the mounting brackets is about 5". I need at least 3 of these for a the thermostat and there's no way they'll fit.
    http://www.amazon.com/Thermostat-Switch-Circuit-100°F-Flange/dp/B0026RYU4W/

    #2 None of the compact devices I can buy like this carry any US certifications.
    http://www.ebay.com/itm/1x-Thermal-...980?pt=LH_DefaultDomain_0&hash=item2a22b34f24

    #3 The compact switches that do carry US certifications can not be purchased individually, only in bulk orders.
     
  7. CocaCola

    CocaCola

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    Apr 7, 2012
    Contact the distributors and/or manufacture direct for 'samples' if they only sell in volume, or inquire about retail outlets for the devices...
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Sorry about that. I didn't know or I missed your size requirements.

    Chris
     
  9. CocaCola

    CocaCola

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    Apr 7, 2012
    If you have strict requirements you might end up having to source a costume design... A small custom thermostat board can be made for a reasonable cost with a temp sensor and a micro, reasonable being subjective... A custom option is going to be a lot more than an over the counter one but you will be getting exactly what you want..
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The mind boggles...
     
  11. CocaCola

    CocaCola

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    Apr 7, 2012
    That would be one in disguise...
     
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