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-16-
The 5 Volt Power Supply
I
n the previous discussion of the variable voltage regulators, the ICs can handle about 1A of current. In the
design of the 5V supply, we need 3A of current. To meet this current requirement, we must add an external pass
transistor capable of delivering 3A.
F
igure 7 shows a simplified 5V regulator with an external PNP pass
transistor. In this circuit, transistor is a power transistor capable of
delivering over 3A. The transistor is biased off until the LM-7805 IC
draws about 0.2A. When 0.2A is drawn by the LM-7805 IC, the voltage
drop across the 3 ohm resistor is 0.6V, enough to turn on the transistor.
The transistor takes over and delivers the current to the output. Note that
if the output voltage goes down, the LM-7805 regulator will draw more
current, forcing the output voltage back to 5V. Thus, the LM-7805
regulator controls the output voltage and keeps it at 5V.
Unfortunately, this circuit has no control of the output
maximum current. If the output is shorted to ground, the
transistor will be overloaded and eventually be damaged.
The LM-7805 IC will only draw the 0.2A it was designed
to handle and never heat up to turn itself off. Another
transistor, Q1, is added to limit maximum current (see
Figure 8). Resistor R5 is added to sense the current in
transistor Q2. When approximately 3A is drawn in
transistor Q2, the voltage drop in resistor R5 will turn on
transistor Q1. This will force more current in the LM-7805
IC. Eventually the LM-7805 IC will overheat turning itself
off and thus limiting the circuit at about 2.6A.
The first 0.2A of current is drawn by the LM-7805 IC. The next 3A are drawn by transistor Q2. Thereafter, the
current is drawn by the LM-7805 IC until it overheats and turns itself off. This is a very effective circuit capable
of regulating the output voltage at a constant 5 volts and yet delivering over 3A of current.
AC Power Supply
The section features a 12.6VAC center-tapped output. Two secondary windings from the transformer are
connected directly to the yellow binding posts. Connecting from one of the outputs to the center black binding
post will give you 6.3VAC. The maximum output current for 12.6VAC and 6.3VAC is 1A.
WARRANTY POLICY
Your power supply has been tested and conforms to our rigid requirements on performance and durability. It is guaranteed to be free of
defects in workmanship, materials and construction for a period of 2 years. If this product should fail during normal use within the first 3
months from the date of purchase, ELENCO
®
will repair or replace the unit at no cost. For the remainder of the warranty period, a nominal
service charge is required to cover shipping and handling.
When returning merchandise for repair, please include proof of purchase, a brief letter of explanation of problem and sufficient packing
material. Before returning any merchandise, please call our service department at (847) 541-3800 to obtain a return authorization number
(RA).
ELENCO
®
Service Department
150 Carpenter Avenue ● Wheeling, IL 60090
Figure 7
Figure 8
XP-720K_REV-H_111114.qxp_XP-720K REV-H 111114 7/31/15 1:08 PM Page 17
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