Re: Inrush current and BFC's


"John Devereux" <jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote in message
news:87abq9bmx0.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxx

Hi,

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

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.



--

John Devereux

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
better


.

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