Q&A
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Figure 3
variable voltage regulator IC. The 5K
resistor is used to set the output
voltage to between 1.2 and 15 volts.
The hard part will be finding a
power transformer, but its rating isn’t
critical. Anything between 15 and 24
volts will work. The output voltage
range will change, though, with the
output being 3 volts less than the
transformer’s rating. Me? I would
keep your present transformer and
wire the secondaries in series.
Remount it to clear the front
panel, if you can. (You don’t need to
keep the choke or old electrolytics.)
Keep the ammeter and voltmeter as
they were — in series and parallel
with the output, respectively. Don’t
be tempted to use the old rectifiers.
Replace them with a 100 volt, 6 amp
bridge rectifier, like the PB61DI from
Digi-Key (800-344-4539; www.
digikey.com). Be sure that the
LM338 and rectifier are well
heatsinked.
Voltage Regulator
Pinouts
Q. Regarding pinouts (“Pinout
Chart,” April 2004), why do all
three-pin voltage regulators use
different pinouts (7805, 78L05, 317,
337, etc.)? I think all the TO-92 and
TO-220 packages should pinout the
same way. I can’t for the life of me
guess why they’re all different. This
seems as dumb as putting diodes in
SOT- 23 packages and randomizing
the pinouts for each part number.
Chuck Larson
Largo, FL
92
A. Actually, SOT- 23 diode packages do vary, but that doesn’t
answer your question. I assume the
pins are changed to protect the
innocent — that is, to safeguard a
circuit in case a negative regulator is
accidentally placed in a positive
socket. Take the 7805 and 7905
TO-220 case, for example. Note that
the GND and input pins are reversed.
This prevents the input voltage from
going to the IC should a wrong polarity
regulator be inserted. No voltage in
JULY 2004
