Tektronix 660A Instruction Manual
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 (except
tubes and batteries) 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 factory, DDD 216.
795-2666. You will be given assistance 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.
To insure prompt repair or recalibration serv-
ice, please contact your local field representatkve or the plant directly before returning the
instrument.
Estimates for repairs, normal recalibrations,
and calibrations traceable to the National Bureau
of Standards are available upon request.
MODEL 660A DIFFERENTIAL VOLTMETER
TABLE OF CONTENTS
CONTENTS
Section
GENERAL DESCRIPTION
1.
1-1.
1-2.
l-3.
l-4.
1-5.
1-6.
OPERATION.
2.
2-l.
2-2.
2-3.
2-4.
2-5.
2-6.
2-7.
2-8.
2-9.
APPLICATIONS . . . . . . . . . .lL
3.
3-1.
3-2.
CIRCUIT DESCRIPTION . . . . . . . .13
4.
4-l.
4-2.
4-3.
4-4.
4-5. Guarding . . . . . , . .15
Description .........
Operating Modes .......
Applications ........
Specifications .......
Accessories .........
Equipment Shipped ... ...
............
Front Panel Controls
and Terminals . . . . . . . 5
Rear Controls and
Terminals . . . . . . . . . 5
Preliminary Procedures . . 6
Operating Procedures . . . . 6
Recorder Output . . . . . . . 8
Effects Due to Kelvin-
Varley Divider Out-
put Resistance . . . . . . . 9
Loading and Off-Null
Resistance . . . . . . . . . 9
Thermal EMF Precau-
tions . . . . . , . . . . .lO
AC Effects on Measurement . .lO
Procedures for Measuring
Resistances . , . . . . .
Theory of Measuring Resis-
tances . . . . . I . . . . .ll
General . . . . , . . . . .13
Reference Voltage Supply . .13
Kelvin-Varley Divider . . . .14
Null Detector . . . . . . . .14
........
Page
1
1
1
1
2
3
3
5
.lL
Section
5-5.
5-6.
5-7. Meter Adjustment . . . . . 19
6. CALIBRATION . . . . . . . . . 21
6-l.
6-2.
6-3.
6-4. Range Calibration , . . . . 23
6-5.
6-6.
7.
ACCESSORIES , . . . . . , . . . . 35
7-L. Model 660LA High Voltage
7-2. Rack Mounting . . . . . . . 35
7-3.
8.
REPLACEABLE PARTS , . . . . . . : 37
8-l. Replaceable Parts List . . 37
8-2. How to Order Parts . . . . 37
$0 Change Notice
Adjusting Hum Bal Control . 19
Zener Diode Replacement . . 19
General . . . . . . . . . . 21
Calibration Schedule . . . 22
Kelvin-Varley Divider
Verification . . . . . . . 22
Reference Voltage Supply
Stability Test . . . . . . 25
Oscillator Adjustment . . . 27
Divider . , . . . . , , . 35
Placing in Rack . . . . . 36
Model 660A Replaceable
Parts List . . . . . . . . 38
Model 6601A Replaceable
Parts List . . . . . . . . 44
Model 66OA Schematic Diagram
182693. . . . . . . . . . 47
Model 6601A Schematic Diagram
1632LB . . . . . . . . . 49
Page
SERVICING . . . . . . . . . . . .17
5.
5-l.
5-2.
5-3.
5-4.
1065R
General . , . . . . . .17
Servicing Schedule . . , . .17
Parts Replacement . . . , . .17
Troubleshooting . . . , . .L7
-'- Yellow Change Notice sheet is included only for instrument modifications affecting the Instruction
Manual.
MODEL 660A DIFFERENTIAL VOLTMETER
GENERAL DESCRIPTION
SECTION 1.
l-1.
contained potentiometer. It measures from 100 millivolts to 500 volts with 0.02% limit
of error, and below 100 millivolts within 20 microvolts. Since the limit of error includes
all stability considerations, the unit need not be periodically calibrated or manually
restandardized for a full year. The 500-volt reference supply permits infinite impedance
at null for measurements from 0 to 500 volts.
