IET Labs, Inc. 1864-1644, User And Service Manual

Page: 29 / 46

THEORY
1864-1644 Positive Polarity Megohmmeter
19
Chapter 5
THEORY
5.1 General
The 1864 Megohmmeter basically consists of a regu-
lated dc power supply, a set of precision resistors,
and a FET-input voltmeter (Figure 5.1). Switch S
1
is
closed in the DISCHARGE position of the function
switch and open in the CHARGE and MEASURE
positions, while S
2
is open only in the MEASURE
position.
The regulated voltage, E, is controlled by a resistance
R
A
. A meter sensitivity resistor, R
B
, is ganged to the
voltage control resistor, R
A,
to make the meter read-
ing independent of applied voltage, (assuming that
the unknown has no voltage coef
fi cient). An inverse
scale is used on a reversed meter to give a reading
proportional to R
X
(and not its reciprocal) and yet
have a scale that increases from left to right (0 to ∞ ).
Metal- fi lm standard resistors are used on all ranges.
The top range of each instrument uses feedback to
effectively multiply the value of the previous stan-
dard resistor by a factor of ten. In the 1864 the 2 G
Ω
resistor is multiplied to 20 G
Ω
. The speci fi cations are
again broadened to allow for the tolerance variations
of this multiplication.
The voltmeter uses a FET-input, four-stage, unity-gain
ampli
fi er (AMP, Figure 5.2) to obtain high stability
and low drift. The SET ∞ control on both instruments
is a voltage balance control, while the SET ∞ HIGH-
EST RANGE control compensates for the FET gate
current on the highest ranges.
5.2 Circuit Description
5.2.1 General
The following paragraphs will relate speci fi c com-
ponents from the schematic diagrams of the 1863
(Figure 7.6) and 1864 (Figure 7.9) to the general
components shown in Figure 5.1.
5.2.2 Type 1863 Megohmmeter (Figure 7.6)
The voltage supply section (RECT.) of the 1863
consists of
fi ve different circuits, three dc and two ac.
One ac circuit is a voltage source for the three pilot
lamps used, two to indicate the measurement range
(P101, P102) and the third to light the
DANGER
indicator (P103). The second supplies fi lament volt-
age to the vacuum tube V101.
The fi rst dc supply is a half-wave recti fi er circuit
with a 24-V Zener diode (CR 111) that supplies
voltages to the ampli
fi er (AMP) circuit. A second dc
supply is a voltage doubler (CR101-CR104, C101-
C102) that supplies the plate voltage to V101. The
voltage to the plate is the same for the 50- to 250-V
ranges but R109 is eliminated from the circuit for the
500 V range. The third dc supply is a half-wave
recti
fi er with a 20-V Zener diode (CR211) to supply
voltage levels to run the unity-gain ampli fi er (+1).
Tube V101 is a series regulator that is controlled by
the 5.6 V Zener diode (CR112, REF) and the setting
of R140.The voltage picked off R 140 is fed into
one side (Q102) of the differential ampli fi er (Q102,
Q103) while part of the output voltage is fed into the
other side (Q103). The output of the ampli
fi er is fed
to the base of Q101 (AMP) and then to the grid of
V101 for controlling the output voltage.
The output selection resistors are R124 through
R133. These resistors determine the TEST VOLT-
AGE level. Resistors R211 through R219 are the
standard resistors (R
S
) that determine the measure-
ment range. The output from this circuit is fed
through the
SET ∞ HIGHEST RANGE control
(R241) to the FET ampli fi er.