AC Time Delay Circuit

• The basic circuit for the AC Time delay circuit is shown in the figure A.  
• Let us consider that the switch is in the reset position. 
• The charging of the capacitor is done through N – (+) C1 – C1 (–) – D2 – R2 – P path during negative half cycle. 
• The upper plate of the capacitor becomes negative whereas the lower plate becomes positive. 
• The SCR does not conduct due to reverse biased of diode D1. 
• The capacitor starts to charge in the reverse direction with upper plate negative and lower plate positive during positive half cycle of alternating supply and switch is in the time position. 
• The charging of capacitor is done through P – R3 – D3 – (+) C1 – C1 (–) – N path. 
• The SCR is turned on due to forward biased of diode D1. 
• The relay coil gets supply due to turn on of the SCR. 
• The time of relay operation depends upon resistance R3. When the resistance R3 is kept higher, the capacitor takes more time to charge resulting firing angle of SCR increases. 
• The relay coil takes more time for its energization.  
• Similarly when the resistance R3 is kept lower position, the relay coil energize is in short time.

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