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.
vacuum tube voltmeter.
to 500 volts with i-0.02% limit of error and from 100 microvolts full scale to LOO milli-
volts within 20 microvolts. As a VTVM, the Model 660A measures from 1 millivolt full
scale to 500 volts with an accuracy of -f3% of full scale.
tances from 10 megohms to 100,000 megohms within i-5%.
DESCRIPTION.
The Keithley Model 660A Guarded dc Differential Voltmeter is a convenient, self-
a.
Features for convenient use include:
b.
OPERATING MODES, The Model 660A can be used as a potentiometer or as a conventional
As a potentiometer,
GENERAL DESCRIPTION
five in-line readout dials with automatically
it can measure from 100 millivolts full scale
It can also measure resis-
1-3. APPLICATIONS.
The Model 660A is used for measurements over a wide range.
a.
the microvolt region are the matching of semiconductors and the monitoring of noise, transients and drift. High level voltage applications include power supply and meter calibra-
tion, and tube potential measurements.
Typical applications in
1.065R
FIGURE I..
Keithley Instruments Model 660A Guarded dc Differential Voltmeter.
1
GENERAL DESCRIPTION
Its Long-term stability makes the Model 660A useful in measurements of extended
b.
duration.
Measurements of Long-term power supply stability, Long-term drift runs, and
MODEL 660A DIFFERENTIAL VOLTMETER
monitoring during environmental and reliability tests are possible uses.
c. The null-detector output permits use with potentiometric recorders and digital volt-
meters equipped with automatic print-out.
product development,
Floating operation to 500 volts is provided for measurements such as plate potential
d.
inspection and production.
The Model 660A is useful in quality control,
differences of balanced amplifiers.
l-4. SPECIFICATIONS.
AS A POTENTIOMETER:
LIMIT OF ERROR:
+0.02% of reading or 20 microvolts,
warm-up.
LONG-TERM STABILITY:
TFXPERATURE COEFFICIENT:
Will operate within stated Limit of error for one year.
Does not exceed 0.002% per OC.
REPEATABILITY: Within 0.005%.
MAXIMUM NULL 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.
LOO microvoltb full scale with 2-microvolt resolution.
RESOLUTION CHART:
Input Voltage
Range >
volts
- 500
50
5 - 50
0.5- 5
0 - 0.5
i
Maximum Dial
Resolution,
millivolts
whichever is greater, after 30-minute
Maximum Usable
Full-Scale Null
Sensitivity,
millivolts
10
1
0.1
0.1
Meter
Resolution,
I
microvoLts
LOO
LO
2
2
AS A VACUUM TUBE VOLTMETER:
VOLTAGE RANGES:
NULL RANGES:
100 microvolts full scale to 100 volts in seven decade ranges.
VTVM ACCURACY:
0.5 volt full
t3% of full scale on all ranges, exclusive of noise and drift.
scale
to 500 volts in four decade ranges.
ZERO DRIFT: Less than 10 microvolts per 24 hours, non-cumulative, after 30-minute warm-up.
2
0665R
MODEL 660A DIFFERENTIAL VOLTMETER
GENEPaL DESCRIPTION
INPUT RESISTANCE:
50 megohms, 0.5 to 500-volt ranges;
10 megohms, O.l-volt range;
1 megohm, 0.1 to lo-millivolt ranges.
LINE FREQUENCY REJECTION:
Greater than 45 db.
GENERAL:
LINE STABILITY:
Better than 5 ppm for 10% change in Line voltage.
RECORDER OUTPUT:
output:
Output Resistance:
Noise:
Note:
Adjustable 10 to 25 millivolts dc for full-scale meter deflection.
300 ohms maximum.
2 microvolts peak-to-peak referred to input up to 1 cps.
Recorder used must have fully isolated input, 1010 ohms minimum to ground.
POLARITY: Positive or negative, selectable by switch.
CONNECTORS:
POWER:
Input:
105-125 or 210-250 volts (switch selected), 50-400 cps, 45
DIMENSIONS, WEIGHT:
Binding posts.
Output:
5-l/2 inches high x 17-l/2 inches wide x 13-l/2 inches deep; net
Banana jacks
weight, 24 pounds.
watts.
l-5. ACCESSORIES.
a. Model 6601A High Voltage Divider is a LOO:L divider which extends the range of the
Model 660A to 5000 volts. The divider accuracy is ?O.Ol% and its input resistance is
10 megohms.
