DC Time Delay Circuit

• The basic circuit for the DC time delay relay is shown in the figure A.
•  The DC supply V_CC depends upon rating of SCR. 
• The zener diode provides constant voltage for UJT. 
• The voltage across load becomes zero when the SCR is in off condition.  
  
• When the supply voltage is given to the circuit, the charging of capacitor is done through Z – R1 – R2 – C – Z path.  
• The UJT is switched on when voltage across capacitor is equal to UJT peak point voltage. 
• As soon as the UJT switched on, the discharging of capacitor is done through (+) C – UJT – R4 – C (–). 
• The SCR is turned on because gate terminal receives gate pulse through voltage across resistance R4. 
• The voltage across load is equal to supply voltage minus voltage across SCR. 
• The time delay of the circuit depends upon time constant ( R1 + R2 ) C and its value can be adjusted by resistance R2. 
• The resistance R3 works as temperature compensation in the circuit and the temperature co – efficient of the circuit can be set by selecting proper value of resistance R3. 

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