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Power-Off Time Delay Relay Circuit

A Communicator Reprise: August 2010
For the original article:

The two circuits below illustrate opening a relay contact a short time after the ignition or light switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts. The common collector or emitter follower has the advantage of one less part, since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input.

The common emitter configuration offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out
voltages less of a concern, but required an extra resistor in series with the transistor base. The common emitter is the better circuit since the series base resistor can be
selected to obtain the desired delay time – I’ve added a variable trim pot for the task to make the delay somewhat adjustable.

The common collector time delay would require changing the capacitor or an additional resistor in parallel with the capacitor to alter the time delay. The time delay for the common emitter will be approximately 3 time constants or 3 x R x C. The capacitor and resistor values can be worked out from the relay coil current and transistor gain. For example, a 120 ohm relay coil will draw 100 mA at 12 volts and assuming the transistor has a gain of 30, the base current will be 100/30 – 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6 volts. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K ohms. The capacitor value for a 15 second delay will be 15/3R = 1327 µf. You can use a standard 1000 µf capacitor and increase the resistor proportionally to get 15 seconds – thus the convenience of a variable trim pot.

This circuit is handy if your APRS tracker is turned off with your ignition switch. Keeps the tracker on long enough to send its last ‘posit’ before shutting down. In hybrid vehicles, the accessory battery does not drop in voltage quickly [or at all] so a tracker that is waiting for a substantial voltage drop to turn itself off or detect a vehicle at rest, will never sense the required voltage drop in the battery... so a power-off timer is put in line
to make sure the tracker has enough time to send a ‘at rest’ posit and then shut down until the vehicle is started.

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