The overall limit of error of the Model 660A with the Model 660lA is ?0.03%.
Section 7 gives operating instructions and Section 8 contains the Replaceable Parts List
and the Schematic Diagram for the Divider.
Model 4000 Rack Mounting Kit,
b.
Model 660A to fit standard 19-inch racks,
high x 19 inches wide x 13-l/2 inches deep.
1-6.
EQUIPMENT SHIPPED.
The Model 660A Guarded dc Differential Voltmeter is factorycalibrated and is shipped with all components in place.
use.
Model 4000 Kit may be ordered for rack mounting; refer to Section 7 for assembly
containing two brackets and a top cover, converts the
Rack mounted, the Model 660A is 5-l/4 inches
Section 7 has assembly instructions.
All units are shipped for bench
instructions. The shipping carton also contains the Instruction Manual.
1066R
3
GENERAL DESCRIPTION
MODEL 660A DIFFERENTIAL VOLTMETER
PIGURE 2.
to Replaceable Parts List and the Schematic Diagram.
Model 660A Front Panel Controls and Terminals.
Circuit Designations refer
4
FIGURE 3.
Model 660A Rear Controls and Terminals.
J
1065R
MODEL 660A DIFFERENTIAL VOLTMETER
SECTION 2. OPERATION
OPERATION
2-1.
a.
FRONT PANEL CONTROLS AND TERMINALS.
Power Switch. A toggle switch turns the instrument on when it is set to the ON
(See Figure 2.)
position.
POLARITY Switch.
b.
The POLARITY Switch selects the input polarity.
The Switch reverses the polarity of the internal reference voltage supply so both positive and negative
voltages may be measured;
c. NULL Switch.
The NULL Switch sets the null detector sensitivity for seven decade
ranges from 0.1 millivolt full scale
the Model 660A operates as a conventional
it does not reverse the meter polarity.
to
100 volts.
vacuum
When the Switch is in the VTVM position,
tube
voltmeter
for the four ranges of the
RANGE Switch.
d. RANGE Switch. The RANGE Switch adjusts the sensitivity of the VTVM in four steps:
0.5, 5, 50 and 500 volts full scale.
Varley divider and the position of the decimal point light -
It also determines the voltage across the Kelvin-
which also serves as a pilot
light - between the five Reference Voltage Dials.
e. Reference Voltage Dials.
reference voltage when the Model 660A is used as a potentiometer.
Five in-line dials at the top of the front panel set the
The last dial is con-
tinuously variable.
METER ZERO Control. The METER ZERO Control adjusts the meter needle to zero. The
f.
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 approx-
imately i30 microvolts.
Input Terminals. At the lower left front panel, the black LO Post is for connections
25.
to the low impedance terminal of the unknown voltage and the red HI Post is for connections
to the high impedance terminal. 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.
2-2.
a.
REAR CONTROLS AND TERMINALS.
Fuse. For 105-125 volt operation, the Model 660A uses a l-ampere 3 AG fuse. FOK
(See Figure 3.)
210-250 volt operation, the Model 660A uses a 0.5-ampere 3 AG fuse.
Power Cord.
b.
unless otherwise specified on the rear panel.
The Model 66OR is designed for a 105-125 volt, SO-&O0 cps line source,
The 3-wire power cord with the NEMA approved
3-prong plug provides a ground connection for the cabinet. An adapter for operation from
2-terminal outlets is provided.
c. NULL DETECTOR OUTPUT.
Two terminals, marked + and -, supply a dc signal from the
null detector.
d. OUTPUT
ADJUST.
+4 screwdriver control next to the OUTPUT terminals adjusts the null
detector output between 10 and 25 millivolts full scale.
1066R
5
OPERATION
MODEL 660A DIFFERENTIAL VOLTMETER
e. 117-234 Switch.
or 234-volt ac power
2-3.
power cord.
The decimal light between the third and fourth Dials will light.
warm up for 30 minutes to meet the specified accuracy on all ranges.
the METER ZERO Control.
