Battery Saver Circuit

Commonly found in simple devices like an IR remote control unit and calculators, the battery protection circuit serves to limit the over-exhaustion of battery power in the event you forgot to switch off the device. Still, the batteries will deplete no matter how low the standby current is in these devices.

This battery saver circuit in discussion automatically cuts off the supply current one minute after power-on, or once the battery voltage drops below the nominal level of operation.

How it Works

A series regulator FET T₁ can permit a maximum current of 150 mA in the circuit. Moreover, we recommend using a more powerful kind of FET than the BS250 in the event more than 100 mA is expected to be utilized by the equipment attached to the output terminals.

This type of FET dips around 0.5 V at a drain current of 0.1 A, and 0.8 V at 0.15 A. Because the T₁ is a p-channel FET, it conducts and supplies the equipment once the output of the Schmitt trigger NAND gate N₃ is low.

In other words, when the gate inputs are high. This occurs at power-up because C₂ is still discharged and the N₄ inputs are kept at logic low level through R₆.

As a result, T₁ is activated and causes C₂ to be charged through R₃. After one minute has elapsed, the voltage across R₃ will be fairly low for N₃ to identify a logic low level at pin 1.

Consequently, it turns off T₁. A locked function of this state is provided by N₂ as otherwise, N₃ could oscillate due to the steadily changing voltage across R₃.

During power-on, N₂’s output is pulsed high from the R-C network R₁-R₂-C₁. Any residual charge in C₂ is cleared and the circuit may be turned on with switches S₂-S₁ immediately after powering down and saving precious battery power automatically.

D₃, R₅, R₆ and N₄ successfully monitor the battery voltage. N₄’s trigger threshold level is positively related to the supply voltage level of the IC.

Whenever there is the supply voltage (battery) and relatively high, N₄ will identify a logic low level at junction R₅-R₆-N₄. In the event, the battery voltage drops, the input voltage to N₄ is maintained at a secure level by D₃.