■ FIGURE 2
The circuit shown in
Figure 2 is a UPS intended
for low power applications.
It is essentially a linear
regulator with battery
backup. It provides a
regulated 5V DC from an
unregulated DC input of at
LED1 is the main power indicator. It remains on as long as the regulator is being supplied by the main power source.
LED2 is the output power indicator. Taken together, LED1 and LED2 are used to determine the status of the UPS.
Diode D1 isolates the main power source from the backup circuit, while diode D2 comprises the switching logic that
will be described later. Capacitors C1, C2, C3, and IC1 form the linear regulator section of the circuit. The battery backup
feature can be eliminated from the circuit via switch S1. Note that S1 and S2, as well as LED1 and LED2 — while useful
for testing the UPS prototype — are optional and need not be included in your application.
The 7805 is a standard linear 5V regulator. It receives
input from either the main power source Vs or the backup
Vbb is provided by either alkaline or non-rechargeable
lithium batteries. To determine the maximum and
minimum values for Vbb, you must consider the forward
voltage drop across D1 and D2, as well as the specified
minimum input voltage for the 7805, which is 7V.
Let’s assume that the forward voltage drop for D1 and
D2 is 1V. If Vs is 9V, then the voltage at the cathode of
D2 (and also the input to the regulator) is 8V. Therefore,
Vbb can’t be greater than 9V if D2 is to remain reverse-biased. In order to ensure that if Vs fails, the input to the
7805 will be at least equal to the specified minimum, Vbb
can’t be less than 8V. If Vs were higher — say 12V — then
there would be a wider range for Vbb (8V to 12V).
The 7805 will provide a regulated 5V output for any
input voltage from its specified maximum and minimum
input voltages. Choosing a value for Vbb that is close to
the minimum input voltage can help to extend battery life
in situations where Vs may frequently fall between its
normal level and the 7805’s minimum input voltage.
However, over time, Vbb will fall below its nominal value.
If you choose Vbb at or close to its minimum possible
value, then this will shorten its usable life. A good choice
for Vbb will be a compromise between the specified
maximum and minimum values.
Construction PARTS LIST
If you intend to use this as a stand-alone power supply, the entire prototype
circuit can be assembled on a piece of perf
board using point-to-point wiring. Mount
the 7805 on a heatsink.
If you intend to incorporate the UPS
circuit into your own application, then the
backup battery configuration will be a
significant factor in your choice of an
January 2011 41