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I need a 12V 3 amp regulator.....small

Discussion in 'Electronic Design' started by geronimo, Feb 25, 2007.

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  1. geronimo

    geronimo Guest

    I want to use a 12volt soldering iron on aircraft repairs. The
    aircraft voltage is 24 VDC. There is a cigarette-lighter type
    receptacle in cockpit, but of course it is 24 VDC. This is what I need
    to plug the soldering iron into. A typical one is 30 watts, so that
    is about 2.5 amps it will draw. So I need a very small 12V regulator
    in plastic box. I think a 7812 regulator would work, with one series
    pass transistor. Should I rate the transistor for 2x expected current
    ? Are there schematics on-line somewhere...or maybe a ready built
    regulator box available already?
  2. You might as well just use a big heatsinked resistor equal to the R
    of the iron, and don't put it in a plastic box, it will probably melt,
    dissipating that much heat

    A more efficient way would be to use a switch mode psu.

    But if your iron is thermostatically controlled, who knows?

  3. PeterD

    PeterD Guest

    Realize the series (pass) regulator will dissapate the same current as
    the soldering iron, which will be about 30 watts! That's a lot of
    heat, certainly not a good 'plastic box' project. (Especially a
    'small' plastic box project!)

    If you have to go pass transistor series regulator, use a heatsink
    large enough to handle the power.

    BTW, this won't be 'very small'...

    Mount the pass transistor and the regulator on the same heat sink, to
    take advantage of the fact that regulator (should) have an internal
    thermal protection circuit. This doggie will definately be larger than
    a pack of cards, probably about the size of a thick paperback book.

    If you must be small, I'd look at a switching regulator which is
    *much* more efficient, should be small and won't generate a lot of
    heat. Better solution.
  4. Jamie

    Jamie Guest

    The best thing to do is make your self a little Pulse width
    modulated supply.
    a 555 Timer at 50% duty cycle driving a Logic Level power Fet.
    With that, it'll run cool enough to put in a plastic box..

    you can most likely place a pot on the box if you would
    like to trim the duty cycle to adjust the heat level.

    This simply type of supply is fine for a soldering iron how ever,
    i wouldn't use it for sensitive electronics due to the 24 Volt peaks
    you will be getting unless you add some regulating components to monitor
    the output.
  5. ehsjr

    ehsjr Guest

    See page 13 in the datasheet at

    The amperage isn't a worry with a big pass transistor,
    the heat is. You'll need a good heatsink in open air - the
    plastic box isn't going to do it for you, unless you use
    a big box with forced air cooling. This style regulator
    will have to waste ~ 30 watts when the iron uses 30 watts.

    You would be far better off just buying a 24V iron.;...ljGr5XDqQLvpAe?sc=10&category=53&it=A&id=2379
    Watch for the line wrap or go here:
    and click on the "spare soldering iron" arrow on the left.

  6. Just use 24V solder, much easier

  7. TT_Man

    TT_Man Guest

    Goto Maplins/Radio shack and get a 24 -12 converter, min 30/40- watts. This
    type of thing is used for Lorries powering 12V radios and other things.
  8. Ross Herbert

    Ross Herbert Guest

    You could simply use a 24V soldering iron. Weller WTCPN, WTCPS and
    WTCPD soldering pencils are usually 24V and their "Magnastat"
    operation ensures temperature control based on the tip used.

    You could also use a 24V to 12V smps step-down converter such as
    with your current model 12V soldering iron. Most alternative energy
    supply stores will have a range of 24V to 12V adapters to suit most

    One way or another, even if you have the ability to build a suitable
    PSU yourself, it is going to mean a reasonable cost outlay whichever
    way you go.
  9. geronimo

    geronimo Guest

    Yea, Weller has a 24 VDC soldering iron. Thats' the way to go! But
    those three soldering irons are AC powered soldering stations! It may
    be that the actual pencil iron connected to it is 24VDC. Guess I need
    to call Weller, as a Google search hasn't turned up the P/N for this,
    and there isn't one for sale on eBay. thanks, Geronimo
  10. Ross Herbert

    Ross Herbert Guest

    The Weller part is TC201 and will set you back about USD90. You might
    be able to score something on Ebay.

    As far as a heating elemnt is concerned it makes no difference whether
    it is supplied 24Vac or 24Vdc.
  11. Except that they have a temp controlled switch inside which may not like DC.



  12. Ross Herbert

    Ross Herbert Guest

    like DC.

