Elenco Variable Voltage Power Supply Kit - Manual (Page 9)

TESTING THE XP-15K POWER SUPPLY
Figure 1
Simplified diagram of positive power supply
120VAC
Input
18VAC 20VDC 0 - 15V
Regulated
Output
Transformer
120V to 18V
AC to DC
Converter
Voltage
Regulator
Figure 2
Voltage Waveform for Supply
A) Transformer
Winding AB
B) Transformer
Winding BC
C) Output of
diode D1.
D) Output of
diode D3.
E) Total of diodes
D1 & D3.
20V
F) Output of capacitor C1
Ripple depends on load
current (expanded).
-8-
Testing the XP-15K Power Supply is very simple.
Before applying power to the unit, be sure that all of
the wiring and soldering is firm. If so, obtain a digital
voltmeter. Apply power to the XP-15K and vary the
voltage control knob.
Next, short the output of the supply. It should turn off
and recover when the short is removed. If you have
a 4.7
Ω 1 watt resistor, set the voltage to 1.4 volts
and place is across the output terminals. The output
of the supply should not change more than 0.1 volts.
Set the output voltage to 15 volts and place a 75
Ω 5
watt resistor across the output terminals. Again, the
output should not change by more than 0.1 volts. In
making these tests, the voltmeter leads should be
clipped directly to the output terminals and not to
the load leads. This is to prevent errors due to
voltage drop in the load leads.
Should any of these tests fail, refer to the
troubleshooting guide.
CIRCUIT DESCRIPTION
INTRODUCTION
The XP-15K Power Supply features an output
voltage variable from 0 to 15V at 0.3 ampere
maximum current. The voltage is regulated to within
0.1V when going from no load to full load. Varying
the input AC voltage from 110 to 130V will have
practically no effect on the output voltage. This is
because of the specially designed IC circuit used in the XP-15K. Severe overloading or even short
circuiting the output will not damage the supply.
Special turn-off circuits in the IC sense the overload
and turn off the output.
Figure 1 shows a simplified circuit diagram of the
power supply. It consists of a power transformer, a
DC rectifier stage and the regulator stage.
TRANSFORMER
The transformer T1 serves two purposes. First, it reduces the 120VAC input to
18VAC to allow the proper voltage to enter the rectifier stage. Second, it isolates
the power supply output from the 120VAC line. This prevents the user from
dangerous voltage shock should they be standing in a grounded area.
AC to DC CONVERTER
The AC to DC converter consists of diodes D1 and D3 and capacitor C1.
Transformer T1 has two secondary windings which are 180 degrees out of phase.
The AC output of each winding is shown in Figure 2A and 2B.
Diodes are semiconductor devices that allow current to flow in one direction. The
arrow in Figure 3 points to the direction that the current will flow. Only when the
transformer voltage is positive will current flow through the diodes. Figure 3 shows
the simplest possible rectifier circuit. This circuit is known as a half wave rectifier.
Here, the diode conducts only half the time when the AC wave is positive as shown
in Figure 2C. Use of this circuit is simple but inefficient. The big gap between cycles
requires much more filtering to obtain a smooth DC voltage.
By the addition of a second diode and transformer winding, we can fill in the gap
between cycles as shown in Figure 4. This circuit is called full wave rectification.