660A is such that no adjustment should be required in eight hours after a 30-minute warm-up.
2-4.
PRELIMINARY PROCEDURES.
Check the 117-234 Switch and the Fuse for the proper ac line voltage.
a.
b. Set the Model 660A as follows:
Power Switch
RANGE Switch 500
NULL Switch
POLARITY Switch
Reference Voltage Dials
c. With the input terminals open,
OPERATING PROCEDURES.
The screwdriver-operated slide switch sets the Model 660A for 117
lines.
Connect the
ON
VTVM
+
zero
Allow the instrument to
set the NULL Switch to 0.1 MV and zero the meter with
Then return the NULL Switch to VTVM.
The stability of the Model
The Model 66OA is used first as a VTVM to determine the approximate value of the un-
a.
known voltage. It is then used in the potentiometric mode to determine the voltage to iO.OZ"/,.
LH LG
A
I
FIGURE 4.
measuring at ground and ,for floating.
In A, the unknown voltage has one terminal
LO and GND Posts of the Model 660A.
In 13,
or off-ground potential
used.
Input Connections to Model 660A.
the unknown voltage has both terminals
must
be less than 500 volts. Also note the shorting link is not
0
I
I
Unknown
Voltage -
The two diagrams show the input circuit for
at
ground. The shorting link is between the
off
ground potential. Note this floating
0000
Off-Ground,
Potential -
B
,
m
-
6
1066R
MODEL 660A DIFFERENTIAL VOLTMETER
The Model 6601A High Voltage Divider extends the Model 660A range to 5000 volts.
Refer to Section 7 for operation instructions.
b. VTVM Operating Procedures.
OPERATION
NOTE
Eleven full-scale ranges are available for VTVM operation.
1.
is at VTVM, the RANGE Switch determines one of four full-scale ranges.
five Reference Voltage Dials at zero,
seven null ranges.
Connect the unknown voltage to the input terminals,
2.
impedance terminal of the unknown.
Switch the RANGE Switch to the most sensitive range for an on-scale meter deflection
3.
c. Potentiometric Operating Prbcedures.
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 5 minutes.
1. Leave the RANGE Switch at the last setting used in the VTVM operation.
VTVM reading is negative,
Set the first two Reference Voltage
2.
Dials to the first two digits of the unknown voltage found in the VTVM operation.
The offset, due to the polarization of the input filter
reverse the POLARITY Switch position.
the Model 660A can then operate as a VTVM on the
using the LO Post for the low
Refer to Figure 4.
NOTE
Initial Most Sensitive
NULL Switch NULL Switch
When the NULL Switch
BY putting the
Setting
If the
3. Set the NULL Switch to the initial
null setting shown in Table 1.
the Voltage Reference Dials progressively
for zero meter deflection while increasing
the null detector's sensitivity with the
NULL Switch. Deflections to the right TABLE 1.
indicate the voltage being measured is and Settings.
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, to obtain the most accurate readings, use
the first two dials as much as possible.
4. The value of the unknwon voltage is read directly from the Reference Voltage Dials.
a) The Dial reading will be within the specified limit of error if the NULL Switch
is at the most sensitive setting (Table 1) for the range used and if the meter indi-
cates as close to null as possible.
1066R
Adjust
Recommended Null Sensitivities
NOTE
Null does not have to be reached.
7
OPERATION
MODEL 660A DIFFERENTIAL VOLTMETER
b) When the first Reference Voltage Dial is used,
to be within specifications (fO.OZ% of reading or 20 microvolts).
may be read as an approximation of a sixth digit.
c) When the first Reference Voltage Dial is not used, read the voltage directly
from the remaining four Dials.
d) Use the meter as a null indicator when balancing voltages. When the first
Reference Voltage Dial is
However,
tative inaccuracies when the meter
2-5. RECORDER OUTPUT.
Recommended recorders for use with the Model 660A are the F. L. Moseley Autograf
a.
680 series recorder and the Minneapolis Honeywell recorder (lOmv-O-10mv scale, 50 kilohms
input resistance).
input must be fully isolated (lOlo
Before attaching the recorder, set all Reference Voltage Dials to zero.
b.
the unknown voltage and short both Model 660A input terminals.
