Filament regulation

Discussion in 'General Electronics Chat' started by eureka_87, Jan 3, 2015.

  1. eureka_87

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    Jan 2, 2015
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    I want to regulate filament with 2A in standby state. And when switch is closed 2A should boost upto 4A. How to design a high frequency circuit for varying current between 2A to 4A in filament using bridge rectifier?
     
  2. MikeML

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    Assuming that you have a switch closure to indicate when the lamp is in standby or operating, how about just a series resistor which is shorted by the switch contacts?
     
  3. eureka_87

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    Does series resistors will work better even for filament firing frequency in kHz?
     
  4. eureka_87

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    which switch can used for switching between standby and ready state?
     
  5. MikeML

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    I dont see why not. The goal is to reduce the heating of the filament, meaning less power, meaning less voltage/current.

    Why are you driving a filament (lamp? tube?) with kHz AC ? DC is prefered.

    What kind of device has such a filament?
     
  6. MikeML

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    Relay, FET, Transistor, etc.
     
  7. eureka_87

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    It is for regulating filament in X-ray tube only. The pre-heat filament current is 2A (in stanby state) and it should vary according selected tube current upto 4.2 A (during exposure state). supply voltagen is 15V pulsating DC (about 12.5 kHz)
     
    Last edited: Jan 4, 2015
  8. eureka_87

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    Can IGBT used for switching? Does it will operate better than FET & others?
     
  9. MikeML

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    You should have mentioned X-Ray tube in the first post. I know nothing about them... Sounds like the tube's filament is being PWMed.

    Generally, IGBTs are used for high-voltage circuits. They are usually used at voltage levels where other devices cannot be used. No reason to use them over others as long as the voltage is low...
     
  10. crutschow

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    IGBTs have a higher maximum operating voltage but switch more slowly and have a larger ON resistance than a MOSFET.
     
  11. ian field

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    Actually DC causes metal migration in filaments, the only filaments with kHz current I know of are in vacuum florescent displays - the filament supply is often derived from the inverter that generates the anode HT.

    Many lighting filaments are coiled - or even coiled-coil, I imagine Xl could creep up towards that of the resistance if you make the frequency too high.
     
  12. MikeML

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    So you are saying that a 12AX7 with its filament operated on DC will have a shorter life than if operated on AC? Please supply a credible reference.
     
  13. ian field

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    Its a well documented phenomenon in lamps - valve (tube) filaments are probably too far below the melting point of the filament metal for it to be as much an issue as other potential failure modes.
     
  14. MikeML

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    Then explain why billions of automobile headlights and lamps seem to work just fine on DC.
     
  15. #12

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    I can imagine why a hot filament would emit atoms when negatively charged. After all, it does emit electrons, and the glass on the ends of fluorescent tubes gets, "dirty" on the inside. An AC signal would reverse polarity half the time, but doesn't that just change which end of the filament is emitting? Personally, I use DC in 12AX7 filaments, but maybe that's just because I'm a fool.
     
    Last edited: Jan 4, 2015
  16. MaxHeadRoom

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    Jul 18, 2013
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    Here is something from a couple of sources.


    Osram.
    With low-wattage, high-voltage (230, 240 V) halogen lamps, a drastic reduction in the service life must be expected with DC operation.
    For example, with the 60 W lamp only approximately 15 % of the nominal value is achieved. The reason for this is the increasing material movements in the tungsten wire of the filament.
    Higher-wattage lamps with their larger wire diameter are less critical in DC operation.
    This is similar for the low-voltage lamps (12 V), as here the critical limit is low due to the generally larger wire diameter of these lamps, i.e. below 20 W.
    Max.
     
  17. ian field

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    If you examine the filament of a well used H4 bulb - you can clearly see the granular appearance of the filament which started life smooth and shiny, the metal has been migrating and you will be able to see that it has formed into clumps along the filament.

    Some of this is due to tungsten evaporation, which boils off the filament and is recirculated by the halogen gas instead of blackening the bulb - but it still happens with lower temperature non-halogen bulbs.

    <snip>

    Moderator Comment: YOU make the assertion, you provide the citations. In over 40 years I've never heard of it either, and am dubious.
     
    Last edited by a moderator: Jan 4, 2015
  18. MikeML

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    And all your wishfull thinking won't make this phenomenon important or critical in thousands of applications. You still never answered the question about the 12AX7 or about billions of car lamps.

    If the pheonomenon was so important, wouldn't car lamps all be running on AC?

    ps, I see Max did...;)
     
  19. ian field

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    Some small motorcycles do.

    <snip>
    Mod edit: Ad Hominem comments are not allowed.
     
    Last edited by a moderator: Jan 4, 2015
  20. cmartinez

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    I worked for GE lighting for a couple of years, and I once developed a program used in the design of filaments for halogen lamps... it was a rather obscure, recursive logical method that was laborious and almost entirely empirical, but that was way back then in 1989...
    Anyway, if I remember correctly, halogen lamps have a higher filament life when they're used with AC because tungsten is a paramagnetic material, and the AC pulses traveling through the filament generate an alternating magnetic field. That field keeps the tungsten atoms that were sublimated from its surface in a "cloud" closer to the filament, preventing most of them from reaching the glass, where they'd adhere to, helping them complete a cycle that eventually allows them to be re-deposited along the filament... don't know if I explained myself clearly...
     
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