and are expensive. Occasionally, they
are used for high-voltage power supplies
where suitable capacitors are expensive.
The formula for calculating the filter
capacitor (C) is quite simple, but you
need to know the acceptable peak-to-peak ripple voltage (V), half-cycle time
(T), and current drawn (I). The formula is
C=I*T/V, where C is in microfarads, I is
in milliamps, T is in milliseconds, and V
is in volts. The half-cycle time for 60 Hz
is 8. 3 milliseconds (reference: 1997
Radio Amateur’s Handbook).
It is clear from the formula that the
filtering requirements are increased for
high current and/or low ripple power
supplies. But this is just common sense.
An easy-to-remember example is 3,000
microfarads per ampere of current will
provide about three volts of ripple. You
can work various ratios from this example to provide reasonable estimates of
what you need fairly quickly.
One important consideration is the
surge of current at turn-on. The filter
capacitors act as dead shorts until they
get charged up. The larger the capacitors,
the greater this surge will be. The bigger
the transformer, the greater the surge will
be. For most low voltage analog power
supplies (< 50 volts), the transformer
winding resistance helps somewhat. The
25. 2 volt, two amp transformer has a
measured secondary resistance of 0.6
ohms. This limits the maximum inrush to
42 amps. Additionally, the inductance of
the transformer reduces this somewhat.
However, there is still a large potential
current surge at turn-on.
The good news is that modern silicon rectifiers often have huge surge current capabilities. The standard 1N400x
family of diodes is usually specified with
30 amps of surge current. With a bridge
circuit, there are two diodes carrying
this so worst-case is 21 amps each
which is below the 30 amp specification (assuming equal current sharing,
which is not always the case). But, this
is an extreme example. Generally, a factor of about 10 is used, instead of 21.
Nevertheless, this current surge is
not something to be ignored.
Spending a few cents more to use a
three-amp bridge instead of a one-amp bridge may be money well spent.
Circuit Filter PIV Needs Factor TransformerUse
Half-Wave Large 2. 82 50%(theoretical)
Full-Wave Small 1.414 50%(theoretical)
Bridge Small 1.414 100% (theoretical)
TABLE 1. A summary of the characteristics
of the various rectifier circuits.
We can now put these rules and
principles to use and start to design a
basic power supply. We will use the 25. 2
volt transformer as the core of the design.
Figure 6 can be seen as a composite of
the previous figures but with practical
part values added. A second pilot light in
the secondary indicates its status. It also
shows if there is a charge on the capacitor. With such a large value, this is an
important safety consideration. (Note
that since this is a DC signal, the 1N4004
reverse voltage diode is not needed.)
It may be cheaper to use two smaller capacitors in parallel than one large
one. The working voltage for the capacitor must be at least 63 volts; 50 volts is
not enough margin for the 40 volt
peak. A 50 volt unit provides only 25%
margin. This may be fine for a noncritical application, but if the capacitor
fails here, the results can be catastrophic. A 63 volt capacitor provides about
60% margin while a 100 volt device
gives 150% margin. For power
supplies, a general rule of thumb is
between 50% and 100% margin for the
rectifiers and capacitors. (The ripple
should be about two volts, as shown.)
The bridge rectifier must be able
to handle the high initial current surge.
A two-amp unit costs about $0.57,
a four-amp bridge is $0.77, and a six-amp device is $0.87. It seems obvious
that spending an additional dime for
improved reliability is worthwhile.
Note that the bridge is specified by
what the transformer can supply rather
than what the power supply is eventually specified for. This is done in case
there is an output short. In such a
case, the full current of the transformer
will be passed through the diodes.
Remember, a power supply failure is a
bad thing. So design it to be robust.
Details are an important consideration in designing a power supply. Noting
the difference between RMS voltage and
peak voltage is critical in determining the
proper working voltages for the supply.
Additionally, the initial surge current is
something that cannot be ignored.
Next month, we will complete this
project by adding a three-terminal regulator. We will design a general-purpose,
current-limited, adjustable voltage power
supply with remote shut-off. Additionally,
the principles used for this design can be
applied to any power supply design. NV
FIGURE 6. Final design of the power supply
with practical parts specifications. Regulating
the power is discussed in the next article.
December 2007 75