Elenco Digital / Analog Trainer in Case - Manual (Page 10)

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Transistor Q5 is called the pass transistor. It controls
the current reaching the output. Transistor Q3 and
Q4 are emitter followers. Their function is to raise the
impedance of the pass transistor. Note that transistor
Q2, Q3, Q4, Q5 and resistor R1 form a close loop.
Also note that the feedback to the base of Q2 is
negative, that is, when the base of Q2 goes positive,
the output at emitter Q5 goes negative. Now if the
2V output voltage goes down because of current
drain at the output, the base of Q2 will drop forcing
the the collector voltage of Q2 to go higher. This will
bring the output voltage back to 2V. This is the basis
of all negative feedback regulators.
Another feature of the LM-317 regulator is to protect
the IC against overload and output shorts. If the IC
is overloaded, the junction of an overload transistor
will overheat. A transistor will sense this overheating
and shut down transistor Q5.
The LM-317 IC basically is a 1.25 volt regulator. To
be able to vary the output from 0 to 20V, you stack
the IC on the negative 1.25VDC voltage as shown in
Figure 6A. When VR1 equals 0, the output voltage
is 0 volts.
THE NEGATIVE VOLTAGE REGULATOR
The theory of the voltage regulator is the same as
the previously discussed positive regulator. The
basic difference is that diodes D1 and D3 are
reversed producing a negative voltage across
capacitor C1. The LM-337 IC is designed to operate
from a negative supply.
THE DATA SWITCHES
There are eight data switches labeled SW1 to SW8.
The circuit is very simple. To perform the desired
functions there is a double throw-double pole switch.
One end is connected to the 5V, the other to ground
and the center lug is connected to the output.
THE LOGIC SWITCHES
The logic switches perform the same function as the
data switch, that is, they produce high or low states.
But there is one big difference. When switching the
data switches, many pulses may be produced due to
bouncing of the contacts.
In the logic switches, only one pulse is produced, no
matter how many times the contacts bounce. This is
extremely important if you are producing pulses for
counting circuits. Figure 7 shows the wiring of the
logic switch. The two NAND gates are connected so
that when X input is grounded the output X goes
high. Opening and closing the ground at X will not
change the output. Only when X is grounded will the
output change to low. Thus, only one output change
is produced with one movement of the X switch.
There are two outputs from logic switch, X and X or
Y and Y.
THE FUNCTION GENERATOR
The function generator frequencies are produced by
an XR-2206 integrated circuit. This IC is capable of
producing high quality sine, square, and triangle
waveform of high stability and accuracy. Figure 8
shows the block diagram of the XR-2206 IC.
0V - 20V
R1
VR1
LM-317
X
X
X
X 6 5
4
1
2
3
1 16
2 15
3 14
4 13
5 12
6 11
7 10
8 9
Symmetry
ADJ.
Waveform
ADJ.
Ground
Sync
Output
Bypass
FKS
Input
Timing
Resistor
Timing
Capacitor
V+
Mult. Out
Sine/Saw
Output
AM Input
+1
Multiplier
and
Sine
Shaper
VCO
Current
Switches
–DC
Figure 6A
Figure 7
Figure 8