INSTRUCTION MANUAL
MODEL 662
GUARDED DC
DIFFERENTIAL VOLTMETER
.
WARRANTY
We warrant each of our products to be free
from defects in material and workmanship. Our
obligation under this warranty is to repair or
replace any instrument or part thereof which,
within a year after shipment, proves defective
upon examination. We will pay domestic
surface freight costs.
To exercise this warranty, call your local
field representative or the Cleveland factory,
DDD 216-248-0400. You will be given assist-
ance and shipping instructions.
REPAIRS AND RECALIBRATION
Keithley Instruments maintains a complete repair service and standards laboratory in Cleveland, and has an authorized field repair facility
in Los Angeles and in all countries outside the
United States having Keithley field representatives.
To insure prompt repair or recalibration serv-
ice, please contact your local field representa-
tive or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bu-
reau of Standa’rds are available upon request.
c
MODEL 662 DIFFERENTIAL VOLTMETER
CONTENTS
_I
section
GENERAL DESCRIPTION ........
1.
TABLE OF CONTENTS
Page
1
Section Page
5-5. Adjusting Hum Bal Control . . 19
5-6. Zener Diode Replacement . . . 19
l-1.
1-2.
1-3.
1-4.
l-5.
1-6.
OPERATION . . . . . . . . . . . .
2.
2-1.
Description ........
Operating Modes .......
Applications ........
Specifications .......
Accessories .........
Equipment Shipped ......
Front Panel Controls
and Terminals. . . . . .
2-2.
Rear Controls and
Terminals . . . . . . .
2-3.
2-4.
2-5.
2-6.
Preliminary Procedures. .
Operating Procedures. . .
Recorder Output . . . . .
Effects Due to Kelvin-
Varley Divider Out-
put Resistance . . . . .
2-7.
Effective Input Resis-
tance . , . . . . . . .
2-8. Thermal EMF Precau-
tions . . . . . . . . .
2-9.
APPLICATIONS . . . . . . . , . . . 11
3.
AC Effects on Measurement
1
1
1
2
3
3
. . 5
. . 5
. . 5
. . 6
. . 6
. . 8
9
. .
9
. .
. . 10
. . 10
5-7. Meter Adjustment . , . . . . 19
CALIBRATION . . . . . . . . . . . . 21
6.
6-l. General . . . . . . . . . . . 21
6-2. Calibration Schedule. . . . . 22
6-3.
Kelvin-Varley Divider . . . .
Verification . . . . . . . . 22
6-4. Range Calibration . . . . . . . 23'
6-5. Reference Voltage Supply
Stability Test . . , . . . . 25
6-6. Oscillator Adjustment . . . . 27
ACCESSORIES . . , . . . . . . , . . 35
7.
7-l. Model 6601A High Voltage
Divider , . . . . . . . . . 35
7-2.
Rack Mounting . . . . . . . . 35
7-3. Placing in Rack , . . . . . . 36
8. REPLACEABLE PARTS . . . . . . . . . 37
8-l. Replaceable Parts List. . . . 37
8-2.
How to Order Parts. . . . . . 37
Model 662 Replaceable
Parts List . . . . . . . . . 38
3-l.
3-2.
Procedures for Measuring
Resistances . . . . . . . . 11
Theory of Measuring Resis-
tances . . . . . . . . . . , 11
Model 6601A Replaceable
Parts List . . . . . . . . . 44
Model 662 Schematic Diagram
185513 , . . . . . . . . . . 47
Model 6601A Schematic Diagram
CIRCUIT DESCRIPTION , . . . . . , . 13
4.
16321B . . . . . . . . . . . 49
4-l. General . , . . . . . . . . . 13
4-2. Reference Voltage Supply. . . 13
4-3.
4-4.
4-5.
SERVICING . . . . . . . . . . . . . 17
5.
5-l.
5-2.
5-3.
5-4.
