Inrush Current and Filter Caps

Discussion in 'The Projects Forum' started by stpx, Mar 11, 2011.

  1. stpx

    Thread Starter New Member

    Dec 31, 2010
    7
    0
    Hello

    This is my first post but I have been reading a lot on allaboutciruits text section, thanks for the great site.

    To protect rectifier bridge/isolation transformer, I first thought of using thermistors alone but if power went out for any reason , an early restart while thermistors still hot won't hold the current rushing to the huge capacitors.

    Would love to hear a second opinion on the matter. I attached a schematic of a pre-charge circuit I thought of. any other suggestions/criticism would be nice :).

    No exact values but to give an idea of the range
    VDC >100 V
    2 Filter caps 1000uF

    Thanks
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    This is a problem, especially on line-powered equipment like switchmode power supplies and motor inverters where the capacitors and rectifiers are supplied direct from the mains supply (not through a transformer). This can often cause circuit breakers to trip on power-up.

    On good quality equipment this inrush surge current is controlled and the peak value is stated on the data sheet.

    Thermistors are commonly used on lower power equipment. I have also seen equipment that has a series power resistor that is shorted with a transistor or an SCR after a short delay. Or better, when the capacitors have reached a certain voltage.
     
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