FIGURE 2. The input conditioning is pretty basic, but it must be
remembered that the RMS voltage is not the same as the peak
voltage. The peak voltage of 120 VAC RMS is about 170 volts.
can see, this is not correct. The
absolute minimum safe working
voltage for your capacitors is 200 volts
(250 volts is better). Don’t forget that if
you expect to see noise/spikes on the
line, you need to add that noise/spike
voltage to the peak voltage.
The input frequency is universally
60 Hz in the USA. In Europe, 50 Hz is
common. Transformers rated for 60 Hz
will generally perform well on 50 Hz
and vice versa. Additionally, the frequency stability of the power line is usually excellent and is rarely a consideration. Occasionally, you may find 400 Hz
transformers available. These are typically military or aeronautical devices
and are generally not suitable for use on
50/60 Hz power (or vice versa).
The output of the transformer is
also specified as an RMS voltage.
Additionally, the voltage specified is
the minimum voltage expected under
full load. Often there is about a 10%
increase in the rated output with
no-load. (My “ 25. 2 volt/two-amp”
transformer measures 28. 6 volts with
no load.) This means that the actual
no-load, peak output voltage for my
“ 25. 2 volt” transformer is 40. 4 volts!
As you can see, it is always important
to remember that the rated RMS voltages for AC power are substantially
less than the actual peak voltages.
Figure 2 provides a typical input
power conversion and conditioning
design. I prefer to use a double-pole
switch although it is not absolutely
necessary. It protects against
mis-wired electrical outlets (which is
rare today) or mis-wired power leads
in the power supply itself (much more
common). It is vital that when the
power switch is off, the hot lead is
disconnected from the power supply.
The fuse (or circuit breaker) is necessary. Its main purpose is to prevent
fires because, without it, a transformer
or primary circuit short will allow
massive currents to flow causing metal
parts to get red or even white hot. It is
usually a slow-blow type rated at 250
volts. The current rating should be
about double of what the transformer
can expect to draw. For example,
the 25. 2 volt two-amp transformer
mentioned above will draw about 0.42
amps of primary current ( 25. 2 volts/120
volts x two amps). So, a one amp fuse is
reasonable. A fuse in the secondary will
be discussed in the next article.
The bypass capacitors help to
filter out noise and are optional. Since
the peak voltage is about 170 volts, a
250 volt rating is better than a marginal 200 volt rating. You may want to
use a “power-entry filter.” There are
many types of these units. Some
contain a standard power connector,
switch, fuse holder, and filter in one
small package. Others may have only
some of these components. Typically,
the ones with everything are fairly
expensive, but surplus units can usually be found at very reasonable prices.
Being able to determine if the
primary circuit is powered is important
so a pilot light is used. Two typical
circuits are shown. The neon lamp has
been used for decades. It is simple and
inexpensive. It has the drawbacks that
it is somewhat fragile (being made of
glass), can flicker if the resistor is too
big, and can actually generate some
electrical noise (due to the sudden
ionic breakdown of the neon gas).
The LED circuit also requires a
current limiting resistor. At 10,000 hms,
about 12 mA of current is provided.
Most LEDs are rated for a maximum
current of 20 mA, so 12 mA is reasonable. (High efficiency LEDs may work
satisfactorily with only 1 or 2 mA, so
FIGURE 4. The full-wave design (top)
produces a nice output. But by
redrawing the circuit (bottom), it can
be seen that it is really just two half-wave
rectifiers connected together. Again,
half the transformer power is wasted.
the resistor can be increased as required.)
Note that LEDs have really poor
reverse breakdown voltages (typically
10 to 20 volts). For that reason, a
second diode is necessary. This must
be able to operate with at least 170
volts of PIV (Peak Inverse Voltage).
The standard 1N4003 is rated at 200
PIV which doesn’t provide much margin. The 1N4004 is rated at 400 PIV
and costs perhaps a penny more. By
placing it in series with the LED, the
overall PIV is 400 plus the LED PIV.
Rectification and Filtering
Figures 3, 4, and 5 show the most
typical rectification circuits with the
FIGURE 3. The half-wave rectifier circuit
is simple but it produces a poor output
waveform that is very difficult to filter.
Additionally, half of the transformer
power is wasted. (Note that the filtering
capacitors are omitted for clarity
because they change the waveform.)
December 2007 73