Kelvin-Varley Divider . . . . 14
Null Detector . . . . . . . . 14
Guarding . . . . . . . . . . 15
General . , . . . . . , . . . 17
Servicing Schedule. . . . . . 17
Parts Replacement . . . . . . 17
Troubleshooting . . . . . . . 17
1165R
e Change Notice . . . . . . . . . . .Last
Page
* Yellow Change Notice sheet is in-
cluded only for instrument modifications affecting the Instruction
MaIlLlal.
MODEL 662 DIFFERENTIAL VOLTMETER GENERAL DESCRIPTION
SECTION 1.. GENERAL DESCRIPTION
1-1.
self-contained, guarded potentiometer.
0.01% limit of error, and below 100 millivolts within 10 microvolts. Since the limit of
error includes all stability considerations,
or manually restandardised for a full year.
impedance at null for measurements from 0 to 500 volts.
age setting has at least 5-dial resolution.
lighted decimal points; 10 to 25-millivolt recorder output; input polarity switch; floating
operation up to 500 volts off chassis ground; line frequency rejection greater than 45 db.
Also, full guarding minimizes leakage problems.
l-2. OPERATING MODES.
vacuum tube voltmeter.
500 volts with 20.01% limit of error and from 100 microvolts full
within 10
500 volts with an accuracy of r3% of full scale. It can also measure resistances from 1
megohm to 100,000 megohms within 'sS%.
(.. 13
- . APPLICATIONS.
DESCRIPTION.
a. The Keithley Model 662 Guarded dc Differential Voltmeter is an extremely precise
It measures from 100 millivolts to 500 volts with
the unit need not be periodically calibrated
The 500-volt reference supply permits infinite
Six dials assure that every volt-
Features for convenient use include:
b.
The Model 662 can be used as a potentiometer
six in-line readout dials with automatically
or
as a conventional
As a potentiometer, it measures from 100 millivolts full scale to
scale to 100 millivolts
microvolts.
As a VTVM, the Model 662 measures from 1 millivolt full scale to
a. The Model 662 meets exacting requirements in research, development and production
for accurate voltage measurements.
It
can be used with a recorder to detect source in-
stabilities down to 25 ppm over weeks of continuous measurements.
1065R
FIGURE 1.
Keithley Instruments Model 662 Guarded dc Differential Voltmeter.
1
GENERAL DESCRIPTION MODEL 662 DIFFFRRNTIAL VOLTMETER
Typical applications include calibrating power supp.lies and meters; monitoring noise,
b.
transients and drift in precise voltage sources , such as sener diodes and electro-chemical
cells.
c. The null-detector output permits use with potentiometric recorders and digital volt-
meters equipped with automatic printout.
The Model 662 is useful in quality control,
product development, inspection and production.
L-4. SPECIFICATIONS.
AS
A POTENTIOMETER :
~'3
LIMIT OF ERROR:
TO.Ol% of reading or 10 microvolts, whichever is greater, after 30-minute
warm-up.
LONG-TEPX STABILITY:
Will operate within stated limit of error for one
TEMPERATURE COEFFICIENT:
REPEATABILITY:
Within 0.0025%.
KAXIHUMNULL SENSITIVITY:
INPUT RESISTANCE:
FLOATING OPERATION:
INPUT ISOLATION:
Infinite at null, from 0 to 500 volts.
500 volts maximum off chassis ground.
Circuit ground to chassis ground: LOS ohms shunted by 0.05 microfarad.
RFSOLUTION CHART:
Input Voltage
Range,
volts
year.
Does not exceed 0.001% per OC.
100 microvolts fuL1 scale with 3-microvolt resolution.
Maximum Usable
Maximum Dial
Resolution,
millivolts
Full-Scale Null
Sensitivity,
millivolts
Meter
Resolution,
microvolts
.\,
./
50 - 500
5
- 50
0.5 - 5
0 - 0.5 0.01
AS A VACLUJM TUBE VOLTMRTER:
VOLTAGE RANGES:
NULL RANGES:
TCVM ACCURACY:
0.5 volt full scale to 500 volts in four decade ranges,
100 microvolts full scale to LOO volts in seven decade ranges.
i3% of fulb scale on all ranges,
sive of noise and drift.