10 MV.
c.
potential to the null detector on the LO-millivolt null range.
scale recorder output which can be matched to the recorder’s range between 10 and 25 milli-
volts by adjusting the OUTPUT ADJUST Control.
Connect the recorder to the OUTPUT terminals on the Model 660A rear panel.
Set the Reference Voltage Dials to 10 millivolts to apply an accurate lo-millivolt
the loading effect of Kelvin-Varley divider on the meter causes some quanti-
Any recorder used must be able to float 500 volts off ground and its
not
used,
ohm minimum leakage resistance to ground).
the meter approximates a fifth dial reading.
is
off null.
only the five Dials need be read
HOWeVer,
(See paragraph Z-6.)
Set the NULL Switch
This will provide a full-
the meter
Disconnect
to
To obtain accurate results and/or to prevent damage to the instruments, the recorder
d.
must be ablt to float off-ground with the
Model 660A. Leakage and pickup between the
two instruments should also be minimized.
Make sure neither recorder terminal
1.
is grounded. Use a 3-wire grounded power
line for the recorder. If a Z-wire line
is used,
the Model 660A chassis with a separate lead.
2.
tween the output terminals, the recorder
and ground. Use polystyrene or Teflon-
insulated wire where possible. If the
connecting wires are shielded, connect
the shield to the LO Post.
3. Avoid long leads between the Model the Model 660A and the recorder may be neces660A and the recorder.
4.
cauwct a lo-microfarad capacitor between the LO and GND terminals on the Model 660A
front panel.
connect the recorder chassis and
Minimize all sources of leakage be-
I
FIGURE 5.
sary when using the O.l-millivolt null detector range.
If difficulty is encountered in off-ground measurements, such as unstable readings,
Model ;
660A ’
Null :
Detector I
output ’
TBlack
Recorder Filter. A filter between
10 pf
i
1
1
T
1
Recorder
Terminals
4
9
MODEL 660A DIFFERENTIAL VOLmTER
NOTE
Do not short either Model 660A output terminal to the case; this may damage the
Kelvin-Varley divider.
If there is substantial recorder jitter on the O.l-millivolt null range, place a
e.
filter between the Model 660A and the recorder.
Note the filter must also be insulated from ground.
Refer to Figure 5 for this connection.
OPERATION
2-6.
ence Voltage Dial may appear to be inaccurate.
across the Kelvin-Varley divider.
and not the accuracy of the Dial setting.
sible, the Reference Voltage Dial setting is correct within the instrument's specifications.
There is no effect present at null.
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 out-
put resistance is significant compared to the shunt resistance across the null detector
meter. The LR 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%. The effect cannot be observed on the other null ranges of the Model 660A.
where R, is the shunt resistance across the meter (50 megohms for the 100 to l-volt null
EFFECTS DUE TO IaLVIN-VARLEY OUTPUT RESISTANCE.
When the Model 660A is used for nulling on the O.l-millivolt range, the last Refer-
a.
The apparent error is due to a voltage drop
This effect involves only the null detector sensitivity
When the Model 66OA is as near to null as pos-
b. The effect is most apparent on the 0.1,
The amount of deflection on the meter is equal to the ratio
c.
ranges, 10 megohms for the 100~millivolt range,
millivolt ranges);
1 and lo-millivolt null ranges. For example,
and 1 megohm for the 10 to O.l-
Rkv is rhe 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 and 2 5 4 5 5 and
a minimum of 100 ohms at settings of 4 9 9 9 8 and 0 0 0 0 2.
2-7. LOADING AND OFF-NULL RESISTANCE.
a. The input resistance of the voltmeter for the seven null ranges varies from 50 to
1 megohm as given in Table 2.
tance of the Model 660A. Its input resistance is considerably higher due to the potentiometric principle of operation.
Where Rin
1066R
is the effective input resistance of the Model 660A;
Ed is the setting of the Reference Voltage Dials in volts;
R, is the shunt or input resistance of the null detector meter in ohms;
V is the null detector meter reading in volts.
