Time delay relay circuit

What is a time delay relay circuit?

In a time delay relay circuit, relay contacts are closed or open after the preset time interval. Whereas in a normal relay contacts are open/closed immediately when coil is energized. It prevents the damaging effect of voltage spikes or input surge current by delaying the relay on/off.

Schematic diagram

Components list

• 12v Relay
• Transistor TIP122
• D1 – IN4007
• D2 – Zener diode 3.3v
• VR1 – 100K
• R1,R2,R3 –  1K
• R4 – 330 Ohm
• C1 – 1000 μF 25 v
• C2 – 100 μF 25 v
• LED – 2 nos

How it works ?

The circuit is based on RC time delay and zener controlled switch. When the power to the circuit is switch on, the 1000 μF capacitor charges through 100 k VR. Once the charge through the capacitor reaches 3.3 v zener diode starts conducting. As the zener is connected to the base of transistor, it triggers the transistor and it is turned on. The relay connected to the transistor is energized. The delay in the relay can be controlled by the VR 100K.