ZERO DRIFT:
INPUT RESISTANCE::
Less than 10 microvolts per 24 hours, non-cumulative, after 30-minute warm-up.
50
megohms, 0.5 to SOO-volt ranges;
10 megohms, O.l-volt range;
1 megohm, 0.1 to lo-millivolt ranges.
2
1
0.1
0.01
1
0.1
0.1
0.1
10
3
3
3
except 5.5% on 100-microvolt range, exclu-
1065R
-l'
-‘.,
!’
\ !
MODEL 662 DIFFERENTIAL VOLTMETER
LINE FREQUENCY REJECTION: Greater than 45 db.
GENERAL:
GENERAL DESCRIPTION
LINE STABILITY: Better than 5
ppm for 10% change in line voltage.
RECORDER OUTPUT:
output:
Adjustable 10 to 25
Output Resistance:
Noise:
Note:
POIARITY:
2 microvolts peak-to-peak referred to input up to 1 cps.
Recorder used must have fully isolated input, 1010 ohms minimum to ground.
Positive or negative, selectable by switch.
CONNECTORS: Input:
POWER:
105-125 or 210-250 volts (switch selected), 50-400 cps, 50 watts.
DIMENSIONS, WEIGHT:
300 ohms
Binding posts.
5-l/2" high x 17-l/2" wide x 13-l/2" deep; net weight, 25 pounds.
millivolts dc for full-scale meter deflection.
maximum.
output: Banana jacks.
1-5. ACCESSORIES.
a. Model 6601A High Voltage Divider is a 1OO:l divider which extends the range of the
Model 662 to 5000 volts.
megohms.
The overall limit of error of the Model 662 with the Model 6601A is 20.02%.
The divider accuracy is 'rO.Ol% and its input resistance is 10
Section 7 gives operating instructions for the Divider.
Model 4000 Rack Mounting Kit,
b.
Model 662 to fit standard 19-inch racks.
high x 19 inches wide x 13-l/2 inches deep.
containing two brackets and a top cover, converts the
Rack mounted,
the Model 662 is 5-l/4 inches
Section 7 has assembly instructions.
l-6.
EQUIPMENT SHIPPED.
The Model 662 Guarded dc Differential Voltmeter is factory-cali-
brated and is shipped with all components in place. All units are shipped for bench use.
Model 4000 Kit may be ordered for rack mounting; refer to Section 7 for assembly instruc-
tions. The shipping carton also contains the Instruction Manual.
1065R
GENERAL DESCRIPTION
MODEL.662 DIFFERENTIAL VOLTMETER
FIGURE 2.
the Replaceable Parts List and the Schematic Diagram.
Model 662 Front Panel Controls and Terminals.
Circuit designations refer to
Newer Model 662's have a toggle
switch in place of the OVEN Pilot Light to turn on the instrument.
4
FIGURE 3. Model 662 Rear Controls and Terminals.
1165R
MODEL 662 DIFFERENTIAL VOLTMETER
SECTION 2. OPERATION
OPERATION
2-l.
FRONT PANEL CONTROLS AND TERMINALS.
POLARITY Switch. The POLARITY Switch selects the input polarity by reversing the
a.
(See Figure 2.)
polarity of the internal reference voltage supply; thus, both positive and negative voltages may be measured. The Switch does not reverse meter polarity. On older models, the
Polarity Switch also turns the instrument on.
NULL Switch.
b.
The NULL Switch sets the null detector sensitivity for seven decade
ranges from 0.1 millivolt full scale to 100 volts.
Newer models have a
When the Switch is in the VTVM posi-
toggle
switch.
tion, the Model 662 operates as a conventional vacuum tube voltmeter for the four ranges
of the RANGE Switch.
c. RANGE Switch.
50 and 500 volts full scale.
divider and the position of the decimal point light - which also
The RANGE Switch adjusts VTVM sensitivity in four steps:
It also determines the voltage across the Kelvin-Varley
serves
as a pilot
0.5, 5, ’
light - between the six Reference Voltage Dials.
Reference Voltage Dials.
d.
