from this month’s questions.
In this column, I answer
✓ Plan on learning about
questions about all aspects
CCFLs, degaussing, and
of electronics, including
computer hardware, software,
circuits, electronic theory,
troubleshooting, and anything
else of interest to the hobbyist.
Fee free to participate with
your questions, comments or
●✓ How about the best way to
build a 78xx power supply
from scratch or putting a PIC
✓ And there’s more.
You can reach me at:
Fail-Safe Power Supply
DC power supply. During battery operation, D2 will prevent
current from flowing back into the wall wart. Your feedback
is greatly appreciated.
■ Fred, via Internet
AYou have a good design, and it will work.
However, if you rearrange the parts just
slightly, you can get better performance
from the 7805 regulator. Specifically, C2 and
C4 should be physically as close to the
regulator as possible. This will improve the transient
response of the power supply.
I would also put C1 on the other side of the D2
isolation diode for the same reason. Take a look at Figure
1 for details.
■ FIGURE 1
BATTERY BACK-UP POWER SUPPLY
QI’m trying to design a five-volt power supply that is
backed up by a nine-volt battery in case of power loss.
Would you please look at my design and let me know
if it makes any sense? D1 is to protect the battery
from being charged by the external wall wart 12-volt
16 NUTS & VOLTS November 2005
SLEEP, PIC, SLEEP
QI have a few projects built around PIC microprocessors, but they all require an on/off switch to apply
battery power. Can you suggest a circuit or IC that
will automatically switch the battery off (or go into a
low power mode) if there is no I/O activity in a set
■ T. Campbell, Austin, TX