Why thermal fuse?

Discussion in 'General Electronics Chat' started by ryancousins, Jun 3, 2014.

  1. ryancousins

    Thread Starter New Member

    Jul 15, 2012
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    I have a "Kill-A-Watt" power meter and I blew the thermal cutoff fuse because I drew too much current. I'm wondering, why not use a regular fuse? The only thing I can think of is that the manufacturer wanted to guarantee that it would trip if it got too hot inside the meter, even if you didn't exceed the 15A max current rating. However, it's rated at 1875VA, and I can't imagine much more than 125v coming down a standard NEMA5-15 outlet (125v * 15a = 1875) , so it seems to me that just using a 15A current limiting fuse would work just as well. What am I missing? It has the standard 5-15 prongs built in, but maybe they considered that someone might find a way to plug it into 240 and thereby upping the VA and therefore heat dissipation without getting near 15A?

    Also, would a thermistor be a suitable or even superior replacement for the thermal fuse?
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Usually they are used where the concern is over heating. Like a dryer or space heater. Doesn't seem like that would be an issue with the kill a watt. But maybe the concern is you plug it in and leave it and then there is an overcurrent that might "fire up" the kill a watt.
     
  3. #12

    Expert

    Nov 30, 2010
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    Trying to find my mind reading hat...

    what kind of, "thermal fuse" The kind you have to solder?
    That alone would stop a lot of people from repairing the device, then they would have to buy another one.

    Still not getting any answers to how a thermistor would work in that circuit.
    Do you have a schematic?
     
  4. ryancousins

    Thread Starter New Member

    Jul 15, 2012
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    I plan on ordering a replacement thermal fuse, ya, it's the kind you have to solder in. My question was just general curiosity about why a circuit designer would use a thermal fuse instead of a current fuse when it's the current that's doing the heating anyhow.

    As far as the thermistor, I know very little about them beyond their Wikipedia article, which says they are often used as over-current devices in circuits. Just curious, again. I'm not planning on replacing the fuse w one.
     
  5. #12

    Expert

    Nov 30, 2010
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    The thermistors I use for thermometers and thermostats wouldn't have any useful role in a Kill-a-Watt, but there are some that change their resistance a lot in a small difference of temperature. I think, "Poly-fuse" is one of them. Still, I'm having some difficulty with imagining how to make one useful in this case without throwing the measurements off.

    Maybe that's the answer. Adding any significant resistance in the power line would alter the readings too much. Is this fuse in the main power path for the thing being measured?

    Here's a page of thermal fuses. You might buy them at Mouser or several other places.

    http://www.mouser.com/Search/Refine.aspx?N=254591
     
    Last edited: Jun 4, 2014
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I normally see the thermal fuse as a (cheap) safety back-up for a device that could pass much more current than it could handle safely in a short period of time causing a component to heat very quickly and become a fire hazard. You might have normal fusing that protects the overall circuit from a overload and a thermal fuse embedded inside a small transformer that could get very hot quickly if shorted but not blow the main fuse.
     
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  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    It is a matter of liability. One component relieves the manufacturer of two issues; excessive current and excessive temperature.
    Did the thermal fuse look like a cylinder with cone shaped ceramic at one end? If it did don't worry about soldering in the replacement. It will survive.
    It is your call if you want to replace it with a simple fuse. You will not have thermal protection.
     
  8. #12

    Expert

    Nov 30, 2010
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    nsaspook brings a good point to the conversation. Some parts can overheat locally without drawing enough current to pop the main fuse. Is there any evidence in how things are arranged in the Kill-a-Watt to suggest this is their motive?
     
  9. Brevor

    Active Member

    Apr 9, 2011
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    I once worked for a company where we had to add a thermal fuse to the power transformer in our products to obtain UL certification.
     
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  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the most common use for thermal fuse is in coffee pots and heaters. they protect against overheating, not over current. an alloy link melts open at a specific temprature, and are not normally resettable. you dont usually find one just hanging out in opoen air, but fastened to something, heat sink, inside transformer, heating element, or whatever that someone wants to protect. if you hold the lead your soldering with needle nose pliers between the soldering and the fuse, they should be ok, if you dont, sometimes they open up and are bad.
     
  11. ryancousins

    Thread Starter New Member

    Jul 15, 2012
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    Thanks for the info, everyone.

    I found the replacement (NTE8098) for $1.84 w/free shipping so that's not an issue!

    It's been sitting around for awhile and I don't remember the exact layout or what they were after. I'll have to study that when I take it apart again to put the new fuse in. There is a current fuse, also. Maybe I'll take a pic of the innards and post it here.
     
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