This resistance, however, is not the effective input resis-
The value is given by
Rin = Ed Rn
V
Equation 1
9
OPERATION
MODEL 660A DIFFERENT,IAL VOLTMETER
b. To find the loading effect the Model
660A will have on a circuit, "se Equation 1
to compute the effective input resistance.
At null, the input resistance is infinite.
Off null, the input resistance is usually
high compared to the internal resistance
of the unknown voltage, and loading will
not be enough to affect the measurement
.XC"iZXY.
Voltage Dials are set at 1.0000 volt on the
l-millivolt null range for a reading off
null by 10% of full scale, the Model 660A
input resistance is lolo ohms.
2-S.
standard thermocouple techniques to reduce
thermal emf errors for measurements using
the most sensitive null ranges.
Model 660A can read to 2 microvolts, thermal
emf's can introduce considerable errors
into the measurements.
pure copper leads throughout the system
when measuring in the microvolt range.
extensive measurements in the microvolt region, request the article, DC Microvolt
Measurements, from Keithley Instruments,
Inc., or its representative.
THERMAL EMF PRECAUTIONS. Observe
For example, if the Reference
Since the
In general, use
FO??
Voltage
Range
500 volts
50 volts
5 volts
0.5 volts
Null
Range
100 volts
10 volts
1 volt
100 millivolts
10 millivolts
1 millivolt
0.1 millivolt
TABLE 2. Model 660A VTVM Input Resistanct
by Ranges. Resistance for Null Ranges is
when the Reference Voltage Dials are set
to zero.
Input
Resistance
50 megohms
50 megohms
50 megohms
50 megohms
Input
Resistance
(Slewing)
50 megohms
50 megohms
50 megohms
10 megohms
1 megohm
1 megohm
1 megohm
2-9.
known voltage, the Model 660A 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,
pick-up will be observed as needle quiver.
Model 660A, additional filtering is required.
filter is effective.
AC EFFECTS ON MEASUREMENTS. To minimize errors from ac signals present in thq un-
may reduce off-null sensitivity.
If ac components affect measurements by the
For a single-frequency ac signal, a twin-T
For a variable frequency signal, "se an ordinary low-pass filter.
Also, heavy 60-cps
10
106611
MODEL 660A DIFFERENTIAL VOLTMETER
APPLICATIONS
SECTION 3.
3-1.
resistances from 1 megohm to 100.000 megohms with an accuracy of is%.
tance,
isolated lead to the HI terminal to prevent measuring leakage between the leads.
RANGE Switch to 500.
adjust the Reference Voltage Dials to obtain a full-scale meter deflection
10.000 from the Dial setting and multiply the difference by 5 to obtain the value of the
resistor in megohms.
volt;
tract 1.0000 from the dial setting and multiply the difference by 50 to obtain the value
of the resistor in megohms.
volt; adjust the Reference Voltage Dials to obtain a convenient deflection on the meter.
Calculate the value of the resistor using,
PROCEDURES FOR MEASURING RESISTANCES. The Model 660A can be used to rapidly measure
connect the unknown resistor across the Model 660A HI and LO terminals.
Then determine the value of the resistor as follows:
For resistances between 1 megohm and 1000 megohms,
a.
For resistances between 1000 megohms and 10,000 megohms,
b. set the NULL Switch to 1
adjust the Reference Voltage Dials to obtain a full-scale meter deflection.
For resistances between 10,000 megohms and 100,000 megohms,
c.
Rx =
50 Ed
V
APPLICATIONS
To measure resis-
Use a short
Set the
set the NULL Switch to 10 volts;
Subtract
*
Sub-
set the NULL Switch to 1
megohms Equation 2
Where & 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 6). If an unknown resistance is across the Model 660A
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;
R, is the i'nput 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
Ed
Measuring Resistances. Rx is the unknown
resistance. R, is the input resistance of
the null detector; V is the null detector;
Ed is the buckout voltage.
1065R
11
APPLICATIONS
If measurements are made on the 1 to loo-volt null ranges, the input resistance, R,,, is
50 megohms. Equation 4 becomes
MODEL 660A DIFFERENTIAL VOLTMETER
Rx = 5~10~ ( 7 -1)
This is thebasis for simplified calculations in paragraph 3-l.
Equation 5
12
1065R
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