    The Weller TC201 uses a magnetically controlled switch which depends
    on tip temperature only. As soon as the tip reaches the design
    temperature (eg. 700 degF) the switch opens thus allowing the tip to
    cool and the switch to close and the process repeats ad-infinitum.

    Why would DC operation affect the tip temperature or any other
    function? 700F is 700F no matter what heats the tip.
  13. jasen

    jasen Guest

    The probelm wit that plan is the regulator gets as much anergy as the
    soldering iron, an consequentually gets as hot as it too.

    For regulating a resistive load like that Pulse width modulation is ideal

  14. The switch will suffer more arc damage when run on DC. It
    may fail dramatically sooner. This is especially true if
    the open gap is very small, and relies on the current zero
    crossings to extinguish the arc.
  15. Hawker

    Hawker Guest

    As many have pointed out a linear, such as the LM78xx is just going to
    get way to hot.
    What I don't see an answer to for you is a better part to use.
    I would suggest a National Simple Switcher.
    Get the adjustable flavor so you can tweak it from 12V to 14.4V. My
    experience with 12V soldering irons is they work better at a bit higher
    than 12V to force the amperage draw (and hence wattage) up a bit.

    I mostly quite using 12V irons though. I use the Weller Butane soldering
    irons for field work with much better luck. They have two flavors. The
    cheap on is cheap (has the big clear tank). The solid color black one is
    much better IMHO.

  16. Ross Herbert

    Ross Herbert Guest

    I would agree with you if there were some inductive component in the
    DC supply circuit. When the TC201 is used with a mains supply it runs
    directly from a 24Vac secondary so I would imagine this would tend to
    produce some arcing when the switch opens anyway - possibly more than
    with DC.
  17. nospam

    nospam Guest

    If you look at some actual switch ratings from the people who make them and
    know what they are talking about you will find it is not unusual for the dc
    current rating to be 1/40th or even 1/80th of the ac current rating at the
    same voltage. You may also find similar current ratings specified at a dc
    working voltage of 1/10th or 1/20th of the ac value.

  18. Yeah, inductive flyback voltage really exaggerates the
    arching at switch contacts. So the switch may last a
    reasonable time. But it still doubt it lasts as long as it
    does with AC.
  19. Ross Herbert

    Ross Herbert Guest

    Yes, I realise this is a common recommendation with switch contacts.
    In the case of the TC201 switch assembly there is a standard 10nF
    ceramic spark quench cap across the dual in-series contacts which may
    or not be adequate to prevent excessive arcing.

    I realise that the average heater current with dc will be higher than
    with ac but I anticipate that this will simply cause the tip to get to
    operating temperature faster than when on ac. The maximum heater
    current will be determined by the resistance of the heater element
    itself and this should be the same for dc as for ac. Since there is no
    inductive kick-back arcing should not be excessive and pehaps less
    than when on ac.

    I have pulled these switch asemblies apart many times and the dual
    in-series contacts look to me to be able to handle the maximum 1.75A
    (42W) current without any problems. Just to be certain I have
    requested technical specs from Cooper Tools on the Weller TC201 switch
    assembly contact ratings and an opinion as to whether dc operation
    will have any deleterious effects on the contacts. Will post back if
    and when it arrives.
  20. John Fields

    John Fields Guest

    I doubt it. With DC there will be no mains return to zero volts, so
    the only way the arc will be quenched is if the cap across the
    switch can drop the voltage from the 24VDC source quickly enough,
    when the switch opens, to a voltage low enough to no longer be able
    to sustain the arc. Either that or the separation of the contacts
    will eventually be great enough to quench the arc.
    That's not true. The average current will be the same.

    That is, the heating effect of 24 volts, RMS, will be the same as
    that of 24 volts, DC
    No. With DC there will be no mains voltage zero-crossing, so the
    arc will only be quenched by the contacts separating far enough
    apart when opening or, perhaps, by the cap across the switch.

    Regarding the cap, since it's in series with the heater and in
    parallel with the switch, and the heater's resistance is:

    E 24V
    R = --- = ------- ~ 14 ohms,
    I 1.75A

    The time constant of the RC will be:

    T = RC = 14R * 0.01µF = 140ns

    Which means that 140ns after the contacts open the voltage across
    them will have risen to about 16V, so there still might be an arc
    struck if the contacts don't separate far enough apart quickly

    Also, if the contacts were to break on the peak of an AC cycle the
    voltage across the RC would be about 34 volts, producing a hotter
    arc, momentarily, that possible with DC.
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