Power-On Time Delay Relay

Here’s a power-on time delay relay circuit that takes advantage of the emitter/base breakdown voltage of an ordinary bi-polar transistor.

The reverse connected emitter/base junction of a 2N3904 transistor is used as an 8 volt zener diode which creates a higher turn-on voltage for the Darlington connected transistor pair. Most any bi-polar transistor may be used, but the zener voltage will vary from about 6 to 9 volts depending on the particular transistor used. Time delay is roughly 7 seconds using a 47K resistor and 100uF capacitor and can be reduced by reducing the R or C values. Longer delays can be obtained with a larger capacitor, the timing resistor probably shouldn’t be increased past 47K. The circuit should work with most any 12 volt DC relay that has a coil resistance of 75 ohms or more. The 10K resistor connected across the supply provides a discharge path for the capacitor when power is turned off and is not needed if the power supply already has a bleeder resistor.

Copyright 2006: Bill Bowden

7 thoughts on “Power-On Time Delay Relay

  1. Paul

    Last transistor should be another NPN, not PNP. Diagram symbol is correct but it’s not a 2N3053. Another 2N3904 will work fine.

    The first 2N3904 can be replaced with a 7-9V zener diode.

  2. Muhammad Shafqat

    i want to make time delay for auto generator off , can any body tell me about the relay, how it work, on for some time and off, or after some time its on

  3. tiago

    i simulate circuit in proteus 8.4 SP0 and don’t works. The circuit works only after replace the first 2N3904 with a 8.2V zener diode!

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