4-20 888-2408-002
WARNING: Disconnect primary power prior to servicing.
Overall System Theory
4.4.2 Low Voltage Power Supply
The Low Voltage Power Supply is a single PC board mounted in the rear of the
transmitter near the main contactor. It is designated A19 (assembly 19). The circuit
board is the same for the 3-Phase and single phase configurations, only the wiring to
and from the board is changed. It supplies DC voltages to all circuits in the
transmitter. These include:
• +20VDC
• -20VDC
• +10VDC
These voltages are sent to the Rectifier boards and then on to the controller boards.
The board contains 3 relays, A19K1, A19K2 and A19K3. When the transmitter is
turned on, A19K1 activates the main contactor (simply called K1) which then
applies AC to the PA power Supplies, while A19K2 applies AC to turn on the
exciter(s). A19K3 selects the low or high fan speed by actuating the fan speed select
relay K2 which is mounted near the main AC contactor.
4.5 Detailed Power Supply Descriptions
4.5.1 3-Phase PA Power Supply
Refer to the Overall System Block Diagram in the schematic package for the
following discussion. The transmitter 3 phase AC inputs connect directly to K1, the
AC mains contactor. The transformer side of the contactor is wired in parallel to
power transformers T1 and T2 primaries in the bottom of the transmitter. T1 and T2
primaries must be tapped for the proper AC input voltage. The tapping chart can be
found on the Overall System Block Diagram. T1 and T2 can accommodate input
voltages in the range of 198 to 250 Vac.
Each transformer has 2, four wire WYE, secondaries with the center connection
being the rectified DC output of 52Vdc. The two secondary outputs are combined
via L1 and L2, for T1, and L3 and L4 for T2. C1 and C2, 120,000uF filter caps,
provide the final filtering. Each of the transformer secondary taps is connected to a
Rectifier Board.
The secondaries of T1 and T2 use the unique tap switching design to give a small
window of coarse regulation (approximately 2Vdc change per tap) to compensate
for 3 phase AC line variations.
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