Fluke 721A User manual

P/N 294066
October 1968
®1982 John Fluke Mfg. Co., Inc., al! rights reserved Litho in U.S.A.

Notwithstanding any provision of any agreement the following warranty is exclusive:
The JOHN FLUKE MFG. CO., INC., warrants each instrument it manufactures to be free from defects in material and
workmanship under normal use and service for the period of 1-year from date of purchase. This warranty extends only
to the original purchaser. This warranty shall not apply to fuses, disposable batteries (rechargeable type batteries are
warranted for 90-days), or any product or parts which have been subject to misuse, neglect, accident, or abnormal
conditions of operations.
In the event of failure of aproduct covered by this warranty, John Fluke Mfg. Co., Inc., will repair and calibrate an
instrument returned to an authorized Service Facility within 1year of the original purchase; provided the warrantor's
examination discioses to its satisfaction that the product was defective. The warrantor may, at its option, replace the
product in lieu of repair. With regard to any instrument returned within 1year of the original purchase, said repairs or
replacement will be made without charge. If the failure has been caused by misuse, neglect, accident, or abnormal
conditions of operations, repairs will be billed at anominal cost. In such case, an estimate will be submitted before
work is started, if requested.
THE FOREGOING WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTY OF MERCHANTABILITY, FITNESS,
OR ADEQUACY FOR ANY PARTICULAR PURPOSE OR USE. JOHN FLUKE MFG. CO., INC., SHALL
NOT BE LIABLE FOR ANY SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WHETHER IN
CONTRACT, TORT, OR OTHERWISE.
If any failure occurs, the following steps should be taken:
1.Notify the JOHN FLUKE MFG. CO., INC., or nearest Service facility, giving full details of the difficulty, and
include the model number, type number, and serial number. On receipt" of this information, service data, or
shipping instructions will be forwarded to you.
2. On receipt of the shipping instructions, forward the instrument, transportation prepaid. Repairs will be
made at the Service Facility and the instrument returned, transportation prepaid.
SHIPPING TO MANUFACTURER FOR REPAIR OR ADJUSTMENT
Ail shipments of JOHN FLUKE MFG. CO., INC., instruments should be made via United Parcel Service or “Best Way"*
prepaid. The instrument should be shipped in the original packing carton; or if it is not available, use any suitable
container that is rigid and of adequate size. If asubstitute container is used, the instrument should be wrapped in paper
and surrounded with at least four inches of excelsior or similar shock-absorbing material.
CLAIM FOR DAMAGE IN SHIPMENT TO ORIGINAL PURCHASER
The instrument should be thoroughly inspected immediately upon original delivery to purchaser. Ail material in the
container should be checked against the enclosed packing list. The manufacturer will not be responsible for shortages
against the packing sheet unless notified immediately, If the instrument is damaged in any way, a claim should be filed
with the carrier immediately, (To obtain aquotation to repair shipment damage, contact the nearest Fluke Technical
Center.) Final claim and negotiations with the carrier must be completed by the customer.
The JOHN FLUKE MFG. CO., INC, will be happy to answer all applications or use questions, which will enhance your
use of this instrument. Please address your requests or correspondence to: JOHN FLUKE MFG. CO., INC., P.O. BOX
C9090, EVERETT,WASHINGTON 98206, ATTN: Sales Dept. For European Customers: Fluke (Holland) B.V., P.O. Box
5053, 5004 EB, Tilburg, The Netherlands.
’For European customers, Air Freight prepaid.
John Fluke yfg. Co., Inc., P.O. Box C909O, Everett, Washington 98206
Rev. 6/81

721A
iii

1-5. MECHANICAL
1-2. The Model 721A Lead Compensator is adevice
designed to compensate for the effects of lead and con-
tact resistance in voltage -divider measurement circuits.
It is particularly useful in those situations demanding
the utmost in accuracy. It is a useful operational ac-
cessory for such high-accuracy voltage dividers as the
John Fluke Model 720A and Model 725A.
1-3. SPECIFICATIONS
1-4. ELECTRICAL
RESOLUTION OF RESISTANCE COMPENSATION
0. 1milliohm.
MAXIMUM RATIO BETWEEN DIVIDER RESISTANCES
4000 to 1.
MAXIMUM ALLOWABLE LEAD RESISTANCE
150 miiliohms.
MAXIMUM DIVIDER VOLTAGE
1500 volts, de or peak ac.
FINE CONTROL
10-turn, 150-milliohm, slide-wire potentiometer.
COARSE CONTROL
18 -position rotary switch.
BINDING POSTS
Gold plated copper.
VOLTAGE SWITCH
Grounds input voltage terminals to permit compensation
for thermal voltages.
DIMENSIONS
3-1/2 high x19" wide x6" deep.
1-7. Figure 1-1 is an outline drawing of the Model
721A presenting the information necessary for instal-
lation.
1-1

