10 TEGAM WAY • GENEVA, OHIO 44041 • 440
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466
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6100 • FAX 440
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466
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6110
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4-1
PRINCIPLES OF OPERATION
SECTION 4
PRINCIPLES OF OPERATION
Power Supply
The internal switching power supply U8 accepts nominal line
voltages between 90 and 250 VAC 50/60 Hz, allowing
operation worldwide. C1, C2, U1, R1, R2 and R101 form a
constant current regulator set to approximately 758 mA for
charging the battery. D5 is used to isolate the AC power for
the battery. D7 serves to clamp the voltage to less than 5 V
when the battery is disconnected. The negative 3.6 volt
power supply for U2 and U10 is developed by U5.
Test Current Path
The test current used to perform the resistance
measurement originates from the b terminal
connected to the +3.6 V power symbol at the emitter of Q1.
It flows through Q1 into the COM1 terminal of SW1 and out
of the terminal selected by the switch for the measurement
range chosen. Each pole of the COM1 section of SW1 is
connected to a different tap on the current sensing resistor
string made up of R13, R48, R50, R52 and R54. The test
current flows into the sense resistor string at the tap
selected, out of R54 into D3 and out of D3 to the I+ test
jack. The external test leads conduct the current through the
resistance being measured and back to the I- test jack,
where it returns to ground and then to the battery -
terminal.
Test Current Regulator
The test current is controlled to a constant value for each
range with a feedback loop. The main loop amplifier is U9A.
The output of U9A controls the conduction of Q1 using Q2,
such that if the output voltage of U9A increases, the test
current increases. The inputs of U9A compare the voltage
across the current sensing resistor string with a 3 volt
reference voltage produced by U4, the voltmeter IC. The
voltage across the sensing resistor string is maintained at
1.5 V, half the value of the reference voltage because R9 and
