Two previous Design Ideas describe simple ways to automatically disconnect a battery from its load after a preset on period, which extends battery life (references 1 and 2). These circuits have little loss in standby operation, but they do draw some current. The circuit in this Design Idea presents a simpler way to perform the same function with fewer components and with no power consumption during standby operation (Figure 1). Moreover, the network comprising R2, D2, and C2 activates and deactivates the circuit. An additional control signal, control on/off, becomes slower than the battery’s on/off cycle.
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| Figure 1 | This on/off management circuit uses a relay to remove battery power. |
Switching S1 to Position 1, the on position, the 24 V battery quickly charges capacitor C1 through diode D1. That voltage drives transistor Q1 into saturation. Q1’s saturation magnetizes and activates relay coil L1, connecting the battery to the main power and control board. Meanwhile, capacitor C2 charges more slowly through 100-kΩ resistor R2, thus generating the control on/off signal with some delay relative to the relay coil’s closing. That scenario occurs after the proper power supply to the power stage and control circuits.
Switching S1 to Position 2, the off position, causes capacitor C1 to slowly discharge through resistor R1 when diode D1 is off. That action delays Q1’s turn-off. Before Q1 turns off, C2 quickly discharges through D2, indicating that the control should shut down the power. The relay switches off with minimum current. Once Q1 is off, the relay coil demagnetizes through R4 and D3. The relay switches off, disconnecting the main power and control board from the battery. During this off state, current flows neither in the on/off circuit of the management board nor to the main board.
