Soft Start For Amps

Posted on Jun 28, 2012

The delay time for all circuits shown has been revised. The optimum is around 100ms - sufficient for around 5 full cycles at 50Hz, or 6 cycles at 60Hz. It is also quite alright to run the transformer to around 200-500% of full load current at start-up, and the formulae have been revised for 200%. Without the soft-start, inrush current can be so high as to be limited only by wiring resistance - in excess of 50A is not at all uncommon for 240V transformers.

Soft Start For Amps
Click here to download the full size of the above Circuit.

If a 12V supply were to be available in all power amps, this would be very simple, but unfortunately this is rarely the case. Most amps will have DC supplies ranging from +/-25V to about +/-70V, and any attempt to obtain relays for these voltages will be

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