Before powering up the
circuit, check all connections.
Make sure that you have a DC
adapter that can deliver the
required maximum current at
the required input voltage. Use
a fresh battery. With the
backup battery out of the
circuit, check out the circuit as
• Set S1 and S2 to their open
• Connect the DC adapter to
the UPS and plug it into the
- Close S1. Both LED
indicators should be on.
If neither LED is on, then
suspect the adapter and
check the voltage across the input to the UPS.
Note: The voltage measured at the adapter output
when it is not under load will most likely be higher
than the rated voltage at the specified load current.
For example, the output of the nine volt adapter
that I used was 11. 2 volts.
- If LED1 is on and LED2 is off, first check the UPS
output voltage. If the UPS output voltage is okay,
then check LED2 and its limiting resistor. If there is
no UPS output voltage, then measure the voltage
on the input side of the 7805 regulator. If there is
no regulator input voltage, then suspect D1.
Otherwise, suspect the 7805.
- If LED1 is off and LED2 is on, then check the wiring
for LED1 and its limiting resistor.
• Measure the current between the anode of D2 and
ground. It should not exceed the specified leakage
current for the diode you have chosen.
• Verify that the regulator portion of your circuit is
operating correctly by varying the UPS output load and
measure the output voltage. The output voltage should
remain relatively constant. Be careful not to exceed the
7805 maximum current limit.
• With S1 still closed, place the backup battery in the
• Close switch S2 and place S1 in the open (off) position
(or remove the adapter from the wall) to simulate a
power failure. LED1 should go off. LED2 should
- Measure the output voltage. It should still be at the
The configuration of LED1 and LED2 indicates the
status of the UPS and the condition of the main power
line. Assuming both S1 and S2 are closed, the status of
the UPS is shown in Table 1.
Off Off Main power off. No backup battery or low backup battery.
Off On Main power failure. UPS in backup mode.
Main power on. However, UPS has
apparently failed. Possible causes
include bad battery, open diode, bad
regulator, or bad indicator LED2.
On On Main power on. UPS operating properly.
Table 1: UPS status conditions assuming switches
S1 and S2 are closed.
There are obviously a number of improvements
that you can make to the basic circuit presented here.
You can replace the standard 7805 with a variable
voltage, low dropout (LDO) regulator. The lower minimum
input voltage of the LDO would extend the range of
Vbb. Additionally, it can be configured for a range of
output voltages. For example, you could use the same
part to build a nine volt UPS to power a DC appliance
or to design a five volt UPS into your next project.
Another possible improvement would be to replace
the linear regulator with the more efficient switching