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Incandescent lamp inrush current vs. regulated power supplies

Discussion in 'Electronic Design' started by John Nagle, Feb 4, 2013.

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  1. John Nagle

    John Nagle Guest

    I'm trying to run two 12V 12W auto lamps off a Tripp-Lite 12V 3A
    linear regulated power supply. One lamp will work, but two
    of them shut down the power supply, presumably because the
    inrush current is too high. Inrush current for an incandescent
    lamp is about 10x the current after warmup.

    Is this likely to be better or worse with low-end switching power
    supplies, of the type usually used to power laptops and such?
    (Assuming I get power supplies with CSA, UL, or GS safety

    John Nagle
  2. Phil Allison

    Phil Allison Guest

    "John Nagle"

    ** You need a regulated supply that goes into current limit, when
    overloaded, to overcome the problem.

    Fed with 3 amps or so the two 12W lamps will then light up slowly - in about
    2 seconds.

    Fraid there is no way to find out if a given SMPS does this ( or shuts down)
    other than trying it.

    ..... Phil
  3. miso

    miso Guest

    Is this the PR-3/UL? It has some margin (like 5A momentary, with
    undefined time). If inrush is really 10x, you are doing good, since that
    is a 10 amp surge and the device is specd for 5 amp surge.
  4. Phil Allison

    Phil Allison Guest

    "John Nagle"

    ** Errr - why are you NOT using a for of SMPS **DESIGNED** for lamp
    loads ???

    Halogen lamp " electronic transformers " are sold in the millions.

    Dirt cheap too

    Wot is your problem ???????????????

    .... Phil
  5. Ecnerwal

    Ecnerwal Guest

    One approach (which also leads to long life for your bulbs) is to PWM
    them gradually on (the digital equivalent of the inrush current limiting
    devices - I'm not over-fond of those myself without adding something to
    bypass them after startup.)

    We have a cheesy (110V) alarm clock that gradually ramps up an
    incandescent this way - thing must be 15 years old, and the bulb has
    never needed to be changed yet. The ultimate in soft-starts.

    In the specific case of two or more bulbs, you could also try a setup
    that ran them in series for a few seconds at turn-on to warm the
    filaments, and then switched to parallel operation.
  6. Phil Allison

    Phil Allison Guest

    "Ecnerwal" = IDIOT

    ** Absolute 100% fucking BULL SHIT !!

    ..... Phil
  7. Guest

    You can buy LED lamps made for replacing the 12 volt auto lamps. They
    are not too expensive and as a friend of mine says " works good,
    lasts a long time. "

  8. tough one. I've run into this very problem with linear power supplies.

    What's happening is the cold filament looks like something close to a
    short, and then the power suppply drops the output voltage to back off a
    bit. It doesn't matter what voltage it's trying to output, it will never
    heat the filament up enough to increase it's resistance to match the load
    rating of the power supply.

    You might be able to defeat the overcurrent protection which is causing
    this- find the current sense resistor and slap another across it.
  9. Tim Williams

    Tim Williams Guest

    I find they work better. A [continuous] foldback limit is guaranteed to
    get stuck forever, each and every time the load is the same. That's
    boring, and dangerous besides.

    Example: suppose you have a converter which draws 12V, 1A. Maybe it's a
    5V buck converter or something, doesn't matter. Set your benchtop supply
    to 1.5A current limit, bring it down below 8V, and just try to get it back
    up without fiddling with that current limit knob.

    The peak current delivered in "burp" mode is typically 1-2 times the full
    load current, enough to push most loads through.

    Additionally, if the heavy load is due, in part, to charging capacitance,
    a few "burps" will deliver enough charge to start it up. The current
    limited supply can do this, too, but if it doesn't deliver enough to cover
    steady state at reduced voltage, it's still screwed.

    Maybe all the "burp" supplies you've bought just suck.

  10. gregz

    gregz Guest

    You need an adjustable current limit type supply. Cost more than most. A
    typical supply will need, say, 10 times the current capability with a built
    in shut down mode. A non regulated supply can work, as well as a variac
    controlled supply.

  11. Yeah, a known voltage, load, R,C and transistor, I think I can make a
    slow start.

    George H.
  12. Phil Allison

    Phil Allison Guest


    ** Why ??

    ** Why ????

    Long as the 12V supply does not shut itself down at switch on it will work
    fine - providing of course it has at least 2 amps current output available.

    ...... Phil
  13. I did a slow start thing for laser diodes... where you care about the
    The circuit was copied from someone else..
    (Little of what I do 'original'.)

    George H.
  14. ehsjr

    ehsjr Guest

    Phil has it right.

    For a less elegant approach that may work with your existing
    power supply, try this:

    5ohm 10W
    +12 ---+---/\/\/---+---+---[Lamp]---+
    | | | |
    +---o o----+ +---[Lamp]---+
    \ |
    Gnd --------------------------------+

    Turn the switch on to short out the resistor after a couple of
    seconds warm up. If it works to your satisfaction you could
    replace the switch with a delay circuit later on.

  15. Tim Williams

    Tim Williams Guest

    We've had good results with MeanWell. Also one of the few manufacturers
    with a 480V input unit (size 100W and up).

  16. Some switchers do need to fold back.

    Here's why: (Assuming a steady output current) at low output voltages
    the duty cycle will be low. This means the synchronous rectifier fet is
    carrying a much larger RMS current than it normally would at full load.

    Adding foldback to reduce the output current when the output voltage is
    low can save money on thermal management for the lower fet in certain

    Of course, foldback can lead to all sorts of system problems and should
    be used with caution.

  17. Oh that's good to know... We've been using the Phihongs.

    George H.
  18. gregz

    gregz Guest

    As long as it does not shut itself down, would work.

  19. gregz

    gregz Guest

    Do you need DC ?

  20. Guest

    But foldback will further reduce output voltage (not current) putting
    you right in that box.
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