TECHNICAL DESCRIPTION
4-3
The AC supply enters the instrument through a connector on the rear panel and passes through a
fuse and supply on/off switch. 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.
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 on/charge switch routes the output from the power supply to either power the instrument or
charge the battery.
The display
The display device is a color transflective TFT liquid crystal display module with an active
viewing area 143 mm x 80 mm. It comprises the display module, a driver board, and a cold
cathode fluorescent tube (CCFT) backlight.
The viewing area is composed of a 1200 X 240 RGB dot matrix. Each pixel is sequentially
addressed with a 6-bit data bus to determine its color and intensity. The display contrast is fixed,
but the brightness can be adjusted over a small range by varying the backlight drive. This is
achieved by generating a programmable voltage on the microprocessor board and feeding it to the
backlight inverter, mounted on the rear of the display module. The inverter produces
approximately 1500 V for the CCFT backlight. In high ambient light conditions the backlight can
be switched off to conserve battery power.
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