Pulse Width Modulating a halogen bulb?

Discussion in 'The Projects Forum' started by urb-nurd, Oct 30, 2014.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Hey guys
    I know that i need to maintain a glass envelope temperature for a successful halogen cycle however i haven't come to any definitives on the use of PWM for halogens and how this can be implemented for the application of a heating element.
    I am looking to use closed loop control to maintain a thermal set point.
    For nichrome wire or a ceramic heater this is as simple as just using PWM to give a variable average voltage for varying power dissipation, however the halogen has constraints imposed on it by its physical composition and i am not too sure on how i can implement PWM control of the halogen for eventual PID control of a set point without reducing the lifespan significantly by fouling the halogen cycle efficacy.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If the PWM frequency is >> than the thermal time constant or a mechanical resonance of the filament, why would controlling the average power into the lamp be any different with DC vs PWM?
     
  3. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    As the life of a halogen is reduced if ran at less than 80% of its recommended voltage.
    I don't want to assume that my PID algorithm and PWM Code from the Pic will successfully control the set point without reducing the lifespan of the bulb.

    If i am using PWM and the temperature falls just below set point, the PID control will exhibit a small influence on the element to maintain the set point (which can be interpreted as an average voltage over a time period).

    If this average voltage is far below the specified operating voltage then the lifespan of the bulb will be reduced significantly (or so i am lead to believe from my research preceding this post) due to the halogen cycle stipulating that each bulb is designed for a particular range of conditions.

    Is there such a thing as bulbs that are designed to work successfully over a range of voltages without drastically reducing lifespan?
    If so, i guess i could just implement PWM as usual but impose lower limits on the duty cycle in order to ensure the voltage seen at the halogen is above its recommended operating ranges.
     
  4. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    http://www.velleman.co.uk/manuals/k8028.pdf
    The link above contains information relating to a commercially available halogen dimmer, so i guess it can be done.
    I just hope to be more certain with how i can do it properly, if anyone has an information or sources of information that may enlighten me - i would thoroughly appreciate it.
     
  5. BR-549

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    Sep 22, 2013
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    Will you at any time be applying less than 80% of recommended voltage?
     
  6. MikeML

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    Oct 2, 2009
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    Remember that 80% of the voltage is 64% of the power, and power is what matters. Dunno about resistance change of the filament as it runs cooler?
     
  7. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Given i am trying to use PID control i can only assume the voltage applied to the halogen will sometimes be lower than 50% to maintain a set point after a small deviation.
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    Ok, I didn't know a PID could control PWM by varying the amplitude of the PWM. My mistake. I was under the impression that PWM always gives 100% voltage, and just varys the time of the give. But it's been a while since I played with it.....and they probably change that too.

    The pdf clearly states that you can not drive a transformer with this board. But then it shows a pic of a connected transformer?
    I would want to talk to someone from that company before I bought that board.

    I read up on that halogen cycle and some on halogen bulbs, but I missed your concern. I didn't come across any 80% voltage figure. But again....I am no expert on these bulbs.

    Also......if your purpose is to achieve maximum lifetime(for warranty)......your must follow bulb specs. If you vary bulb specs in any way.....you lose that lifetime spec. Life time spec is a legal term, not a physical one.

    No matter what you decide.......you will have to try it. And you will have to adjust how bad the effect will be.
    In any setup use choose........it will take you several tries.....to get the maximum physical lifetime of a bulb. You might find the settings different from the bulbs specs. This could void warranty, but increase lifetime.
    The bulbs will have a tolerance which should be close, but you never know.......some bulbs might work better at different settings.

    And...if your goal is lifetime and not dimness.......you might consider increasing bulb V to 150% or even higher. This will give you a much better range of power transfer(increase average power). Keep an eye on your duty cycle.

    I hope I didn't confuse you. Good luck and keep us informed.
     
  9. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    PWM technically speaking will deliver full voltage, however as the frequency of PWM is usually high - then applying this full voltage periodically over a second gives an average voltage for that second.
    So if the duty cycle is 50%, in one second for 500ms the voltage is at its full value, then for the remaining 500ms it drops to zero.
    This method is used to dim LED's by emulating a reduced voltage at the terminals by applying full voltage for a shorted time.
    So my fear is that if i PWM, then this rapid switching will emulate a reduced voltage at the terminals of the halogen (enabling dimming) but also reducing the lifespan.

