You often need to include a timed
automatic-turn-off circuit in
battery-powered equipment to extend
battery life. Previously published Design Ideas
for this function all involve many
components (references 1 through 7
The circuit in Figure 1
is a simple automatic-shutoff add-on circuit featuring
no quiescent current.
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=−R1C1loge(3/VBAT), 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.
Baddi, Raju, “CMOS gate makeslong-duration timers using RC components,”
EDN, March 1, 2012, pg 43.
- Chenier, Glen, “RC-timed shutofffunction uses op amp and momentaryswitch,” EDN, Feb 16, 2012, pg 45.
- Espí, José M; Rafael García-Gil;
and Jaime Castelló, “Circuit extendsbattery life,” EDN, July 29, 2010, pg
- Xia, Yongping, “Battery automaticpower-off has simpler design,” EDN, March 31, 2005, pg 80.
- Gimenez, Miguel, “Schemeprovides automatic power-off for batteries,”
EDN, May 13, 2004, pg 92.
- Xia, Yongping, “Timer automaticallyshuts off,” EDN, Aug 17, 2000, pg 128.
- Elias, Kamil, “Timer provides poweroff
function,” EDN, May 22, 1997,
- “Si4435BDY P-Channel 30-V (D-S)MOSFET,” Vishay Siliconix, May 4,