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With this circuit built into the power supply of a battery powered device, it will prevent the rechargeable cells from being completely drained when you forget to turn the equipment off. When the battery voltage drops below a preset limit (9.5 V in this example) the circuit will automatically disconnect the battery. Power is re-connected when the voltage rises above an upper threshold level (10.5 V here), this will typically occur after the equipment has been plugged into its recharging station.

The circuit is designed to use as little power as possible.The ICL7665 from Intersil forms the heart of the circuit. This IC contains two comparators together with a voltage reference and consumes just 3 μA. The circuit only uses one of the comparators, the values of resistors R1 to R3 shown in the diagram will cause the circuit to switch at the levels mentioned above. The comparator output switches the P-channel MOSFET T1 which in turn controls power to the load RLOAD.

The switching threshold levels and hysteresis can be changed by using different values of resistor for R1 to R3. Increasing the value of R3 to 300 KΩ will raise the upper thresh-old level to 12.5 V. The ICL7665 data sheet gives examples of suitable resistor values that can be used here.  The PCB layout uses SMD components so the finished circuit takes up very little space  when installed in the equipment.  A fine-tipped soldering iron should be adequate to mount the components and there shouldn’t be any problems provided you do not choose to use very small resistor packages. Once the circuit has been tested the entire PCB can be protected by encapsu-lating it with a short length of heat shrink sleeving.

Data sheet IRL7665:  www.intersil.com/data/fn/fn3182.pdf
Data sheet IRLML6402:  www.irf.com/product-info/datasheets/data/irlml6402.pdf

Author : Tilman Küpper Copyright : elektor elector  -  7-8/2007