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Delay Relay Circuit: 12V, 15s
Description
The two circuits below depict the process of opening a relay contact shortly after the ignition or light switch has been deactivated. A capacitor accumulates charge, and the relay remains closed when the voltage at the diode's anode rises to +12 volts. The circuit on the left utilizes a common collector configuration, also known as an emitter follower, and presents the benefit of a reduced component count because a resistor is not required in series with the transistor's base. Nevertheless, the voltage across the relay coil will be two diode drops less than the supply voltage, approximately 11 volts for a 12.5 volt input. The emitter follower (circuit on the left) is favored due to its simpler design. The right-hand circuit employs an emitter configuration, which provides the complete supply voltage across the load for most of the delay period, minimizing concerns about relay pull-in and drop-out voltages, yet necessitates an additional resistor in series with the transistor’s base. The emitter configuration (circuit on the right) is considered superior as the series base resistor can be chosen to achieve the desired delay time while the capacitor selection for the common collector (or a parallel resistor used with the capacitor) remains fixed. The delay duration for the emitter configuration will be around 3 time constants, calculated as 3*R*C. The capacitor and resistor values can be determined from the relay coil current and transistor gain. For instance, a 120-ohm relay coil draws 100 mA at 12 volts, and assuming a transistor 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, equaling 12 - 1.4 = 10.6 volts. The resistor value is then calculated as voltage/current = 10.6/0.003 = 3533 ohms, or approximately 3.6K ohms. The capacitor value for a 15-second delay is 15/(3*R) = 1327 uF. A standard 1000 uF capacitor can be utilized, and the resistor value should be adjusted proportionally to achieve a 15-second delay.
Circuit diagram
