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13-28
GROUP
13-CHARGING SYSTEM
this energises the base circuit of the
power transistors T2 through resistor
Rl. Refer in Lucas circuit diagram.
These transistors then become con-
ducting by virtue of the transistor
action characteristic of the semi-con-
ductor material employed in the
transistor and will permit current to
flow in the collector-emitter section
of the transistor and thus acts as a
closed switch to complete the field
circuit.
As the alternator rotor speed in-
creases the rising voltage generated
across the alternator field-feed diodes
is applied to the potential divider as
illustrated in the diagram and identi-
fied as R3
,
R2 and R4. According
to the position of the tapping point
on R2, a proportion of this potential
is applied to the Zener Diode
(ZD).
This latter device which opposes
the flow of current through itself
until a certain voltage (known as the
breakdown
voltage)
is
reached,
beyond which the current increases
very steeply for small increases in
voltage. The Zener diode can thus be
considered as a voltage conscious
switch which closes when the voltage
reaches
its "breakdown" voltage
(about 10 volts), and since this is a
known proportion of the alternator
output voltage as determined by the
position of the tapping point, the
breakdown point, therefore, reflects
the value of the output voltage. When
this point is reached, the Zener diode
conducts and current flows in the base
circuit of the driver transistor T.l.
Again by transistor action, current
will now flow in the collector-
emitter portion ofTl, so that some of
the current which previously passed
through Rl and the base circuit of
T2 is now diverted through Tl. Thus
the base current of T2 is reduced and,
as a
result,
so also is the alternator
field
excitation.
Consequently, the
alternator output voltage will tend
to
fall:
and this in turn will tend to
reduce the base current in T1 , allow-
ing increased field current to flow
in T2. By this
means,
the field
current is continuously varied to
keep the output voltage substantially
constant at the value determined by
the setting of
R2.
NEGATIVE FEED-
BACK CIRCUIT
Basically, this is the principle of
operation of the regulator, but there
are certain desirable additions. If the
field current were varied continuously
as described
,
considerable power dis-
9ATR Control
FIG. 36- Alternator
Vo·
l tage Test
-14
AC.
sipation would occur in the power
transistor, leading to problems of
overheating. For this reason, it is
desirable to operate the transistor
either in the fully-on or
fully~ff
condition, and this is the purpose of
the positive feed-back circuit com-
prising RS and C2. As the field
current in transistor T2 starts to fall,
the voltage of F rises and current
flows through resistor RS and cap-
acitor C2, thus adding to the Zener
diode current in the base circuit of
transistor Tl. This has the effect of
increasing the current through T1
and decreasing the current through
T2 still further.
This effect is cumulative and the
circuit quickly reaches the condition
when T1 is fully-on and T2 fully-off.
As capacitor C2 charges, the feed-back
current falls and eventually reaches
a value at which the combination
of Zener diode current and feed-back
current in the base circuit of Tl is no
lbnger great enough to keep T1 fully-
on. Current then begins to flow
again in the base circuit of T2, so that
the field current again begins' to
flow through T2. The voltage at
terminal F now commences to fall,
reducing the feed-back current even-
tually to zero. As T2 becomes yet
more conductive and the voltage at
F falls further, current in the feed-
back circuit reverses in direction, in
effect reducing still further the base
current in Tl. This effect also is
cumulative and the circuit reverts
to the condition where T1 is fully-off
and T2 fully-on. This condition is
only momentary, since C2 quickly
charges to the opposite polarity when
feed-back current is reduced the
current again flows in the base of Tl.
The circuit thus oscillates, switching
the voltage across the alternator field
winding rapidly on and off. This
method of operation results in con-
siderably low power
dissipation.
SURGE QUENCH DIODE
Because switching is achieved so
rapidly, any sudden collapse of the
field current would result in a very
high induced voltage being applied
to transistors T2. They are protected
by means of the surge quench diode
D connected across the field
winding.
This also serves to provide a measure
of field current smoothing, since
the current continues to flow in the
diode after the excitation voltage is
removed from the field
.
This current
decays by only a small amount be-
fore the excitation is again
applied.
RADIOINTEREFERENCE
SUPPRESSION
The elimination of radio inter-
ference caused by rapid switching
within the regulator is achieved by
connecting condenser Cl between
the base and collector terminals of
T1 to provide negative feed-back.
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