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

    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

  2. JDT

    Well-Known Member

    Feb 12, 2009
    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.