-15-
In practice, the current through the diodes is not as shown in Figure 2C. Because capacitor C5 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 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 5C.
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 peak current rating of 10 amps.
Regulator Circuit
The regulator circuit in the Model XP-620 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. Transistor Q1 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 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. Transistors Q3 and Q4 are emitter followers. Their function is to raise
the impedance of the pass transistor. Note that transistors Q2, Q3, Q4 and 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 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.
Figure 5
Figure 6
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.