Six in-line dials at the top of the front panel set the
reference voltage when the Model 662 is used as a potentiometer.
e. METER ZERO Control. The METER ZERO Control adjusts the meter needle to zero. The
Control is needed on only the 0.1 and l.O-millivolt null ranges; on the other ranges, the
needle will normally be on zero without adjustment. The Control has a range of approxi-
mately 230 microvolts.
Input Terminals.
f.
The red HI Post is for connections to the high impedance terminal
and the black LO Post is for connections to the low impedance terminal of the unknown
voltage. A second set of binding posts marked LO and GND is provided for grounding the
LO input terminal to the chassis when desired. The LO terminals are connected together
internally.
NOTE
Older models have an OVEN Pilot Light which indicates when the oven controlling
the temperature for the zener diode in the reference voltage supply is on.
The
slight clicking sound is characteristic of the oven cycling.
2-2.
REAR CONTROLS AND TERMINALS. (See Figure 3.)
Fuse. For 105-125 volt operation, the Model 662 uses a l-ampere 3 AG fuse. For
a.
210-250 volt operation, the Model 662 uses a 0.5-ampere 3 AG fuse.
Power Cord.
b.
The 3-wire power cord with the NEXA approved 3-prong plug provides a
ground connection for the cabinet. A 3:2 prong adapter is also provided.
NULL DETECTOR OUTPUT. Two terminals, marked + and -, supply a dc signal from the
C.
null detector.
/
OUTPUT ADJUST.
d.
A screwdriver control next ‘to the OUTPUT terminals adjusts the null
detector output between 10 and 25 millivolts full scale.
106SR
5
OPERATION
MODEL 662 DIFFEREXTIAL VOLTMETER
e. 117-234 Switch.
or 234~volt ac power lines.
2-3. PRELIMINARY PROCEDURES.
Check the 117-234 Switch and the Fuse'for the proper ac line voltage.
a.
power cord.
b. Set the Model 662 as follows:
BARGE Switch
NULL Switch
POLARITY Switch
Reference Voltage Dials
The decimal light between the third and fourth Dials will light. Allow the instrument to
warm
the METER ZERO Control.
662 is such that no adjustment should be required after a 30-minute warm-up;
2-4. OPERATING PROCEDURES.
up for 30 minutes to meet the specified accuracy on all ranges.
With the input terminals open,
c.
The screwdriver-operated slide switch sets the blodel 662 for 117
Connect the
500
VTVM
+
Zero
set the NULL Switch to 0.1 MV and zero the meter with
Then return the NLTLL Switch to VTVM. The stability of the Model
-3
a. The Model 662 is used first ,as a VTVM to determine the approximate value of the
unknown voltage,
20.01%.
It is then used in the potentiometric mode to determine the voltage to
[7] 000000
E” 0000
Potential -
A
FIGURE 4.
measuring at ground and for floating.
In A, the unknown voltage has one terminal at ground.
the LO and GND Posts of the Model 662.
In B, the unknown voltage has both terminals off ground potential. Note this floating
or off-ground potential must be Less than 500 volts.
used.
Input Connections to Hodel 662.
The two diagrams show the input circuit for
Also note the shorting link is n&
B
The shorting link is between
.-*
6
0665
MODEL 662 DIFFERENTIAL VOLTMETER
OPERATION
Cl
NOTE
The Model 6601A High Voltage Divider extends the Model 662 range to 5000 volts.
Refer to Section 7 for operating instructions.
VTVM Operating Procedures.
b.
Eleven full-scale ranges are available for VTVM operation.
1.
is at VTVM, the RANGE Switch determines one of four full-scale ranges. By putting the
six Reference Voltage Dials at Zero, the Model 662 can then operate as a VTVM on the
seven null ranges.
Connect the unknown voltage to the input terminals,
2.
impedance terminal of the unknown. Refer to Figure 4.
Switch the RANGE Switch to the most sensitive range for an on-scale meter deflec-
3.
tion.
c. Potentiometric Operating Procedures.
