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Current limiter or current booster

Discussion in 'Electronic Basics' started by Claudio Bonavolta, Jan 26, 2004.

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  1. Dear All,

    I want to feed the lamp (halogen 12V/75W) of a photographic enlarger
    with stabilized DC (voltage variations give exposure/color
    I bought for this a 12V/100W switching supply.
    The problem is the cold filament has a much lower resistance than when
    hot, this forces the supply to furnish a much higher current than it
    is intended and the protection switches the output off and on,

    I may, of course, replace the supply by a much oversized model, but
    I'd prefer to find a better solution ...

    I've tried to put electrolytic capacitors up to 100'000uF but it is
    still not sufficient. I may try super capacitors in serial/parallel
    configuration but they are not really cheap.

    My second option is to put a battery after a diode (to avoid the
    discharge through the supply) but I'm not sure it is very good for
    battery's life to be connected in such a way.

    Another option was to insert serially a choke but their DC resistance
    is to high (resulting in heat and voltage drop) and for such currents,
    they are pretty heavy and expensive.

    Is there a current limiter I can set to, say 8A, with a very low
    resistance when not limiting current ?

    Any other idea ?

    Claudio Bonavolta
  2. Mebbe you should try a diode or two in series with the lamp, put switch
    across them so you could short them out when the lamp is heated up. They
    would drop 1.4volts which may well be enough for a soft start. Or maybe a
    resistor ??
  3. Bill Vajk

    Bill Vajk Guest

    I would do the startup with an inexpensive unregulated power supply
    and switch over to the regulated one after 1 second or so. Use a
    mechanical shutter to prevent exposure during the warm up phase.
  4. Ian Bell

    Ian Bell Guest

    I think a soft start is what you need. very many years ago I worked for a
    recording mixer manufactuerer. There were so many capacitors across the
    supply in the circuits in it that the off the shelf Coutant power supply
    went straight into current limit as they charged up when it was first
    turned on. We had to get specially modified power supplies with a soft
    start circuit to overcome it.

  5. Thanks for your comments,

    I continued to search the web and found two other solutions:
    - there are thermistors intended for this purpose, they limit the
    inrush current to a predefined value.
    An example: Sheets/SL12 1R308.pdf

    - the switch I use between the supply and the lamp is a MosFet driven
    by a TTL pulse, I didn't realise that I could simply add a RC network
    to increase slowly the gate voltage and thus the current flowing
    through it. The time constant should be tuned according to the
    supply/lamp characteristics.
    I found this article:

    Another interesting article on inrush current problems:

    Again, thanks to all and best regards,
    Claudio Bonavolta
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