Q7804 through R7809, causes Q7804 to cut
low-resistance path to them. When tran-
off. When transistor Q7804 is cut off, the
sistor Q7801 is biased into conduction, it
voltage at the base of dc switching tran-
provides a much lower path to ground than
s i s t o r Q7805 is increased. Transistor
the diode network, and the diode network
Q7805 starts conducting and causes a
is effectively removed from the circuit.
relatively low resistance path to ground
through R7815, CR7802, Q7805, and GREEN
10. Schmitt Trigger Circuit
indicator lamp DS7901 and causes DS7901
to light.
The Schmitt trigger circuit consists of a
d. Resistors R7806 and R7808 and lamps
dc amplifier, a Schmitt trigger, a dc switch,
DS7902 and DS7901 are the collector loads
and a pair of indicator lamps. The circuit
for Q7802 through Q7805 respectively.
is used in all positions of selector switch
Resistor R7807 is the emitter-base bias
S7902.
resistor for Q7802. Resistor R7811 es-
a. Transistor Q7802 is an emitter-fol-
tablishes the emitter bias for Q7803 and
lower, dc amplifier. Transistors Q7803
Q7804. Resistors R7809 and R7812 estab-
and Q7804 form the schmitt trigger. Tran-
lish the base bias for Q7804, and R7813
sistor Q7805 is used as a dc switch.
and R7814 establish the base bias for
Q7805. Resistor R7815 establishes the
b. In the absence of an input signal (de)
emitter bias for Q7805. Diode CR7802 pre-
or when a small input signal is present,
vents any small signal present at the base
transistor Q7804 conducts and transistors
o f transistor Q7805 from biasing the
Q7802, Q7803, and Q7805 are cut off.
transistor into conduction while Q7804 is
Transistor Q7804 conducts and a relatively
conducting. When Q7805 is conducting,
low-resistance path to ground is estab-
diode CR7802 presents a very small im-
lished through R7811, Q7804, and RED
pedance to the current.
indicator 1amp DS7902, causing lamp
DS7902 to light.
11. Line Voltage Sensing Circuit
C. When a large input signal (de) is ap-
plied to the base of Q7802, the transistor
s biased into conduction. Since the base of
The line voltage sensing circuit consists
Q7803 in the Schmitt trigger is directly
of a voltage divider, a pulse generator, and
connected to the emitter of Q7802, tran-
a dc switch which senses the minimum
sistor Q7803 is also biased into conduction.
limit of the power source voltage for the
When Q7803 conducts, the voltage at the
TS-1777/VRM-l. The circuit is used when-
base of Q7803 decreases. This decreased
ever power is applied to the TS-1777/
voltage, which is applied to the base of
VRM-1.
Figure 6. Voltage supply limits sensing circuit, schematic diagram.
