Revised August, 2004
10 TEGAM WAY • GENEVA, OHIO 44041 • 440-466-6100 • FAX 440-466-6110
•
4-3
PRINCIPLES OF OPERATION
changes gain with the test voltage. With 50 V, its gain is set
by S1B and R22 at 4.23x so the output at pin 1 is 106 mV.
With 500 V, the gain is decreased to 0.423x so the output
remains at 106 mV, even though the increased test voltage
increases the signal current by ten times.
As indicated above, the output at pin 1 of U1 is 106 mV for
full-scale resistor values of any range and with any test
voltage. For smaller resistors, this voltage increases. At 5%
of full scale, this voltage is approximately 2.12 V,
approaching its upper limit. If the value of resistance under
test is less than 5% of the selected range, the voltage at pin
1 of U1 will exceed 2.12 V. This voltage is connected to pin 5
of U1, where it is compared with the 2.5 V reference from
U2. If the voltage exceeds 2.5 V, the output at pin 7 of U1
goes positive, increasing the base current to the shunt
regulator Q1 and reducing the output high voltage. When
this happens, the normal regulating section of U1, with
output pin 8 (which is normally somewhat positive to control
Q1) turns off and pin 8 goes full negative, trying to increase
the value of the high voltage. This negative voltage is
coupled to the input pin of the A/D IC, U4, via diode D4.
Thus, the display will show a minus sign to indicate that the
resistor is too small for the selected range and a smaller
range should be selected.
U4 is a 3 ½ digit A/D converter and LCD driver, functioning
as a digital voltmeter. However, since the test current and
voltage are inversely proportional to the test resistance, the
digital voltmeter is operated as a ratio meter, with a fixed DC
voltage as the normal input to pin 31, and the test voltage
signal connected to the normal reference input, pin 36.
U5 is a 3 ½ digit liquid crystal display, driven segment by
segment by U4. S2B provides the proper logic levels to the
quad exclusive-or gate U6 to locate the decimal point on the
display. R33, 34 and 35 are pull-down resistors.
