Page 72 - Electromechanical Devices and Components Illustrated Sourcebook

34 Electromechanical Devices & Components Illustrated Sourcebook

Next we must consider the diodes to be used. If diodes are to the base with two machine screws. The base has four
subjected to voltages higher than their maximum rating, they ordinary rubber feet attached to the four corners. Before
will fail. The voltage rating on a diode is given in peak mounting, take a moment to lay out all of the components
inverted volts (PIV). Select diodes with a PIV rating of at in a logical manner. Be sure to provide ample space for
least five times higher than the output voltage. This is wiring and soldering.
intended to protect the diodes from high transient voltages For internal power supplies, the AC connection is gener-
that may be encountered while switching any equipment con- ally a solder joint or modular connector. Desktop computers
nected to the supply. The current rating should also be con- use a standard connector into which an inexpensive AC cord
sidered when selecting the diodes. The +10% rule is also can be plugged. Be certain that the selected cord is large
applicable for this selection. enough to carry the current that is required.
If we go back to the bench supply, we should select diodes The bench supply has a 250-watt output and this translates
with 400 PIV and 5.5 amp ratings. The higher PIV rating is to 2.1 amps at 120 volts. Almost any AC cord will carry this
because this is a bench supply and, at times, it may be sub- current. A more important consideration is the rough service
jected to very high transient voltages. that the cord will probably receive, so a heavy, durable AC
The filter capacitor size is based on the level of filtration cord with a molded plug is recommended.
desired. There are two parameters that are of interest to the Protecting a power supply from overloading is always a
power supply designer, the capacity, given in microfarads good idea. In the case of a nonregulated supply, a fuse is gen-
or f, and the maximum voltage rating. As with diodes, erally placed on the primary winding of the transformer.
overvolting a capacitor will cause severe damage. If enough Transformers are generally hardy pieces of equipment, so a
current is available at the moment of over-voltage, some slow-blow fuse, with a current rating no higher than the max-
capacitors can actually burst because of excess internal imum current draw of the transformer, is usually recom-
temperatures. Direct current electrolytic capacitors are gen- mended. Voltage regulated supplies should also have a fast-
erally selected for the primary filter on any DC power sup- blow fuse on the output of the regulator.
ply. The filtration should be as high as possible. For most For our bench supply we use two fuses, one on the AC
power supplies, the principal limit is the physical size of the input and one on the AC output. The input fuse should be a
capacitor. When selecting the filter capacitor, the highest f 2 amp slow-blow fuse and the output should be a 5 amp fast-
rating that will fit into the physical limits should be blow fuse.
selected. Our bench supply also uses a variable autotransformer to
In the case of the bench supply, a capacitor with a voltage adjust the output voltage. Most variable autotransformers
rating between 150 and 250 volts at 2000 to 5000 f should have an output range from 0 volts to the line voltage, in this
be selected. A capacitor of this size can be difficult to find, case 0 to 120 volts. Alternating current line voltage is con-
and when found can be rather cost prohibitive. It should be nected to the input of the autotransformer and the output is
noted, however, that the surplus marketplace offers a wide connected to the input of the power supply’s transformer.
selection of these types of capacitors at rock bottom prices. Using the autotransformer to vary the line voltage has a direct
If the filtration requirement is high enough to warrant a effect on the supply’s output voltage.
voltage regulator, then a device should be selected with the The last significant item to consider is the panel meter.
desired output voltage and current capabilities. It should also This item should be a 0- to 50-volt AC meter. The most sig-
be noted that voltage regulators will have an input voltage nificant item to consider when selecting a meter movement is
range and a transformer should be selected that falls roughly how easy it is to read. It should have bold numbers and grad-
in the middle of that range. uations and an easy-to-see pointer. It should also be large
For a power supplies that are permanently installed into a enough to comfortably read during operation.
piece of equipment, the output terminals will most likely be a
soldered connection or some type of modular connector.
For our bench supply it is best to select a standard binding UInterruptible Power Supplies (UPS)
post for the terminals. The type of terminals that are most
often used on bench supplies are combination banana plug One other type of power supply that warrants a brief discus-
and screw type binding post with an insulated thumb nut. sion is the uninterruptible power supply (UPS). These sup-
These posts are generally supplied with insulating washers so plies have exploded onto the market in the past decade and
that they may be installed into a metal panel. are primarily intended to provide emergency back-up power
Oftentimes power supplies do not even have their own for computer systems. Figure 3-57 shows a block diagram of
chassis. They are commonly built into the chassis of the a typical UPS. The AC is passed through a transient suppres-
equipment that the supply serves. This technique provides sor to provide basic surge protection. The output of the sup-
a great deal of flexibility when selecting and arranging pressor is routed to a full-wave bridge, filter capacitor, and
components. voltage regulator, which all act as a battery charger. The out-
Our bench supply uses a rather simple chassis. The base put of the suppressor is also routed to an electronic switch
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is a piece of / inch thick, paper-based phenolic. The front and utility outputs. During normal operation, the utility out-
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panel is a piece of / inch aluminum, which is attached lets receive power and the electronic switch directs AC power
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