NOTE
Avoid large overload voltages on the null detector. No permanent damage will
occur even with 500-volt overloads, but some open circuit offset will be caused
in the null detector.
capacitors,
will disappear after about five minutes.
The offset, due to the polarization of the input filter
using the LO Post for the low
When the NULL Switch
Leave the RANGE Switch at the last setting used in the VTVM operation. If the
1.
VTVM reading is negative,
Set the first two.Reference Voltage Dials to the first two digits of the unknown
2.
voltage found in the VTVM operation.
Set the NULL Switch to the initial
3.
null setting shown in Table 1, Adjust
the Voltage Reference Dials progressively
for zero meter deflection while increasing
the null detector's sensitivity with the
NULL Switch.
indicate the voltage being measured is
more positive than the Reference Voltage
Dial setting.
The most accurate resistors in the Kelvin-Varley divider are in the first two
Reference Voltage Dials. Therefore,
the first two dials as much as possible.
The value of the unknown voltage is read directly from the Reference Voltage Dials.
4.
Deflections to the right
reverse
the POLARITY Switch position.
RANGE Initial Most Sensitiv
Switch
Setting
500 v 100 v
50 v 10 v
5v 1v
0.5 v 100 MV
TABLE 1.
and Settings.
NOTE
to obtain the most accurate readings, use
NULL Switch NDLL Switch
Setting Setting
Recommended Null Sensitivities
10 MV
1.0 Mv
0.1 MV
0.1 MV
1065R
a) The Dial reading will be within the specified limit of error if the NDLL Switch
7
OPERATION MODEL 662 DIFFERENTIAL VOLTMETER
is at the most sensitive setting (Table 1) for the range used and if the meter indicates as close to null as possible.
Null does not have to be reached.
b) Readings can be made from only five Reference.Voltage Dials to be within spec-
ifications (20.01% of reading or 10 microvolts).
Using the sixth Dial allows the
instrument to approach null closer, further reducing loading effects upon the source.
2-5. REcoBsm OUTPUT.
a. Recommended recorders for use with the Model 662 are the F. L. Moseley Autograf
680 series recorder and the Minneapolis Honeywell recorder (LOmv-0-1Omv scale, 50 kilohms
input resistance). Any recorder used
input must be fully isolated (1.010
Before attaching the recorder , set all Reference Voltage Dials to zero. Disconnect
b.
must
be able to float 500 volts off ground and its
ohm minimum leakage resistance to ground).
the unknown voltage and short both Model 662 input terminals. Set the NULL Switch to
10 MV. Connect the recorder to the OUTPUT terminals on the Model 662 rear panel.
Set the Reference Voltage Dials to LO millivolts to apply an accurate LO-millivolt
c.
potential to the null detector on the lo-millivolt null range. This will provide a fullscale recorder output which can be matched to the recorder’s range between 10 and 25 milli-
volts by adjusting the OUTPUT ADJUST Control.
To obtain accurate results and/or to prevent damage to the instruments, the recorder
d.
must be able to float off-ground with the Model 662. Leakage and pickup between the two
instruments should also be minimized.
wake sure neither recorder terminal
1.
is grounded. Use a 3-wire grounded power
line for the recorder. If a 2-wire line
is used, connect the recorder chassis and
the Model 662 chassis with a separate lead.
Model
1 Red
662
Minimize all sources of leakage be-
2.
DEigj =zf + ;:j:::
tween the output terminals, the recorder
and ground. Use polystyrene or Tefloninsulated wire where possible. If the
connecting wires are shielded, connect
the shield to the LO Post.
Avoid long leads between the Hodel
3.
662 and the recorder.
.
FIGURE 5. Recorder Filter. A filter be-
1 Black
tween the Model 662 and the recorder may
4.
ground
ings ,
If difficulty is encountered in off-
measurements,
such as unsW7le read-
connect a lo-microfarad ca:?scitor
be necessary when using the O.l-millivolt
null detector range,
between the LO and GND terminals .rn the
Model 662 front panel.
.
j
/
NOTE
Do not short either Node1 662 ,.,‘:tput terminal to the case; this may damage the
Kelvin-Varley divider.
