iET 1863, User And Service Manual

THEORY

1863 and 1864 Meghommeters

21

Chapter 5

THEORY

5.1  Theory Overview

The 1863 and 1864 Megohmmeters consist of a 
regulated 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. Switch S

2

 is open 

only

 in the 

MEASURE

position.

The regulated voltage, E, is controlled by a resistance 
R

A

.  A fraction, E

M

 of the meter output voltage, 

E

X

R

S

/R

X

 is added to E to keep the voltage on the 

unknown, E

X

, more constant and thus improve the 

meter accuracy. A meter sensitivity resistor, R

B

 , is 

ganged to the voltage control resistor, R

A,

 to make 

the meter reading independent of applied voltage, (as-
suming 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 standard 
resistor by a factor of ten. In the 1863 the 200 M

Ω

resistor is multiplied to 2 G

Ω

; 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

The following paragraphs relate speci

fi

 c components 

from the schematic diagrams of the 1863 (Figure 7.6) 
and 1864 (Figure 7.9) to the general components 
shown in Figure 5.1.

5.2.1 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 
R127 (R

A

). These resistors along with the voltage 

(E

M

) developed across R138, determine the TEST 

VOLTAGE level. Resistors R211 through R219