Elenco Deluxe Digital / Analog Trainer with Tools Kit Version - Manual (Page 24)

In practice, the current through the diodes is not as
shown in Figure 2C. Because capacitor C1 has a
charge after the first cycle, the diode will not conduct
until the positive AC voltage exceeds the positive
charge in the capacitor. Figure 5 shows a better
picture of what the current flow looks like assuming
no loss in the diode. It takes a few cycles for the
voltage to build up on the capacitor. This depends
on the resistance of the winding and the diode. After
the initial start-up, there will be a charge and
discharge on the capacitor depending on the current
drawn by the output load. Remember, current only
flows through the diode when the anode is more
positive than the cathode. Thus, current will flow in
short bursts as shown in Figure 5.
The DC load current may be one ampere, but the
peak diode current may be three times that.
Therefore, the diode rating must be sufficient to
handle the peak current. The 1N4001 has a peak
current rating of 10 amps.
REGULATOR CIRCUIT
The regulator circuit in the power supply consists of
a LM-317 integrated circuit. This IC is specially
designed to perform the regulation function. Figure 6
shows a simplified circuit of how the LM-317 IC
works.
Transistors Q1 and Q2 form a circuit known as a
differential amplifier. The base of transistor Q1 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 of current. Transistor Q2 sees the
current source as a very high resistor of about 1 meg
ohms. Thus, the gain of transistor Q2 is extremely
high.
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
transistors 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, when the base of Q2 goes
positive, the output at emitter Q5 goes negative. Now
if the 2 volt output voltage goes down because of
current drain at the output, the base of Q2 will drop,
forcing the collector voltage to go higher. This will
bring the output voltage back to 2 volts. This is the
basis of all negative feedback regulators.
Another feature of the LM-317 regulator if 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 is basically a 1.25 volt regulator. To
be able to vary the output from 0V to 20V, you stack
the IC on the negative 1.25VDC voltage as shown in
Figure 7. When VR1 equals 0, the output voltage is
0 volts.
THE NEGATIVE VOLTAGE REGULATOR
The theory of the negative 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-317 IC is designed to operate
from a negative supply.
-23-
Figure 5
Figure 6
Figure 7
A) Transformer
winding
B) Voltage C1
C) Current
through diodes
20V
Peak
20V
2V
Output
R1
R2
Divider
Q1
Q2
1.5V
Q3
Q4
Q5
Current
source
equalized
to 1 meg.
0V - 20V
R1
VR1
LM-317
–DC