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Thermostat spark suppression

  1. Feb 9, 2012 #1

    AlephZero

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    An elderly neighbour has just bought a cheap store-branded electrical convector heater. Basically it's a mains power switch, a mechanical thermostat switch, and a resistive heater element in series. (There isn't a fan, it relies on natural convection).

    The thermostat contacts are producing some impressive looking flashes when it switches, I guess it isn't going to have a very long life. I wonder it it's worth wiring a suppressor across the switch, e.g. this http://www.maplin.co.uk/contact-suppressor-498 which appears to be a 0.1uF X-rated capacitor in series with a 100 ohm resistor.

    A bit of research on the web suggests these work best for fairly low currents (< 2A) whereas the heater has a maximum power of 2 kW, i.e. about 8.5A at 230V (UK mains supply).

    Any advice?
     
  2. jcsd
  3. Feb 9, 2012 #2
    Yes, a varistor would probably work but they lose their effectiveness over time. A non-polarized capacitor might also work. Too large a value and the heating element will draw current even with the thermostat open. Too small a value and it won't do anything. I'd start with a small capacitor and increase the value until it reduced the spark significantly. Maybe a combination of a varistor and capacitor would work the best.
     
  4. Feb 9, 2012 #3

    jim hardy

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    i've used a similar product by Cornell Dubelier to suppress relay noise.
    It worked very well.

    Here's their application guide.
    http://www.cde.com/catalogs/igbtAPPguide.pdf
    and datasheet
    http://www.cde.com/catalogs/Q-QRL.pdf
    looks like at line frequency you can't hurt them.

    you could parallel them to get more arc quenching effect.
    it's a race between contacts getting open and voltage across capacitor getting high enough to jump the gap between the retreating contacts. More capacitance(parallel them) gives the contacts more time to separate further.
    For your high current get the device with lowest internal resistance they offer - i'd use the Cornell 104M06QC22; dont know what choices you have in that line.
    i'd experiment to see what visible effect one had, then two, maybe even four.
    if i calculate right each one will bypass roughly seven milliamps around the switch steady state.



    That's nice of you to help old folks.:approve:
     
    Last edited: Feb 9, 2012
  5. Feb 10, 2012 #4

    vk6kro

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    It was probably cheap, but it is going to cost a lot to run and not be very effective.

    The real solution, I think, is to persuade the owners that a fan heater would be much more effective.

    Then, if they agreed, the sparking could be cited as a fire hazard and an exchange requested.
     
  6. Feb 10, 2012 #5

    sophiecentaur

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    If the sparks are not causing noticeable RF interference then why worry? The appliance will almost certainly have been 'type approved' is the supplier was legit.
    If the arcing bothers you then take the heater back and they should give you another one - no questions asked. If that one arcs too then that's what they're like so don't worry. Life's too short.

    BTW, why should a fan heater be any better value per-se? 1kW tends to be 1kW, however it's delivered. A fan can help in some circs but they do make a din. There are good radiant (halogen) heaters at a reasonable price and they are directive.
     
  7. Feb 10, 2012 #6

    vk6kro

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    Unless you live in a telephone box, a convection heater will accumulate heat straight up and under the ceiling while a fan heater will spread it around the room.

    The difference is surprising. A fan heater will heat up a room in 10 minutes while you hardly know the convection heater is on.
     
  8. Feb 10, 2012 #7

    sophiecentaur

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    Yes, I take your point about the short term effect. However, the majority of rooms are, in fact, heated by convection. (Central Heating 'radiators'). A fan heater has its place but they make a ridiculous amount of noise and the bearings can be a maintenance issue.

    I had to laugh when I ordered a new fan for an old fan heater, many years ago. The girl in the Electricity Board Showroom (that dates it) wrote on the order form "Tangenital Fan Assembly". Freudian slip if ever there was one.
     
  9. Feb 10, 2012 #8

    AlephZero

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    Sure it has CE approval, but it's not my heater! Actually it was bought as a replacement for a fan heater was prbasbly 30 or 40 years old, and died with a loud bang which not only blew the fuse in the mains plug, but also destroyed the inside of the wall socket (so far as I can tell, the fan motor went short circuit). So the old dear who bought the replacement is understandably a bit suspicious of anything electrical that makes sparks.
     
