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SoftStart Build Guide

The designer of a powerful amplifier or any system where there exists a large capacitor bank to be charged faces two design issues:

Inrush current - When power is first applied to the system, the capacitor bank is uncharged, empty if you will. The system will draw large amounts of current from the AC line to initially charge the capacitor bank. This current may be several times larger than the normal operating current. This presents a problem for the protection circuit (fuse or circuit breaker). If the fuse is specified for normal operation then the inrush current will pop the fuse. If the fuse is specified for inrush, then abnormal current consumption under normal operation may not pop the fuse.

Switch Selection - In addition to the fuse that has to handle the inrush current, so too does the power switch. Switches that can handle large amounts of power are not necessarily the most visually attractive. Often the switch that is the best aestheticaly, is either not sufficient in terms of current capability or is a momentary switch and not an ON-OFF switch.

The solution to these issues as a soft-start device. With this device, when power is first applied, there is a current limiter in the AC path keeping the inrush current to a minimum. Once the capacitor bank has charged, the limiter is removed and the system functions normally. Since this device controls the power, a low power switch can be used to control the device. Appropriate circuit design can allow ON-OFF switches, momentary switches, even CPU or remote control.

Follow along to discover how you can make one of these soft-start devices.

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Ken Seibert
Redondo Beach, CA