Elenco 015V Power Supply Kit - Manual (Page 4)

FILTERING
Filtering is the process of
smoothing
out AC hills
produced by the full wave
rectifiers. The circuit consists
of the 1000µF capacitor C1. A
capacitor is an electrical device
that stores electrical energy. If
two metal plates are placed
very close to each other and
are connected to a battery, a
current will flow momentarily,
even though there is no
connection between the plates
(see Figure 8A). If the battery
is removed, a voltage will remain on the plates. These
plates are now charged and will act like a battery.
Current can be drawn from this capacitor until the
charge is consumed. Typical capacitors are shown in
Figure 8B.
The capability of storing a charge on a capacitor is
measured in farads (F). Most capacitors used in
electronic power supplies are rated in microfarads
(µF) which means one millionth of a farad. Typical
values are 100 to 10,000µF. Capacitors used in power
supplies are called electrolytics because of their
design. An electrolytic is used in a power supply to
smooth out the rectified AC voltages. During the peak
voltage, energy is supplied to the capacitor and
released during the valleys between the peaks (see
Figure 9). The current through the diode will initially
be very high until the electrolytic is charged. Then,
little bursts of current are
needed to supply the
current going to the load.
Remember, current will
only flow through the
diode when the driving
voltage is greater than
the
DC across the
electrolytic.
REFERENCE VOLTAGE
An important element of a well regulated power supply
is a dependable reference voltage. This gives a stable
voltage which does not vary with the load current. The
component used to make this voltage in Elenco’s
TM
supply is called a zener diode (D3).
In our discussion of diodes, we talked about current
flow in one direction, but none when we reverse the
voltage. If, in the reverse direction, we keep
increasing the voltage, the diode will breakdown and
current will eventually flow. This breakdown is called
zener voltage. Manufacturers have learned to control
this breakdown voltage and supply zener diodes for
almost any voltage. Figure 10 shows the zener circuit
used in our power supply. The DC filter output voltage
will vary between 18 minimum and a higher voltage,
depending on load current. The voltage across the
zener diode will remain constant at 17V. This constant
voltage is important to control the output of the
regulators as
you will see in
the following
section.
VOLTAGE REGULATOR
Figure 11A shows the circuit of the voltage regulator. It
consists of two transistors and a variable resistor. Our
objective is to control the output of Q1 to give the
desired voltage of 0-15 volts. This output should not
change with different loads.
To understand the circuit operations, we must have a
little knowledge of transistors. Briefly, the voltage at
the emitter (Figure 11B) of a transistor will be .7V less
than the voltage on the base. If we put 10 volts on the
base, then 9.3 volts will appear on the emitter. The
emitter voltage will stay fixed, even though the
collector voltage may vary. Thus, increasing or
decreasing the collector voltage will not effect the
emitter voltage, providing the base voltage stays fixed.
If we increase the load on the transistor, more current
will flow. This increased current will produce a drop in
the filter DC voltage across the electrolytic, but the
zener diode will hold the reference fixed and thus the
output voltage of Q1 will be constant. This process is
called Regulation. A good regulator will not change
the output voltage with load current changes.
To vary the output voltage of transistor Q1, we place a
variable resistor R2 to divide the output from the zener
diode. Thus, we can vary the 17 volt zener output
between 0-17 volts and control Q1’s output. Transistor
Q2 is used to reduce the current drawn from the zener
circuit.
-3-
Reference
Voltage
18V min. 17V
R1
D8
Filtered Output
Voltage at C1
Figure 8B
Battery
Figure 8A
Radial
Axial
Figure 9
Voltage without
Capacitor
Voltage with
Capacitor
Current in
Diode
18VP
Figure 10
Figure 11A
Figure 11B
Collector Emitter
Base
Variable
Resistor
Q1
R2
Load
Filtered
17 Volts min.
Q2
Zener
Res. Volt