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Inrush current and BFC's

Discussion in 'Electronic Design' started by John Devereux, Oct 23, 2007.

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  1. Hi,

    I am putting together a high current supply and wondering about inrush

    The situation is a center-tapped 110V 3kVA transformer, feeding a
    bridge rectifier and a large, high-ripple current capacitance:

    .. -------- +70V
    .. ----| |-------o------> 20A rms >---
    ..|| / | AC + | | |
    ..|| / 55 | | ----- 20,000uF [LOAD]
    ..|| / | | ----- |
    ..|| /-- | | | |
    ..|| / | | | |
    ..|| / 55 | | | |
    ..|| / | AC - | | |
    ..|| /----| |-------o------< 20A rms <---
    .. -------- -70V

    Simulation shows hundreds of amps for the first mains cycle!

    Do I need to do anything to stop the local substation tripping out
    when I plug it in?

    Or will the transformer limit the current to a sane value?

    I have looked at thermistor based "inrush current limiters" but they
    don't seem to protect against short disconnections.
  2. Paul Mathews

    Paul Mathews Guest

    Energy storage in 20K uF at 140 V is 196 Joules (i.e., watt-sec),
    which means that charging the cap is probably not going to blow a
    thermal breaker rated at 30 amps or more (which is what you'll need
    for your 2.8 kW load). You can't count on leakage inductance to limit
    the current much, either, because the initial core state can include a
    flux offset at the moment that you energize the circuit.
    A common way to limit this type of inrush is to add a series power
    resistor and a time delay relay. The relay contacts short out the
    resistor after a delay. The relay can be electromechanical or SSR
    Paul Mathews
  3. [...]

    The good solution is a resistor + triac. The resistor limits the inrush
    current to a reasonable value. After that the resistor gets shorted by a

    The substation will survive; the diode bridge and the power switch will
    burn out eventually.
    I wouldn't rely on that.
    True. The thermistor limiter is a simple solution for the low power.

  4. john jardine

    john jardine Guest

    A much bigger problem is the switch on transients of the big transformer
    (occasional core saturation). Spice does not model this. Two switch on's out
    of five maybe OK, the other 3 can (try to) draw many thousands of amps .
    If it's a one off design, I'd be inclined to fit a small, current limit
    series resistor in one of the supply leads, which is shorted out by a
    relay, 1/4 second delayed (say) after power up. You then kill 2 birds with
    one stone.
    For production, a softly ramped up triac in place of the resistor, would be
  5. Phil Allison

    Phil Allison Guest

    "John Devereux"

    ** See:

    ....... Phil
  6. Jamie

    Jamie Guest

    might be a good idea to use a time delay on set of contacts that would
    bride a power resistor for a soft start.
  7. Yes (bridge) a power resistor. You should also turn the load during
    this process to avoid the potential of welding the contacts. I guess
    you could use a SPDT relay if there is no electronic way to turn the
    load off.

    Best regards,
    Spehro Pefhany
  8. John Larkin

    John Larkin Guest

    I've used the disc-type thermistor inrush limiters at the kilowatt
    level, two units, one in series with each of the two primary windings
    of a 120/240 power supply. They seemed to work fine, even when I
    teased the power switch various ways; don't quite know why.

  9. That's what they do for power factor correction caps.
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