Elenco Variable Voltage Power Supply - Manual (Page 4)

-3-
In practice, the current through the
diodes is not as shown in Figure 2E.
Because capacitor C1 has a charge
after the first cycle, the diode will not
conduct until the positive AC voltage
exceeds the positive voltage 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 a peak
current rating of 10 amps.
Figure 5
A) Transformer
Winding
B) Voltage C1
C) Current
through diodes
20V
Peak
20V
REGULATOR CIRCUIT
Figure 6
The regulator circuit in the Model XP-15 Power Supply consists of
a LM317 integrated circuit. This IC is specially designed to perform
the regulation function. Figure 6 shows a simplified circuit of how
the LM317 IC works.
1.25V
Output
R1
R2
Divider
Q1
Q2
1.5V
Q3
Q4
Q5
Current
Source
Equalized
to 1 Meg.