Elenco Digital / Analog Trainer in Case - Manual (Page 6)

-5-
Simplified diagram of positive power supply
120VAC
Input 17VAC 20VDC 0V - 15V
Regulated
Output
Transformer
120V to 17V
AC to DC
Converter
Voltage
Regulator
TESTING THE LOGIC SWITCHES
There are two logic switches and four conditions to
be checked out. Connect a wire from the “X” test
pin to the “A” logic indicator test pin. Connect
another from the “X” test pin to the “B” test pin.
Apply power and note that the “A” LED indicator
should be lit when the logic switch is in the “X”
positions and the “B” LED is not lit. Moving the logic
switch to “X” should reverse the indicator LEDs, that
is, the “B” LED should light and the “A” LED not light.
Check the Y logic switch in the same manner.
TESTING THE DATA SWITCHES
There are eight data switches to be checked. The
output of the switches are at 5V or ground depending
on position. Connect a wire to SW1 terminal and the
“A” test pin, the “A” LED should light when the switch
is placed toward the top case. Repeat the same test
on SW2, SW3, SW4, SW5, SW6, SW7 and SW8.
This completes the testing of the trainer.
The XK-550 power supply features two variable
output voltages and three fixed 12V, –12V and 5V,
variable output voltages are 0V to 20V and 0V to –
20V at up to 1 ampere maximum current. All
supplies are regulated to better than .2V when going
from no load to full load. Varying the input AC voltage
from 105 to 135V will have practically no effect on
the output voltages. This is because of the specially
designed IC circuits used in the XK-550 circuits.
Severe overloading or even shorting the output
circuits will not damage the supplies. Special turn-
off circuits in the IC sense the overload and turn off
the output.
THE POSITIVE 0V TO 20V POWER SUPPLY
Figure 1 shows a simplified circuit diagram of the
positive supply. It consists of a power transformer, a
DC rectifier stage and the regulator stage.
TRANSFORMER
The transformer T1 serves two purposes. First, it
reduces the 120VAC input to 17VAC to allow the
proper voltage to enter the rectifier stages. Second,
it isolates the power supply output from the 120VAC
line. This prevents the user from dangerous voltage
shock should he or she be standing in a grounded
area.
AC TO DC CONVERTER
The AC to DC converter consists of diodes D1, D3
and capacitor C1. Transformer T1 has two
secondary windings which are 180
O out of phase.
The AC output at each winding is shown in Figure 2A
and 2B.
Diodes are semiconductor devices that allow current
to flow in one direction. The arrow in Figure 3 points
to the direction current will flow. Only when the
transformer voltage is positive will current flow
through the diodes. Figure 3 shows the simplest
possible rectifier circuit. This circuit is known as a
half-wave rectifier. Here the diode conducts only half
of the time when the AC wave is positive as shown in
2C. Use of this circuit is simple but inefficient. The
big gap between cycles require much more filtering
to obtain a smooth DC voltage.
CIRCUIT DESCRIPTION
Figure 1