    I could always just wing it, i guess i could change the PID and PWM code to limit the duty cycle available.
    So the PID control can only force a PWM duty of say 50% or above
     
    Last edited: Oct 31, 2014
  10. MikeML

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    Why not just add a dc current to the output of the PWM where the dc component meets the "stay lit" requirement and the PWM modulates the current between 80% power and 100% power?
     
  11. urb-nurd

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    Jul 9, 2014
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    This...... this could work.
     
  12. GopherT

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    Nov 23, 2012
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    The halogen in the bulb causes the vaporized tungsten to fall back on the filament as it cools (vs. the glass). This means the bulb can be run much hotter than a standard tungsten / vacuum bulb. The problem is that the filament and glass need to be hot for the vapor to preferentially fall on the filament vs. the glass. Now, this is only a problem if the bulb is never about 80% (or infrequently operated about 80%). It is possible to re-vaporize the tungsten and have it fall back on the filament several additional long-cycles. The tungsten bulb is a classic chemical solution to an engineering problem. Tunsten halides (WBr6) makes a reversible reaction with tungsten metal at high temperatures.
     
  13. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    So one method of lengthening the lifespan of the bulb is to periodically condition the filament by running full power in order to redeposit the tungsten gathered on the envelope?
    This would be easy enough to code into my mcu and would simply involve forcing full duty cycle for a brief period before shut-off.
    Thanks!
     
  14. RichardO

    Well-Known Member

    May 4, 2013
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    Sounds like a good idea. Just remember that the lamp will have to run at full power long enough for the envelope to get to full temperature. (Assuming that I understand what others have said).
     
  15. BR-549

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    Sep 22, 2013
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    If the blackening doesn't reduce in several hours....tap the bulb once........and give it a few more hours.
     
  16. Alec_t

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    Sep 17, 2013
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    Does this mean halogen hobs have a short life if run at 'simmer' settings rather than maximum?
     
  17. BR-549

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    Sep 22, 2013
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    I thought air leakage was the determining factor in the lifetime of a regular incandescent bulb. And I thought the darkening was from oxidation, due to the air leakage.

    After reading all this stuff....there'r saying it's filament separation due to particle depletion at hot spots.

    And the halogen bulb has a little(temp dependent) side action to keep the glass clear. Halogen bulbs turn about 10% of the power into light.
    Isn't this like working with tube equipment?

    It's a lot of bother for so little efficiency.
     
  18. Sensacell

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    Jun 19, 2012
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    If you run the halogen significantly below the rated power level, the rate of filament evaporation will be commensurately lower.
    The most problematic scenario is probably running the halogen at 80% power, not hot enough for the halogen cycle, but hot enough for filament evaporation.
     
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I worked in the lighbulb manufacturing industry a while ago, and have a little experience regarding halogen bulbs.
    Their lifespan can be affected by many factors, even the filament's orientation (horizontal or vertical) has an effect on them (horizontal is better). They're designed to run hotter so as to complete the tungsten redeposit cycle on the filament in the most efficient manner.
    However, if you were to run them below certain voltage (or low RMS voltages) and somewhat dimmed, then their lifespan would not just be unaffected, but they might also last a bit longer, since almost no tungsten would be sublimating from the filament. Remember that tungsten redeposit takes place randomly along the filament, and not in an evenly smooth and well ordered way. In the long run some sections of the filament would grow thicker and some thinner, and in the end it will invariably fail.
    It is temperatures in between that affect them the most, since they could allow tungsten sublimation but not redeposit. You'd be surprised, but there's more guesswork going into a filament's design than most engineers would be willing to admit.

    Here's an incandescent light bulb dimmer (that could also be used to control any resistive load) that you might find interesting. I've actually built and tested this circuit myself, adding a phase control SCR in series to it that allows you to not only perform pwm on the bulb, but also lets you control the maximum voltage that is being delivered. This mode of operation lets you power the filament in a way that allows you to watch how it glows a very dim cigarette-like red in the middle, but it's completely opaque (colder) at its extremes, right where it's crimped to the electrodes.
     
    Last edited: Nov 1, 2014
  20. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Thanks for that insight!
    My intended method is still up in the air but progress is being made!
     
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