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Flame detection

Discussion in 'Electronic Design' started by Peter Amey, Sep 7, 2006.

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  1. Peter Amey

    Peter Amey Guest

    Electronics isn't really my field so this is an appeal for help for
    something which is probably embarrassingly easy if you know what you are

    I want to detect the presence or absence of a small gas flame (caravan
    fridge) and light an led if the flame is there and turn it off if the
    flame isn't there. A reasonably quick response time is needed.

    Presumably a thermocouple (I think I have read the term "K type"
    somewhere), a transistor an led and a resistor or two will be involved!
    It would be nice if I could run it off 12v DC and it would be nice if
    the current drain could be zero or at least tiny if the led is off.

    Any thoughts welcome.


  2. Detecting the light of the flame with a phototransistor would be more
    reliable and permit a simpler circuit. Thermocouples give very little
    output (40 uV per kelvin), so you need at least two transistors, and
    erode quickly in the flame unless you use the specific type for
    flame-failure detectors. The sort of circuit you need doesn't have zero
    current drain in the 'off' mode.

    There are two sorts of phototransistor, with very different
    sensitivities. You need the more sensitive type. I doubt if it will
    light the LED itself, since you need to put it some distance from the
    flame so as not to get too hot, which means you need another transistor
    and at least two resistors. 12 V DC is ideal, and the 'off' current
    drain would be very low.

    I don't have time to draw you a schematic, but I expect others here have
    them already drawn.
  3. Luhan

    Luhan Guest

    These units usually have this function built in as part of the
    temperature control circuit and ignitor. Is your unit broken or just
    made without this feature?

  4. Peter Amey

    Peter Amey Guest

    Made without - flame detection on the fridge consists of small window
    through which you can see if the flame is alight. The ignitor is manual
    push piezo unit. This arrangement has 3 disadvantages: 1. you have to
    grovel on the floor to light the fridge; 2. you have to have the door
    open during the operation (losing all that hard-won coldness) and 3. You
    often have to partly unload the fridge to see the little window.

    I just fancied a little light to replace all of the above!

  5. Donald

    Donald Guest

    Your goal is understood.

    There have been many people before you who have had to solve this problem.

    Now getting this done with safety and cost in mind, is what others are
    trying to offer. Also understanding what you really have.
    ( I for one have never heard of a gas fridge !! )

    So, people here are not trying to stop you from building this project.
    We all would hate to hear that your fridge caught fire because of some
    idea presented was done just wrong !

    Finding a commercial product seems your best way to go.

    Good Luck

  6. Dave

    Dave Guest

    Ugh - you must have flame failure detect, or you could end up dead !! There
    is usually a button you must hold in until it is warm enough - flame failure
    turns the gas off.

    Otherwise, it sounds like an application for a piece of fibre optic cable

  7. It's a very old technique - evaporation cooling. The working fluid is
    usually ammonia. Whereas modern coolants evaporate at room temperature
    without assistance, ammonia doesn't do so well, and need a very small
    amount of hat input, The flame is hardly bigger than a match flame.
    It would be quite difficult for even for some of the clowns here to set
    an evaporator fridge on fire.

    My optical solution has the advantage that nothing is placed in the
    flame and everything that is added is added at a safe distance.
  8. Peter Amey

    Peter Amey Guest

    Yes, of course, there is some sort of capilliary tube set up that shuts
    off the gas if the flame blows out but I can't see any way of getting an
    "output" from that.
    Now that might just be a brilliant idea!
  9. That is actually a thermocouple. The 'tube' is miniature MICS cable.
    DON'T even TRY to get an output from it. That WOULD be dangerous, like
    BANG!, or wake up dead.
  10. Donald

    Donald Guest

    I would like to see the flame involved. Just to satisfy my lack of
    Would you be looking for IR or a blue-white gas light ??

  11. Rick

    Rick Guest

    Do a search on absorption refrigeration...
  12. Neither; these things use a 'no air' smoky yellow flame. Only a very
    small amount of heat is involved. It's easy to detect the visible light.
  13. Is the flame to be manually lit (like a pilot light) or controlled by
    demand automatically?

    Gas appliances have various safety mechanisms which cut off the pilot
    gas supply in the event that it is blown out. Many of these use a sealed
    tube in which the gas (air?) expands when heated and operates a piston
    (or bellows) to hold the gas supply on as long as the flame is burning.
    This needs no power source and has a reasonably fast response time (15
    seconds to a minute).

    You could arrange to have the piston pop up an indicator flag for a
    visual signal.

    If you want a more rapid indication of the presence of a flame, you
    could aim a fiber optic tube at the flame (make the 'hot end' out of
    Pyrex?) and place the output where its convenient to observe.
  14. Traditionally a UV sensitive detector - perhaps a transistor - would be
    used. However for this an LDR and LED might work fine.
  15. Maybe a length of 1/4" clear plastic rod bent to shape?
  16. Flame failure devices usually work on the basis of detecting the flame by
    passing a current through its ionised gases. Some designs in gas
    boilers and camping fridges use a common electrode for spark ignition and
    flame detection. Depending on how the spark electrode in your own fridge
    is designed you might be able to convert it to electronic ignition using
    and off-the-shelf ignition unit intended for a more modern fridge fitted
    in place of the piezo igniter. You can buy these (not cheaply) from
    caravan/camping/boating supply shops (probably cheaper online).

    They incorporate flame detection and spark generation ccts and keep
    sparking the electrode until they detect a flame. They have the advantage
    that if the flame is blown out they'll try to reignite it straight away,
    and if they succeed in doing so before the flame failure device (operated
    by the thermocouple) cuts off the gas save you a possibly lenghthy period
    of fridge warming up & food going yucky while you're out enjoying the

    To test if your fridge's spark electrode might lend itself to
    operating as a flame sensor you could probably try sticking an ohmmeter
    onto it while the flame's alight, to see if it's 'seeing' any ionised
    gases from the flame.
  17. Joerg

    Joerg Guest

    Hello Peter,

    Some thoughts: Optical recognition has been done with mixed results on
    other gear, for example pellet stoves. After some time the optical
    surface closest to the flame can soot over and give a false alarm. IOW
    signal a flame loss when in reality the flame is there.

    Another option that is often more reliable is to sense the heat of a
    shroud around that area. Stoves often use "thermo disks", probably
    containing a simple bi-metal type switch. If the shroud doesn't get hot
    enough over ambient for a clean detection there are thermo switches with
    a "feeler" that is bent to be near the flame. Similar to pilot light
    sensing. Actually a pilot light sensor might be the ticket here because
    they can be bought off-the-shelf as spare parts.
  18. Barry Lennox

    Barry Lennox Guest

    I know the problem, I toured much of Europe 30 years ago with a hired
    campervan. The bloody fridge had the "flame viewing" window about 1
    inch above the floor with a viewing angle of perhaps 15 degrees. The
    only way to view it was with a mirror and the lights out.

    There's a number of systems in place, I had a heatpump with gas
    booster that used a TC sensor. It never failed in the 5 years I was
    living there. Or they use a gas-filled capillary tube that shuts off
    the gas with no flame.

    More modern systems use an optical system, as other have suggested,
    and it's possibly the easiest to home-build. The best detectors are
    possibly the Hamamtsu ones. See:

    Barry Lennox
  19. JoeBloe

    JoeBloe Guest

    Even better, a resistor bolometer. A transistor sized package that
    actually detects the temperature. Any simple IR activated light
    switch will do usually as they are not typically clamped to too tight
    an IR band.
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