BATTERY BACKUP FOR DIGITAL CLOCKS
IN VCR'S AND OTHER DEVICES


It is most frustrating to experience a momentary power outage
and see your digital clocks blinking and begging for a reset.

Fortunately, it is easy to provide a battery backup for a
couple of dollars that will prevent this annoyance.

All you need is a 1N914 diode, an ordinary 9-volt transistor
radio battery, and a little wire. Everything you need can be
obtained at Radio Shack or any other electronics supply house.

The circuit is connected as shown:



From To
Power oo Clock
Supply

+ 1N914 Diode

+
9-Volt Battery

-


Ground




The line from the VCR power supply to the clock module should
be evident from looking at the clock module. The input power
line is generally indicated. You then tap into that line and
arrange the diode, battery, and ground as shown above.

Once the circuit is set up, test it by turning on the ac power,
resetting the clock, and then cutting the ac power. If all is
well, the battery will prevent loss of clock.

A few devices with digital clocks use a different voltage from
the standard 9 to 12 volts. If that is the case substitute the
proper voltage battery.

This backup circuit works because during normal operation of
the VCR, the clock operating voltage will be slightly higher
than the battery voltage. When ac power is lost, the clock's
voltage drops below the battery voltage, and current then flows
from the battery to the clock. Battery life will essentially
be determined by the shelf life, which can be quite long.

This backup circuit and a slightly more complicated one that
powers only the clock timing circuits (not the display) are
discussed in the January 1983 issue of Radio-Electronics
magazine.