Inrush current at power on or power resumption after a power failure is the major cause of data loss and errors in Microcontroller based electronic circuits. Even the SMPS based power supply is vulnerable, if the inrush current is too heavy. This circuit gives atime delay of 2 minutes to stabilize the power before the device switches on. It is an ideal circuit to protect the LCD and Plasma TV, since the delicate circuitry inside can be damaged instantly if there is an inrush current at power on.
Circuit works on the principle of charging/discharging the capacitor. When the circuit is switched on, capacitor C1 charges slowly through R1 and VR. When its voltage increases above 4 volts, (this takes around 2 minutes) Zener diode ZD conducts and activate the NPN transistor T1. Relay switch is connected to the collector of T1 so that when T1 conducts, relay energize and LED lights. Power to the Device can be connected through the Common and NO contacts of the relay so that when the relay energize, device gets power supply. When the circuit switches off, C1 discharges through R2.
Diode D1 protects T1 from back e.m.f since there will be high current when the relay switches off. Capacitor C2 helps for the clean switching of the relay and prevents relay clicking.
The circuit and the 9 volt transformer power supply can be enclosed in a plastic switch box. Use a 6 volt PCB relay.
Filed Under: Electronic Projects