Simple automatic-shutoff circuit uses few components

When you press the pushbutton switch, C₁ charges rapidly through the low-value R₂ to the zener voltage of diode D₁, and P-channel MOSFET Q₁ immediately conducts. After the pushbutton is released, C₁ discharges slowly through the high-value R₁ with a time constant of R₁C₁ seconds. During this time, C₁ loses 63% of its initial voltage— from 9V to 3V after the delay. Reference 8 shows the on-resistance versus the gate-to-source voltage of a Vishay Siliconix Si4435. As long as the gate-to-source voltage is greater than approximately 3V, the device’s on-resistance remains lower than 0.1Ω, yielding a dropout voltage of less than 0.1V for a load sinking as much as 1A.

The 9.1V zener diode, D₁, keeps the shutoff time delay independent of the battery voltage and ensures that the gate-to-source voltage does not exceed Q₁’s rated maximum of 20V. Thus, you can use this circuit with a choice of battery voltages; only the maximum drain-to-source voltage of transistor Q₁ limits the choice. With 3.6 to 9V batteries, D₁ and R₁ are useless (remove D₁ and short-circuit R₂), and you must compute the time delay with the classic equation T=−R₁C₁log_e(3/V_BAT), as Table 1 shows. With battery voltages as low as 1.5V, instead use a bipolar transistor with a low saturation voltage as well as a modified circuit scheme.

Editor’s note: With no feedback for rapid shutoff, as C₁ slowly discharges below 3V, Q₁ goes through a period of gradually increasing the on-resistance, which temporarily increases its power dissipation and heating during the shutoff action. Be sure to consider this effect, size Q₁ adequately for the load current, and use adequately sized heat sinks.

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2. Chenier, Glen, “RC-timed shutofffunction uses op amp and momentaryswitch,” EDN, Feb 16, 2012, pg 45.
3. Espí, José M; Rafael García-Gil; and Jaime Castelló, “Circuit extendsbattery life,” EDN, July 29, 2010, pg 42.
4. Xia, Yongping, “Battery automaticpower-off has simpler design,” EDN, March 31, 2005, pg 80.
5. Gimenez, Miguel, “Schemeprovides automatic power-off for batteries,” EDN, May 13, 2004, pg 92.
6. Xia, Yongping, “Timer automaticallyshuts off,” EDN, Aug 17, 2000, pg 128.
7. Elias, Kamil, “Timer provides poweroff function,” EDN, May 22, 1997, http://bit.ly/MlbQxg.
8. “Si4435BDY P-Channel 30-V (D-S)MOSFET,” Vishay Siliconix, May 4, 2009.