NX15/NX25 Operations and Maintenance Manual
Description
Page 1-2
Issue 6.0 2019-01-31
Ac-Dc Power Stage
See electrical schematic
SD-1
. The ac/dc power supply stage contains the input power
transformer that receives the main ac input to the transmitter. It also contains the transmitter’s
low voltage power supplies and rack interface PWB. The ac/dc power supply stage also
includes a three-phase SCR rectifier assembly, B+ current sensor, B+ distribution assembly,
capacitor tray assembly and arc detector assembly.
Ac Power Transformer
The NX15/NX25’s power transformer can be set to use a range of input voltages. See
3, “Installing the power transformer” on page 3-1
in the
NX15/NX25 Installation Manual
.
High Voltage B+ Power Supply
The secondary of the power transformer is applied to the SCR rectifier assembly to create the
transmitter’s B+ voltage (normally 400 Vdc). The output of the rectifier assembly is fed
through the B+ current sensor, which supplies a B+ current sample to the rack interface PWB.
The rectifier assembly output is applied to a choke input filter and the B+ distribution
assembly, which then provide the B+ voltage to the RF power modules.
The B+ discharge relay, which is part of the capacitor assembly, opens when ac power is
applied and closes and ac power is removed, applying a high power discharge resistor across the
B+ bus to quickly discharge the voltage. When the discharge relay closes, a microswitch
ensures the SCR rectifier is immediately inhibited.
Low Voltage Power Supplies
One phase of the power transformer’s secondary is also used to provide a source for the low
voltage dc power supplies and an ac sample for the rack interface PWB. The low voltage power
supply outputs (+48 V, +15 V and +12 V) are distributed throughout the transmitter via the
rack interface PWB. The +48 V power supply sources the transmitter’s cooling fans. An
inhibit signal, controlled by the rack interface PWB, is applied to the +48 V power supply to
turn the fans on and off. The +15 V power supply sources all of the low voltage control
circuitry. The +12V power supply sources the SBC and the touchscreen monitor.
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