Elenco Variable Voltage Power Supply - Manual (Page 5)

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Transistors Q1 and Q2 form a circuit known as a differential
amplifier. Transistor Q1’s base is connected to a stable 1.5V
reference voltage. The base of Q2 is connected to the regulator
output circuit through a voltage divider network. The collector of
transistor Q2 is connected to a current source. This basically is a
PNP transistor biased to draw about 1mA current. Transistor Q2
sees the current source as a very high resistor of about 1 meg
ohms. Thus, the gain of transistor Q2 is very high.
Transistor Q5 is called the pass transistor. It controls the current
reaching the output. Transistors 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 closed loop.
Also, note that the feedback to the base of Q2 is negative, that is,
the output at emitter Q5 goes negative. Now, if the 1.25V output
voltage goes down because of current drain at the output, the base
of Q2 will drop, forcing the collector voltage of Q2 to go higher. This
will bring the output voltage back to 1.25V. This is the basis of all
negative feedback regulators.
Another feature of the LM317 regulator is to protect the IC against
overload and output shorts. If the IC is overloaded, the junction will
overheat. A transistor will sense this overheating and shut down
transistor Q5.
The LM317 IC is basically a 1.25V regulator. To be able to vary the
output 0 - 15V, we stack the IC on the negative 1.25VDC voltage
as shown in Figure 7. When R3 equals 0, the output voltage is 0
volts.
0V - 15V
R2
R3
LM-317
–DC
Figure 7