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Inrush current smps.

Discussion in 'Power Electronics' started by sureshot, Feb 17, 2018.

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  1. sureshot


    Jul 7, 2012
    Hello to the community.
    Just looking for some, what might be simple information.

    I have an 18 Amp 12 Volt smps, I'm not 100% certain, but have an idea of what's going on.
    On adding load (halogen lamps) the psu can handle a single 50 watt 12 Volt lamp. But if i add a second 50 watt lamp in parallel it shuts the supply down.

    I was thinking its seeing the combined load as a short circuit, by the cold resistance of the lamps. I'm not completely sure that's the problem, but suspect it might be. I was wandering if it would behave the same with an inductive load. Any help appreciated.
  2. AnalogKid


    Jun 10, 2015
    Inrush current is the startup transient current into the input. What you are describing is load transient current. If the overcurrent protection circuit in the supply is too sensitive, it is entirely possible that your thinking is correct.

    Did you add the second bulb after the first bulb already was on? If not, try that to confirm.

  3. sureshot


    Jul 7, 2012
    Thank you for the help AK. Yes I've tried adding a second lamp whilst the first lamp is already on. That trips the psu off. I've also tried two lamps from cold and connected, it won't power on from a loaded state with two lamps either.

    Testing the lamps resistance is approximately 1R0 each. So i expect cold start is just to much for the power supply. Thank you for putting me straight on the current load terminology.

    I don't have a big enough inductive load at the moment to try it. Thinking about it, i could try my 12 Volt drill by adding physical load to the chuck. With a meter in series with the drill and power supply.

    I have seen retail bench supply information sheet stating not to use halogen lighting with the unit. That was a Maplin bench psu. I'm wandering if its also for the same reason.
  4. kellys_eye


    Jun 25, 2010
    Is it a 'real' 18A SMPS or a 'Chinese' specc'd 18A SMPS?
  5. sureshot


    Jul 7, 2012
    Yes it's a real smps, it was a surplus to requirements dell power brick, picture below. I thought I'd have a go at converting it for cb radio use. For that it works fine with an 80 channel radio and a 35 watt amplifier. I was just wandering if it could supply a larger amplifier with current. Say 100 watt amplifier, I don't have one at the moment. I've read heavy resistive loads can be an issue for some smps. There's very little data on the web for converting these old smps, I've nodded a few Xbox, atx , and server psu's with really good results.

    Attached Files:

  6. sureshot


    Jul 7, 2012
    Thanks all for the help.
  7. Bluejets


    Oct 5, 2014
    For future reference, one can add a low ohm resistance for a few seconds to bypass the inrush effect.

    Had a similar problem with 15 Line isolation transformers in a hospital installation where the above cured the problem.


    May 20, 2017
    Very common method for reducing inrush current.
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