Tektronix 485, Instruction Manual

Page: 20 / 197

Circuit Description—485/R485 Service
DS1801 and DS1802 are line voltage transient pro­
tectors. With S I803 in the 115 V position, only DS1801 is
connected across the line. If a peak voltage surge in excess
of approximately 230 V is present on the line, DS1801 will
break down and conduct sufficient current to open the line
fuse F 1801. In the 230 V position, DS1801 and DS1802
are in series across the line to protect against voltage surges
exceeding 460 V.
a nominal resonant frequency of about 25 kilohertz. To
provide regulation of the voltages induced in the secondary
circuit, the action of regulator transistor Q1900 varies the
frequency of oscillation on the low side of resonance by
holding both Q1834 and Q1844 o ff for a time during each
half cycle as determined by the Regulator circuit (see
Inverter Regulator for a more detailed discussion of regula­
tion).
Transformer T1801 provides a sample of the line voltage
for triggering at line frequencies. It also provides a signal to
the Line Stop circuit to indicate the presence of line voltage
at the input to this circuit. T1825, C1825, and C1827
provide common-mode filtering. Cl 826 and LI 825 act as a
differential filter.
Start Network
Resistive divider R1828 and R1829 is connected be­
tween the input line and the negative side of C l 823. When
the line voltage goes positive. C l829 charges. A t the same
time CR1842 conducts, charging C l835, and CR1843
conducts to charge Cl 848. When the voltage on C l829
reaches approximately 32 V, VR1831 conducts, discharging
Cl 829 thru the base of Q1844. As Q1844 turns on. C l835
discharges thru LI 835 to start the Inverter. After operation
has begun, CR1842 discharges C l829 with each cycle of
the Inverter. This disables the start network.
Inverter
Refer to the simplified schematic shown in Fig. 3-2.
Once the Inverter has been started by the Start Network, the
Inverter is self-oscillating. Feedback necessary for oscilla­
tion is provided by base-drive transformer T1831. The
series-resonant circuit, consisting of L1835 and C1835, has
Transistors Q1834 and Q1844 are a switching pair,
where only one transistor can conduct at a time. The
direction of current flow in the feedback winding of T1831
determines which transistor w ill conduct. Transistors
Q1834 and Q1844 change states every half cycle. The
switching action provides a square-wave voltage at the
emitter of Q1834, which has a peak-to-peak voltage about
equal to the DC voltage from the Line Input circuit. This
square-wave voltage supplies the drive necessary to main­
tain oscillation in the resonant circuit. When both Q1834
and Q1844 are being held o ff by Q1900, resonant circuit
current flows through CR1834 or CR1844. The resonant
circuit current drives the primary of the power transformer
T1960 and thus supplies power to the Secondary circuit.
In normal operation, the sequence of events during one
cycle of operation is as follows:
1. Assume the current in the resonant circuit is at 0
ampere and beginning to increase in the direction to cause
conduction in CR1834. At the time the current reaches 0
ampere, regulator transistor Q1900 is turned on by the
Inverter Control circuit. The Regulator transistor holds
both Q1834 and Q1844 off for a controlled amount of
time. During this time, resonant circuit current flows
through CR1834.
1193-06
REV. C, AUG. 1975
3-7