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The DC-DC inverter circuit has two built-in functions for
its safer operation, which are described below.
(i) Overload preventive function
After passing through F770, the supply voltage (12V
DC) is separated into three currents i,, i, and i;. the
current i is applied to the output transistor Q770 to
make it ready to be in operation. The current i, is
sent via R651 to pin @) of IC5O1 so that 1501 is in
operation the output of which appears at the point @):
at the same time, the transistor Q2 turns on to cause
an output signal at its collector. Further the current i;
is applied via C774 to the primary side of T770 the
output of which is instantly impressed onto the base of
Q770. With Q770 turned on, there is an instant output
of DC voltage between pins @) and (5) of T771 which
is applied to D775 for the rectification. At the same
time, there is also an instant output of DC voltage bet-
ween pins
and @) of T771 and is fed to D770 for
the rectification.
The output current from D770 is separated into two
currents i, and is: the former is sent via D773 and
R651 to IC5O1, and the latter to Q3.
Usually
the rectification
DC
output
from D770
is
about
18V, and since it is higher than the power
supply voltage (about 12 V), the voltage at the cathode
of D772 becomes higher than its anode so that this
diode D772 turns off.
However, if there occurs an overload at the output
circuit (due to its shortcircuit, for example), no voltage
is generated between pins
and (6) of T771 so that
the inverter circuit gets in a stop automatically: at the
time, however,
the oscillator circuit [501 is still in
operation since it is given i, from D772.
When the inverter circuit is stopped in this way, first
turn off the power
switch to discharge C774, and
about 10 second thereafter, turn on the power switch
again to cause the inverter circuit to restart.
(ii) Input reverse-connection preventive function
The diode D771 is connected in parallel to the power
supply circuit (12V DC): actually, the cathode and
anode of the former are connected respectively to the
plus (+) terminal and minus (—) terminal of the latter.
With this design, the diode is usually kept turned off as
far as the input connection of the car battery cord is
made in a proper way.
But if the input connection with reverse polarity is
made as shown in Fig. 4, D771 turns on and F770 is
shorted to be blown out. With F770 blown out, the
inverter circuit gets in a stop not suffering any further
trouble.
Figure 5 shows the other way of connection in which a
diode is connected in series to the power supply circuit
(12V DC). With this design, the diode turns off, with the
fuse being not blown out, if the reverse connection of the
car battery cord is performed. With the diode turned off,
the inverter circuit stops its operation thus suffering no fur-
ther trouble.
_ _
To vaoid this unnecessary
power consumption,
this set
employs the protective circuit shown in Fig. 4 instead of
that shown in Fig. 5.
Inverter
circuit
Figure 4.
Inverter
circuit
Figure 5.
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