Passive circuit limits inrush current

摘要

You would normally limit the inrush current during start-up of a I rectifier circuit with a capacitor output filter by using the circuit in Figure 1. You insert a high resistance in series with the ac input or the dc output of the rectifier and then short-circuit the resistance with a switch once the filter capacitor is sufficiently charged. In this scheme, you need an additional timer relay or sensing circuit to control the closure or opening of the switch. Moreover, the switch carries the full load current during normal operation. As an alternative, the simple, passive circuit in Figure 2 for inrush-current limiting uses commercially available components and presents advantages in size and cost. The resistance-switch arrangement inserted in series with the filter capacitor, instead of in the main power line, limits inrush current in this circuit. The current rating of the switch can therefore be much lower. A switch, S_1, short-circuits the charging resistor, R_1. This switch represents the contact of a commercially available dc relay. The relay senses the voltage on capacitor C; thus, the switch operates automatically.