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operation close to a source of high electromagnetic fields should be avoided to prevent pick-up
which will be evident as noise in the output signal.
12 DETAILED CIRCUIT DESCRIPTION (SEE FIGURES 1 to 5)
12.1 POWER
An unregulated voltage from the battery charger appears via the 2.1 mm charger connector
between Pins 1 and 2 of PC36. Diode D1 provides protection against polarity reversal. IC1
provides a constant voltage of 6.7 V ±0.15 V at a maximum current of 1 A for battery charging.
Diode D2 prevents discharge of the battery when a charger is not in use.
Current is supplied to the voltage regulator and alarm control IC2 via the ON/OFF switch SW1.
This provides a regulated 5 V supply for the voltage converter IC3. Should the battery voltage
fall below 5 V, alarm A1 will sound under the action of connection RS output of IC2. IC3
provides a nominal voltage of ±12 V to power the magnetic field sensor. Additional filtering is
provided by the pi network comprising C5, L1, C6 and C7, L2, C8.
The power return is via the cable screen to Pin 6 of PC36. The power return contains a significant
dc offset error due to circulating current. For this reason the analog signal return is via a separate
conductor appearing on Pin 11 of PC36. The analog return is shared by X, Y and Z channels. To
prevent dc crosstalk the common return is buffered via an operation amplifier which is one quarter
of the quad Op Amp IC4. Capacitors C16 and C15 provide a boot strapped high impedance ac
connection to the low ac impedance cable screen.
12.2 FILTERS
Each analog channel X, Y and Z is connected to its respective buffered output via a high and low
pass filter. Each filter is a single section RC network providing -6dB per octave roll-off beyond
the -3dB cut off point. The 4.5 kHz low pass filter provides filtering of HF noise components and
precedes the 0.1Hz dc isolation high pass filter. The series capacitor of the high pass filter may be
short circuited by a link to provide a dc response.
13 FAULT FINDING
13.1 EQUIPMENT REQUIRED
The following test equipment will be required:-
(i)
A 3½ digit multimeter (DMM) preferably with a continuity tester.
(ii)
A cross head screw driver - medium size.
(iii)
6.8ohm 10W resistor.
If a fault is suspected in the magnetic field sensor or magnetic field sensor/power supply
combination, the following investigation of symptoms, together with fault diagnosis (i, ii, etc.) can
be carried out:-
OM0065 ISSUE 5 PAGE 8 of 21
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