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Low loss/dissipation xistor or ...

Discussion in 'Electronic Basics' started by Bob Engelhardt, Dec 20, 2008.

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  1. The project:
    I am making electrically heated socks & gloves. For power I'm using the
    9.6v NiCad batteries from my cordless drill. The working assumption is
    that each sock & glove will need 1w, but that's very tentative & I'll
    have a regulator to adjust as needed. The critical parameter is the
    9w-hr available from a battery before needing to be recharged. So the
    regulator must have minimum losses. The 2 socks & the 2 gloves will be
    in series & independently regulated.

    The current plan is to use a 555 timer for each pair & vary the duty
    cycle to achieve regulation. But the 555 can't drive the heaters
    directly - they'll require about an amp. I'm thinking of using SMT
    BJT's from ON Semi:
    http://www.onsemi.com/PowerSolutions/parametrics.do?id=808
    because the very low VCEsat (as low as 50mv) will minimize the regulator
    losses.

    The questions:
    - is there another device that I could use instead of BJT's? Switching
    10v & carrying an amp at low loss (1/10w +-). (OK, now it's obvious - I
    don't know anything about semiconductors <G>)

    - are there through-hole BJT's with very low VCEsat? (I've not used
    SMT's & I suspect that they'll be a fabrication challenge.) Googling on
    "low VCEsat BJT" was not helpful.

    Thanks,
    Bob

    BTW - I hang around rec.crafts.metalworking mostly & when I subscribed
    here I was REALLY impressed the the minimal amount of OT posts. No
    political OT that I recall. Keep up the good work.
     
  2. Eeyore

    Eeyore Guest

    Well there are Mosfets but if your Vce is only going to be 50mV, I don't see
    much advantage.

    Graham
     
  3. neon

    neon

    1,325
    0
    Oct 21, 2006
    now 1w is significant drain on any small battery it will not last very long at that rate. anyhow why using a timer to control current? I know you don't know. use a lm317 T0220 REGUALTOR good for an amp and all you need is a pot of 100 ohm to regulate the current essentialy the heat. the idea of design is to make it simple less parts more reliable.
    get an LM317 add a 100 ohms [pot] from out to adjust apins and there you go. the adjust pin is now your power source for. you need current so adjust that screw the voltage is immaterial.
     
  4. Ah, good idea. I'll make it a consideration.
    The heater is not a discrete resistor - it's heating wire from an
    electric blanket. About 2 ohms per foot. Better heat distribution.

    And the 1 watt I'm figuring with is very tentative - I'm sure that it
    will change once it's working & in "trials".
    Actually, I'm not concerned with constant power at all. I'm assuming
    that occasional adjustment will be desirable.

    I did some measurements on my battery with a 1 w load: fully charged
    it's about 11v, declining over a hour (linearly) to about 9v & then
    going to hell very quickly. Knowing that, I was planning on having a
    low battery detector.

    Thanks,
    Bob
     
  5. You're right - good "catch". I wrote about the battery from memory &
    had it wrong. I was just using a 10 ohm load - 1A. And fully charged
    was 10v, not 11 - just to get all the facts righted.
    ....

    I've read that too, but had forgotten it. My plans for a low battery
    detector was motivated by a desire to keep the heater going & my toes
    warm <G>. Thanks for the reminder.

    Bob
     
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