Other reasons for separate AC and
DC power switches are to eliminate
transients presenting themselves at the
font panel jacks upon AC power up/
power down, which can occur at the
first few milliseconds of AC switching.
Another reason is that when using
a negative reference voltage such as
Z1(-1.225V) for the regulator and due
to unequal charge discharge times of
C1, C2, a 1.5 volt step-up in voltage
can occur upon AC shutdown only.
This is not an easy problem to
overcome to cover all situations.
Again, DC switches block this
All my power supply designs have
incorporated separate AC and DC
switching for these and also to give
you a setup procedure while the DC
power switch is off, as the metering
will still be active. If one were to just
drag this supply to the test bench for a
couple of hours at a time and then
return it, there really wouldn’t be any
need for an AC switch — just plug and
go! But for my use, this supply will
become a permanent fixture on my
test bench, so I decided it needed AC
Let’s discuss the five volt supplies
that are shown in Figure 2. There are
five of these, but it’s not as bad as it
sounds. There are some options here.
The first of these is the 5V at 1A front
panel supply. This is so straightforward, it almost needs no explanation.
T2, BR1, C1 provide raw B+ to the
regulator chip IC5 (7805), which then
does a fine job of regulating it to + 5
VDC. T2 has a single 9 VAC secondary at 1.5 amps and was chosen for
minimum voltage differential across
IC5 for reduced wattage dissipation.
The other four five-volt supplies
are for the four front panel meters.
These are all meters requiring isolated
power supplies. Two of these supplies
could be eliminated for the front panel
voltmeters, since the meter’s negative
input is connected to common.
A 78L05 (TO- 92 case) could be
connected to the raw B+ supply and
power these meters directly from its
positive and common. You will need
one of these for each supply you are
metering. The meters will have to be
the type that operate with common
grounds (negative input lead and
negative power lead tied to common).
Another option is to use only one
meter per supply and switch them
from volts to amps, with regards to
their input leads. These will require
isolated power supplies, but only two
instead of four. I have to admit that the
meters used in this project are by far
the greatest cost of constructing it. I
wanted the luxury of full metering all
■ FIGURE 2