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Old 25th November 2005, 05:58 AM   #1
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Default how to calculate in rush current limit circuit

hi all..i would like some help as to calculate in rush current limit circuit values and also how much time (cycles in 50 or 60hz) is required to have this settle..any help would be much appreciated..
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Old 25th November 2005, 06:37 AM   #2
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Measure the DC resistansce of the primary winding. This is the absolute max.

230 VAC:
300 VA gives you 30-50 A, 600 VA = 77 A in my amp. This is valid for toroid transformers.
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Old 25th November 2005, 01:13 PM   #3
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excellent thank you..now how to calculate the time i need for this circuitry to charge....does it have something to do with i = C dv/dt
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Old 25th November 2005, 01:34 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
find the PSUD11 design software. It's free and really helpful.

You can select all the PSU components and simulate a resistive load over any timescale you want to investigate.

It will show peak and average currents and the gradual charging (with big peak currents) of PSU smoothing caps.

The slow start circuit seems to be a special case using some kind of current limiting circuit and may not simulate a solid state slow start accurately, it was designed by a valve (tube) fanatic.

I have seen a reference stating 3 to 5 cycles (60ms to 100mS) may be enough to achieve close to steady state conditions but I generally look at the first 500mS using this software or 1500ms to 2000mS for steady state.
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Old 25th November 2005, 02:31 PM   #5
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First: The soft start is mainly to avoid blowing your mains fuses (in the wall) and also not be forced to have unnecessary big fuses in the amp.

You don't need a softstart for charging the caps, only for "charging" the magnetic field in the transformer.

The time is 0.3-0.6 seconds only. EDIT as Andrew says only a few mains periods is enough.
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Old 26th November 2005, 05:57 AM   #6
Wynand is offline Wynand  South Africa
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Where can I find a circuit like this?
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