../
0665
:
MODEL 662 DIFFERENTIAL VOLTMETER
OPERATION
i-:: e.
filter between the Model 662 and the recorder.
Note the filter
2-6. EFFECTS DUE TO KELVIN-VARLEY OUTPUT RESISTANCE.
a. When the Model 662 is used for nulling on the O.l-millivolt range, the last Reference Voltage Dial may appear to be inaccurate.
drop across the Kelvin-VarLey divider, This effect involves only the null detector sensitivity and not the accuracy of the Dial setting. When the Model 662 is as near to null
as possible, the Reference Voltage Dial setting is correct within the instrument's specified limit of error. There is no effect present at null.
b. The effect is most apparent on the 0.1,
on the O.l-millivolt range a O.l-millivolt off-null setting of the reference voltage will
not produce a full-scale meter deflection. This is because the Kelvin-Varley divider output resistance is significant compared to the shunt resistance across the null detector
meter. The IR drop across the divider will cause the meter to be off up to 6%, depending
upon the Reference Voltage Dial settings. On the lOO-millivolt range the maximum error
is 1%.
c. The amount of deflection on the meter is equal to the ratio
If there is substantial recorder jitter on the O.l-millivolt null range, place a
Refer to Figure 5 for this connection.
must
also be insulated from ground,
The apparent error is due to a voltage
1 and lo-millivolt null ranges. For example,
The effect cannot be observed on the other null ranges of the Model 662.
R*
%-I
+
NW
5
where R,, is the shunt resistance across the meter (50 megohms for the 100 to l-volt null
ranges, 10 megohms for the lOO-millivolt range,
and 1 megohm for the 10 to~O.l.-
millivolt ranges);
Rkv
is the output resistance of the Kelvin-Varley divider, which is a maximum of
62.4 kilohms at Reference Voltage Dial settings of 2 4 5 4 5 5 and 2 5 4 5 5 5
and a minimum of 100 ohms at settings of 4 9 9 9 9 8 and 0 0 0 0 0 2.
NOTE
'For a fuller treatment of the effect source resistance has on measurements,
send for the Keithley Instruments Product Note, "The Effective Input Resis-
tance of Potentiometric Voltmeters."
2-7.
EFFECTIVE INPUT RESISTANCE.
The input resistance of the voltmeter for the seven null ranges varies from 50 to
a.
1 megohm as given in Table 2. This, however, is not the Model 662's effective input resistance. Its input resistance is considerably higher due to the potentiometric principle of operation. When the reference voltage (Reference Voltage Dial setting) is much
greater than the meter reading, the value is:
Fin"
&I %
V
Equation 1
where R.
Eh*is the setting of the Reference Voltage Dials in volts;
Rn is the shunt or input resistance of the null detector meter in ohms;
V is the null detector meter reading in volts.
1065R
is the effective input resistance of the Model 662;
9
UPERATION
To find the loading effect the Model 662 will have on a circuit, use equation 1 to
b.
compute the effective input resistance.
finite.
Off null, the input resistance is usually high compared to the.internal resis-
At null, where V=O, the input resistance is in-
MODEL 662 DIFFERENTIAL VOLTMETER
tance of the unknown voltage, and loading will not be enough to affect the measurement
accuracy.
The graph in Figure 6 shows the Model 662 effective input resistance for the
three most sensitive null ranges as a percent of meter reading off null (V/Ed).
'2
2-a.
THRlWAL KMF PRECAUTIONS.
thermal emf
errors
for measurements using the most sensitive null ranges. Since the
Observe standard thermocouple techniques to reduce
Model 662 can read to 3 microvolts, thermal emf's can introduce considerable errors into
the measurements. In general, use pure copper leads throughout the system when measuring
fn the microvolt rsnge. For extensive measurements in the microvolt region, request the
article, DC Microvolt Measurements, from'Keithley Instruments, Inc., or its representative.
