Microamps monitor dual-supply batteriesBRUCE ANDERSON, UNIVERSITY OF WISCONSIN—MADISONThe low-power circuit in Figure 1 monitors two 9V batteries in
a dual-supply configuration and turns on the Battery Low LED
ifeitherbatteryvoltagedropsbelowitslimit.Italsoprovidestwo
shutdown signals you can use to turn off voltage regulators,
such as Maxim s MAX663/664 positive and negative regulators.
By using low-power voltage references and op amps, the
circuit holds the current drain to approximately 45 mA from each
battery, with the positive drain rising to approximately 1 mA
when the LED turns on.
Each battery voltage undergoes comparison with a Motorola
LM385Z 1.2V reference, using a Maxim 7612 op amp with
hysteresis via a 2-MV resistor (R5 and R11). Cross-coupling via
the 3-MV resistors (R6 and R12) ensures that if either shutdown
signal goes true, both do, and the circuit locks up in the shutdown
state with the Battery Low LED illuminated. C1 and C3
delay the reference voltages so that when the batteries switch
on, the circuit comes up in the proper state. Positive Shutdown
is at the positive rail when true. Negative Shutdown is at the
negative rail when true. The values shown allow you to adjust
the battery-low limits over a range of approximately 3.8 to 8.1V.
For our applications with Eveready EN22 alkaline batteries, we
typically set the limits at 6.5V. This setting uses a good portion
of the battery life, yet allows some reserve for continued operation
after the LED comes on.
The circuit has two convenient features that were unforeseen
before testing. One is that when the batteries switch off,
the LED flashes briefly as the decoupling capacitors discharge.
The flashing indicates that the batteries are not so totally dead
that they cannot light the LED. The other is that the LM385 has
an initial turn-on voltage about 10% higher than the steadystate
1.2V reference. Thus, when you switch the batteries on,
they must have a voltage about 10% higher than the steadystate
threshold to be considered good. So if the Battery Low
LED stays off when the device turns on, the batteries will
remain good for a while. Of course, if you are not using the shutdown
signals to turn off regulators, you can set the threshold
so that your device will continue to operate for a period after the
LED comes on. Using the battery-discharge characteristics and
your circuit s voltage and current requirements, you can select
a threshold that gives appropriate reserve for your application.
(DI #2169)
If either battery voltage in a two-battery supply drops below a preset limit, a Battery Low LED turns on.
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