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.
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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