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Timer provides power-off function

2015-03-17 07:08  
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The circuit in Figure 1 is a simple, timed automatic power-off circuit for 12V-battery-powered systems. It's suitable for portable designs that consume current as high as 1A. You can configure the circuit to automatically shut off power from seconds to several minutes after the system becomes idle. When you set the power switch to the on position, capacitor C1 starts to charge through resistor R3. The voltage drop across R3 momentarily turns on Q2, which energizes the Set coil in relay K1. The relay contacts close and switch on the power to the rest of the circuit. The relay contacts remain closed even after the Set coil is de-energized because K1 is a latching relay that needs only momentary energizing to change states. The circuit has good power efficiency; the relay contacts have low resistance compared with that of solid-state relays. The only input required from external circuitry is an active-low logic signal, DEVICE_IDLE, that indicates that the system is idle. Whenever this logic signal goes high, it resets the 74HC4060 counter. When DEVICE_IDLE goes low, the 74HC4060 starts the count from zero. If DEVICE_IDLE remains low continuously for the entire time-out period, the most significant bit of the counter connected to the gate of Q1 goes high. Q1 turns on, energizes K1's Reset coil, opens K1's contacts, and switches off the power. To switch the power back on, you must first switch the power switch to its off position for approximately 0.5 sec to reset the R3C1 charging circuit. If you turn off the power switch before the time-out, K1's contacts remain closed and the switching comes only from the power-switch contacts. This feature extends the life of the relay contacts because no relay switching occurs unless the power-off circuit times out. The timing is a function of the selection of R1 and C4 and the output stage of the 74HC4060 counter connected to the gate of Q1. Recommended values for R2 are 10R1>R2>2R1. If the oscillator period, TOSC, is 2.5 msec>TOSC>2.5 µsec, the value of TOSC is 3R1 C4. The total time-out period, TOFF, is 2(n- 1)TOSC, where n is the number of the output stage (4 to 14). These formulas may vary for other oscillator periods; you may have to determine the relationships empirically. For example, the values shown in Figure 1 provide a time-out period of approximately 5.5 minutes. You can easily adapt the circuit for other voltages by selecting the appropriate relay type (5, 6, 9, or 24V). For Q1 and Q2, you can substitute any MOSFET type that has a low on-voltage and provides approximately 30 mA for the relay coil. A CD4060 counter can replace the 74HC4060, thereby eliminating the need for the 7805 5V regulator in designs that don't require 5V logic levels. (DI #2026)  74HC4060 You can set an interval from seconds to several minutes for automatic power-off switching in idle battery-powered systems.