Power circuit of DC time delay relay
The basic circuit for the DC time delay relay is shown in the figure A. The DC supply VCC 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.
Working of DC time delay circuit
Firing of UJT & Switching SCR
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.
Time delay of the circuit
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. The
firing angle control is done by resistor R2.
Disadvantages of DC time delay circuit
In this circuit, commutation circuit of SCR is not given.
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