Operating instructions

OPERATING MANUAL 2945A
46882-311D 4-3
An additional requirement is for a charging facility to provide a trickle charge to the DC supply
when the instrument is working from the AC supply and a recharge facility from the AC supply
when the instrument is not operating.
The AC supply enters the instrument through a connector on the rear panel and passes through a
fuse and two poles of a triple pole, double throw switch. This switch selects the operate condition,
the charge condition or off. The supply then enters the power supply module where it is fed to a
bridge rectifier in the AC-DC converter to produce an unregulated DC supply. The voltage of this
will depend on the supply voltage as the full range of AC input voltage is covered without range
switching.
The second stage of the AC-DC converter produces semi-regulated DC supplies of 12 V or 24 V
using a 60 kHz switched mode oscillator and transformer coupling. This transformer also provides
the safety isolation barrier.
The DC external or the DC supply from the AC-DC converter is used to drive the DC-DC
converter.
The DC output circuits producing the four output supplies are each fed from an individual winding
on the DC-DC converter output transformer.
Regulation is applied to the DC-DC converter from the output current and voltage sensing circuits.
The charging supply circuits are contained within the DC-DC converter.
Current monitoring to provide regulation is obtained from the three common-return supplies and
voltage monitoring from the +5 V supply.
The 36 V is generated by adding a 24 V floating supply onto the +12 V supply rail.
The floating 24 V supply has a voltage regulator configured within it.
A control circuit PCB contains the components for frequency control and regulation of both
converters.
The third pole of the power on-off and charge switch is connected to the DC-DC converter circuits
through plug and sockets. The DC voltage range selector switch, fitted to the rear panel, is
similarly connected.
In the ‘charge’ position the DC-DC converter is turned off, allowing the full output of the AC-DC
converter to be available for charging a 12 V lead-acid battery.
The display
The display device is a module containing:- a liquid crystal display with an active viewing area
156 mm x 78 mm; a logic board which processes data from the microprocessor to assemble the
display information; and a back lighting system to provide illumination for the viewing area.
The viewing area is composed of a 400 X 200 dot matrix with a white background which turns blue
where activated. The logic within the display assembly, using latches and shift registers, addresses
the 80,000 pixels of the display from 3 control lines and 4 data lines.
The display contrast is dependent on the voltage supplied to the liquid crystal display matrix. This
AC voltage is derived within the module from a DC voltage supplied from the microprocessor
board B2. It is an infinitely variable voltage fed from a digital to analogue converter, allowing the
contrast to be set for optimum viewing.
Illumination for the display is provided by a cold cathode fluorescent tube (CCFT) built into the
display module. The supply for the back light is from three op-amps located on the microprocessor
board B2. The op-amps are gated by the microprocessor through a latch to provide four levels of
brightness and an off condition.
The DC supply is fed to a CCFT invertor which provides the control voltages for the tube.