RFC-1 Accessories and Miscellaneous Circuits page 4.6
4.2.3 Battery Backup
Do not under any conditions apply a DC voltage greater than 19.9 volts (19.9 VDC peak if
significant ripple is present) to the RFC-1. Prolonged exposure will cause the protection circuitry
in the RFC-1 to overheat and be damaged. This maximum voltage rating precludes the use of
some rechargeable batteries.
The user settings in the RFC-1 are stored in non-volatile memory. No user settings are lost when the RFC-1 loses
power. The time and date are the only operational data that are lost as a result of a power failure.
The RFC-1 can be operated on a small uninterruptible power supply. A small UPS designed for small-office, home-
office use should power the RFC-1 for 30 minutes to several hours depending on the capacity of the UPS.
Alternately, the RFC-1 can be operated during power failures by an external 16 to 18 VDC power supply connected to
the 12 VAC supply input. The DC source must be "floating" (neither side connected to ground) to allow the telemetry
section of the RFC-1 to work properly.
Polarity of the voltage is unimportant since the input connector applies power to the bridge rectifier in the RFC-1. The
following simple circuit provides battery backup for a few dollars plus the cost of batteries.
Figure 4.5; Battery backup switching circuit
The value of C1 should be just large enough to keep RY1 pulled-in during normal power conditions. If its value is too
large, the switchover will take too long and the RFC-1 will reset. A good starting value for C1 is 47 µF.
The 18-volt battery can be as simple as three 6-volt heavy-duty lantern batteries wired in series. This type of battery
will power the RFC-1 for several hours. The batteries should be changed at intervals of 12 to 18 months even if they
are seldom used due to their limited shelf life.
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