  10. Feb 10, 2012 #9

    sophiecentaur

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    She is being reasonably anxious, of course. If you have seen the arcing and you think it's excessive then the supplier would probably replace it. I have one for frostproofing my conservatory. The thermostat is not at all dramatic so that implies her one is not right. Take it back with a sob story. They should be helpful.
    Be quick if it's Comet, though.
     
  11. Feb 10, 2012 #10

    vk6kro

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    You could be kind to old ladies and go to the store with her. Tell them how dangerous sparks like that could set solvent fumes on fire.
    These places try to bluff old folks, but back off quickly when they are outgunned.
     
  12. Feb 10, 2012 #11

    AlephZero

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    Yep, the paperwork says "Do not use in areas where flammable liquids are stored or where flammable fumes are present. Do not use in rooms with explosive gas or while using inflammable solvents, varnish, or glue".

    Actually they cover pretty much everything: Warning #6 says "The heater is hot when in use". Wow, I'd never have thought of that.
     
  13. Feb 10, 2012 #12

    Ivan Seeking

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    I do EE work as a part of my consulting work. Liability is always my first concern. I'm accustomed to US law, so with that in mind:

    Aleph modifies a CE approved heater. A month later, there's a fire. Guess who's the number one suspect; the guy who made an illegal [unapproved] modification to an approved device. And being an EE, you would carry additional responsibility.
     
    Last edited: Feb 10, 2012
  14. Feb 11, 2012 #13

    NascentOxygen

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    Some can be noisy. I've used a couple that were whisper quiet. :approve: You need to shop around, and try before you buy.

    I've found the fan heater is the best plug-in heater to quickly take the chill off a room. And by circulating the air, it keeps the bathroom glass from "steaming up".

    OP, find out the store, and ask for a demo of the brand. If it has better contacts, then consider returning the original for a replacement. Chinese made, presumably?
     
  15. Feb 11, 2012 #14

    sophiecentaur

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    Absolutely agree.
    Cover your backside even when you want to be helpful.
     
  16. Feb 11, 2012 #15

    jim hardy

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    give them a better heater? random kindness always comes back..
     
  17. Feb 11, 2012 #16

    vk6kro

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    It shouldn't be necessary to give them a heater, although that is a kind thought.

    This lady is not only within her rights to refuse to accept this heater, she is obliged to refuse.

    This is part of her obligation to behave sensibly.
    For example, if she saw that the power cord was damaged, even though it was new, she would have an obligation not to use it. She would be reckless to do so.

    This is the same. The heater is behaving in an alarming manner.
    It is probably generating Ozone, which is a poisonous gas.
    A naked spark is capable of igniting vapour from solvents and other volatile chemicals.

    Nobody can say it is safe as it is and she should refuse to accept it.

    If you could be there to provide moral support, she could end up with a nice quiet fan heater while this piece of junk goes back to wherever it came from.
     
    Last edited: Feb 11, 2012
  18. Feb 11, 2012 #17

    jim hardy

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    sorry if i was out of line.

    vk6 is of course right.

    a decent merchant will exchange or give a refund.
     
  19. Feb 11, 2012 #18
    I use a couple of these. The sparks are harmless unless there's a gas leak. But then you'd have much bigger problems.

    I second Ivan's comment. At work we can't add safety devices to things that are UL approved or they loose their safety certification. This applies even to the products we design and make ourselves. They have to be retested for safety if any changes are made.
     
  20. Feb 11, 2012 #19

    AlephZero

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    Well, the links in your first post were the only thing in the thread that actually answered my question. I wasn't looking for advice on the best way to heat a room, or on health and safety laws and trading standards in other countries.
     
  21. Feb 11, 2012 #20

    jim hardy

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    thanks for the kind words.
    Applying those things has a couple subtleties that those links make clear (at least they weren't intuitively obvious to me)
    and we are here to share our experience.

    This was an interesting observation you posted :
    ""died with a loud bang which not only blew the fuse in the mains plug, but also destroyed the inside of the wall socket (so far as I can tell, the fan motor went short circuit). """ bold mine

    The fireball propagated inside the receptacle box?
    To me that infers the fault was likely very near the receptacle.
    Had it started inside the heater one would think it had to propagate the length of the power cord which should have given the house fuse time to clear, and should have turned the whole length of the cord into a charred string.
    Perhaps the fault started at the plug where the insulation has accumulated forty years worth of flexing and strain?

    Doesn't matter much, just old troubleshooter instincts flaring up here. dont mind me.

    As 'Trustee from the Toolroom' would say, Bon Mechanique !

    thanks again, old jim
     
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