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Circuits > Power > Power-On Time Delay Relay
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.
Power-On Time Delay Relay
Title: Power-On Time Delay Relay
Source: unknown
Published on: 2007-07-21
Reads: 430
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