2-9.
unknown voltage,
AC EFFECTS ON i%ASDREMENTS. To minimize errors from ac signals present in the
the Model 662 employs a chopper-stabilized null detector operating at a
42-cps chopping rate with a 3-section R-C filter at the input. Very large ac components
on the measuring lines, however, may reduce off-null sensitivity. Also, heavy 60-cps
pickup will be observed as needle quiver.
If ac components affect measurements by the
Model 662, additional filtering is required. For a single-frequency ac signal, a twin-
T filter is effective.
For a variable frequency signal,
use an ordinary low-pass filter.
Voltage Input
Range
Resistance
,
'
300 volts 50 megohms
50 volts 50 megohms
5 volts 50 megohms
0.5 volts 50 megohms
Input
Null
Range
100 volts
10 volts
1 volt
100 millivolts
10 millivolts
1 millivolt
0.1 millivolt
TABLE 2.
by Ranges.
Model 662 VTVM Input Resistance
Resistance for Null Ranges is
Resistance
( Skwing)
50 megohms
50 megohms
50 megohms
10 megohms
1 megohm
1 megohm
1 megohm
when the Reference Voltage Dials are set
to zero.
FIGURE 6.
Model 662 Off-Null Effective Input Resistance. The graph shows the effective input resistance for the 0.1 to lo-
millivolt ranges.
Percent off null is V/Ed "
as defined in Equation 1.
IO
1265R
MODEL 662 DIFFERENTIAL VOLTMETER
APPLICATIONS
APPLICATIONS
The Model 662 can be used to rapidly measure
3-l.
SECTION 3.
PROCEDURRS FOR MEASURING RRSISTANCES.
resistances from 1 megohm to 100,000 megohms with an accuracy of 25%. To measure resistance,
isolated lead to the RI terminal to prevent measuring leakage between the leads.
RANGE Switch to 500.
a.
adjust the Reference Voltage Dials to obtain a full-scale meter deflection.
connect the unknown resistor across the Model 662 RI and LO terminals. Use a short
Set the
Then determine the value of the resistor as follows:
For resistances between 1 megohm and 1000 megohms,
set the NULL Switch to 10 volts;
Subtract
10.000 from the Dial setting and multiply the difference by 5 to obtain the value of the
resistor in megohms.
b.
volt;
For resistances between 1000 megohms and 10,000 megohms,
adjust the Reference Voltage Dials to obtain a full-scale meter deflection.
set the NULL Switch to 1
sub- '
tract 1.0000 from the dial setting and multiply the difference by 50 to obtain the value
of the resistor in megohms.
For resistances between 10,000 megohms and 100,000 megohms, set the NULL Switch to
c.
1 volt; adjust the Reference Voltage Dials to obtain a convenient deflection on the meter.
Calculate the value of the resistor using,
Rx = 50 Ed megohms
v
Where Rx is the unknown resistance;
Ed is the Reference Voltage Dial setting in volts;
V is the meter reading in volts.
3-2.
THEORY OF MEASURING RESISTANCES. The
above method for determining the value of an
unknown resistor is based upon the equation
for the circuit.
(See Figure 7;) If an unknown resistance is across the Model 662
input terminals, then
Ed = i (Rx + Rn)
Equation 3
Where Ed is the Reference Voltage Dial set-
ting in volts;
i is the current in the circuit;
Rx is the unknown resistance;
Rn is the input resistance of the null
detector meter in ohms.
The current can be written i = V/R,,, where
V
is the null detector meter reading in
volts.
Equation 3 now becomes
Equation 4
Equation 2
Ed
I
FIGURE 7.
Simplified Model 662 Circuit for
Measuring Resistances. Rr is the unknown
resistance. Rn is the input resistance of
the null detector; V is the null detector;
Ed is the buckout voltage.
1065R
11
APPLICATIONS
MODEL 662 DIFFERENTIAL VOLTMETER
If measurements are made on the 1 to LOO-volt null ranges, the input resistance, Rn, is
50 megohms.
Equation 4 becomes
Rx"5x107
Equation 5
This is the basis for the simplified calculations in paragraph 3-1.
.'-J
~2
7
12
1065R
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