Page 313 - 671940 Digital Edition

TRANSFORMERS
Center-tapped, power supply or filament transformers.
NOTES:
It is the users responsibility to determine which
transformer is appropriate for his use, as well as
which circuit provides an adequate and safe power
supply. These notes are given as general guide-
lines and are not intended for use by persons lack-
ing technical expertise.
The current rating is the absolute maximum
current output at the rated voltage. The trans-
former’s rated current should be about twice your
circuit’s current needs. This will assure plenty of
reserve power and a cool running power supply.
Only for brief and infrequent peaks of a few sec-
onds should your circuit draw as much current as
the peak rating.
In short, if you have an amplifier (for exam-
ple) that requires 250mA current during normal
operation, then use at least a 500mA or larger
transformer. Audio circuits in particular will require
power above the circuit’s average and there must
be spare power to prevent distortion as well as
over-heating.
DIODES; the current rating of diodes used
in a full wave rectifier circuit should be at least
twice the secondary current to be able to handle
turn-on surge current. The reverse voltage rating of
diodes should be at least 1.5 times the voltage of
the secondary; most engineers prefer a bit more
than that.
In half wave circuits, the diode’s reverse rat-
ing must exceed 2.8 times (or more) the AC rating.
Don’t forget to take into account voltage
drop in your chosen components, especially diodes
which can drop a half volt each.
Capacitors; the voltage rise in a capacitor
will increase due to transformer regulation; use a
capacitor rated about three times the DC load cur-
rent.
DC REGULATORS; probably one of the
best devices ever to come from the semi-conductor
manufacturers are voltage regulators. Connected at
the output of your power supply, a regulator can
provide a fixed DC voltage . Look for the 78L series
for small regulators (100mA) and the 78XX series
can provide one amp at a fixed voltage. (7805=five
volts, 7812=twelve volts etc.) The LM317T IC can
provide an adjustable voltage; see industry texts for
typical circuits.
FULL WAVE Capacitor Input Load.
A typical power supply circuit using two diodes. As a
general rule of thumb, multiply the transformer’s
secondary AC voltage times 0.45 to determine out-
put average DC voltage.
FULL WAVE BRIDGE
Typical full wave bridge rectifier; multiply the second-
ary ACV output by 0.90 to determine DC voltage.
DC current is about 0.94 of the secondary current,
or “rated current” in the part numbers chart on the
preceding page.
POWER SUPPLY COMPONENTS
available in this catalog.
86-1105 5V regulator 78L05 100mA
86-1112 12V regulator 78L12 100mA
86-1117 LM317T variable regulator IC
86-1100 2 amp bridge rectifier
86-1217 Zener diode assortment
86-1225 1N4001 50V PIV diodes
86-1230 1N4004 400V PIV diodes
86-1235 1N4007 1000V PIV diodes
86-1137 5V regulator 7805 1Amp
86-1140 12V regulator 7812 1Amp
86-1237 3Amp diodes 1N5400
86-1239 6Amp diodes 6A05
86-1280 1Amp bridge rectifier.
85-1401 Electrolytic ass’t 50V
SPT1182-6 series power cord w/plug
FH55 or 526 Fuse holders
FH503 Fuse holders
3AG series fuses
Switches-, binding posts etc. hundreds to choose
from in this catalog.
313