Bose 1800-V Manual page 14

2.3 Magnetic Field Power Supply
The main power supply for the 1800-V is a dual-voltage design which provides no-load voltages of +/-106
and +/-53 volts DC. The triac drives the primary of the magnetic field power transformer by operating as a
phase controlled switch; its gate signal depends on the signal supplied to opto-isolator U3 located on the
regulator board. U4B provides steering for the photodiac in U3, allowing the triac to fire on both
alternations of the power line.
U2 on the Regulator Board provides AC to DC conversion, with the AC line voltage providing the input to
the converter through limit resistors R3/R4, and the 12.5VDC output determined by feedback resistor R6.
This voltage provides the positive supply for U4, DC reference for comparators U4C and U4D, and the
current through opto-isolator U1 which sets the voltage for the voltage-to-current converter U4A. Note that
U4 (MC3405) is a dual op-amp and dual-voltage comparator in a singe package (U4A/U4B are op-amps
and U4C/U4D are open collector comparators).
2.4 Start-up Sequence
When the power switch is OFF (S1 across E1 and E2) and the linecord is connected to an AC voltage, D1
illuminates (STANDBY LED). When the power switch is turned ON (S1 across E2 and E3), the LED goes
off and AC H1 is supplied to the Regulator Board.
D2 and D3 on the Regulator Board provide overvoltage protection to U4. U4B-14 is a full-wave rectifier
that outputs positive pulses to comparator U4C. The reference voltage is set at 0.7VDC by R10 at pin 3.
Where pin 2 crosses the threshold, the output of U4C-1 goes open and C6 begins to charge through R21
and U4A-8.
U4A is a voltage-to-current converter (Howland current pump), whose output current is determined by the
voltage at the junction of R12 and R13. As the voltage increases, the charging current to C6 increases.
The voltage on C6 is connected to comparator U4D-6. The reference voltage is set at 0.7VDC by R22 at
pin 5. When pin 6 crosses the threshold, the output of U4D-7 goes open and base current is supplied to
Q1 through R14. Q1 turns on, and current flows through the LED portion of U3, illuminating it and turning
on the diac. This applies voltage to the gate of triac Q1 on the Power Supply Board which fires it and
allows it to conduct current through the primary side of the power transformer.
When the output of U4B-14 drops below the threshold voltage of U4C-3, U4C-1 goes low and C6
discharges rapidly through it. When the voltage on C6 drops below the threshold voltage of U4D-5, U4D-7
goes low and Q1 and U3 turn off, removing the gate voltage to the triac momentarily interrupting the
current through the primary side of the transformer.
The triac is switched on and off every half-cycle of the 60Hz AC line. Thus, the triac switches the AC line
current off at a rate twice the line frequency, at the instant the line current crosses the zero axis. The triac
will then remain off for a number of degrees of the sinusoid, before switching on again. The phase angle
at which the triac switches on is the "firing angle" of the triac.
This produces enough voltage to the primary of the power transformer to allow the secondary regulator
stage to begin to operate.
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