72 1A.
Figure 1-1. MODEL 72 1A OUTLINE DRAWING
1-2

Some semiconductors and custom !C’s can be
damaged by electrostatic discharge during
handling. This notice explains how you can
minimize the chances of destroying such devices
by:
1. Knowing that there is aproblem.
2. Learning the guidelines for handling them.
3. Using the procedures, and packaging and
bench techniques that are recommended.
The Static Sensitive (S.S.) devices are identified in the Fluke technical manual parts list with the symbol
The following practices should be followed to minimize damage to S.S. devices.
2. KEEP PARTS IN ORIGINAL CONTAINERS
UNTIL READY FOR USE. 4. HANDLE S.S. DEVICES BY THE BODY
Page 1of 2

USE ANTI-STATIC CONTAINERS FOR
HANDLING AND TRANSPORT
7, AVOID PLASTIC, VINYL AND STYROFOAM1
IN WORK AREA
PORTIONS REPRINTED
WITH PERMISSION FROM TEKTRONIX, INC,
AND GENERAL DYNAMICS, POMONA DIV,
WHEN REMOVING PLUG-IN ASSEMBLIES,
HANDLE ONLY BY NON-CONDUCTIVE
EDGES AND NEVER TOUCH OPEN EDGE
CONNECTOR EXCEPT AT STATIC-FREE
WORK STATION. PLACING SHORTING
STRIPS ON EDGE CONNECTOR USUALLY
PROVIDES COMPLETE PROTECTION TO
INSTALLED SS DEVICES.
9. HANDLE S.S. DEVICES ONLY AT A
STATIC-FREE WORK STATION
10. ONLY ANTI-STATIC TYPE SOLDER-
SUCKERS SHOULD BE USED.
11. ONLY GROUNDED TIP SOLDERING
IRONS SHOULD BE USED.
Anti-static bags, for storing S.S. devices or pcbs
with these devices on them, can be ordered from the
John Fluke Mfg. Co., Inc.. See section 5in any Fluke
technical manual for ordering instructions. Use the
following part numbers when ordering these special
bags.
®Dow Chemical
John Fluke Part No.
453522
453530
453548
454025
Pink Poly Sheet
30"x60"x60 Mil
P/N RC-AS-1 200
$20.00
Description
6" X8" Bag
8” X12" Bag
16" X24" Bag
12" X15" Bag
Wrist Strap
P/N TL6-60
$7.00
Page 2of2J0089B-07U7810/SE EN Litho in U.S.A.

2-1. INTRODUCTION
2-2. In most resistance measurement setups, the
resistance of test leads, switch contacts, etc. is dif-
ficult if not impossible to determine with high accuracy.
In high-accuracy ratio measurements, these resistances
can contribute asignificant portion of the total error. A
resistance measurement setup is aproportional bridge
circuit in which the ratio of one divider is compared to
the ratio of another divider. The undesired resistance
of test leads and contacts causes an unbalance in the
bridge circuit which may be seen easily when making
both the low end and the high end comparisons with a
null detector. The Model 721A is designed to compen-
sate for this unbalance at both ends permitting linear
comparison of the divider ratios. With the Model 721A
used correctly to balance the circuit, the accuracy of
ratio measurement is limited primarily by the accuracy
of the standard voltage divider.
2-3. CONTROLS AND TERMINALS
2-4. STANDARD DIVIDER TERMINALS
2-5. The three STANDARD DIVIDER terminals are
used for making the connections to the standard resist-
ive divider. The bottom terminal is aground terminal.
It is electrically connected to the similar terminals for
the TEST DIVIDER and the VOLTAGE INPUT.
2-6. VOLTAGE SWITCH
2-7. When the VOLTAGE switch is in the ON position,
voltage is applied to the test circuit. When it is in the
OFF position, the voltage is removed from the test cir-
cuit and the wipers of the fine adjust potentiometers
are grounded to permit monitoring the circuit for ther-
mal voltages.
2-8. MODE SWITCH
2-9. The MODE switch controls the placement in the
circuit of the coarse balance resistance. With the
switch in the RSTD <RTEST position, the coarse
balance resistance is placed in series with the test di-
vider; with the switch in the RSTD >RTEST position,
the coarse balance resistance is placed in series with
the standard divider.
2-10. LOW BALANCE CONTROLS
2-11. The COARSE control, an 18 -position switch,
controls application of the relatively large amounts
of resistance in series with the low (black) terminal
for either the standard divider or the test divider, re-
quired to compensate for differences in divider input
resistance. The FINE control potentiometer provides
high-re solution balance (centering) adjustment over
anarrow range.
2-12. HIGH BALANCE CONTROLS
2-13. The COARSE control, an 18-position switch,
controls application of the relatively large amounts of
resistance in series with the high (red) terminal for
either the standard divider or the test divider, required
to compensate for differences in divider input resist-
ance. The FINE control potentiometer provides high-
resolution balance (centering) adjustment over anarrow
range.
2-14. TEST DIVIDER TERMINALS
2-15. The three TEST DIVIDER terminals are used for
making the connections to the test divider. The bottom
terminal is aground terminal. It is electrically con-
nected to the similar terminals for the STANDARD DI-
VIDER and the VOLTAGE INPUT.
2-16. VOLTAGE INPUT TERMINALS
2-17. The three VOLTAGE INPUT terminals are used
for connecting the voltage source to the test circuit.
The upper (red) terminal is normally connected to the
positive (+) terminal of the power supply and the mid-
dle (black) terminal is normally connected to the neg-
ative (-) terminal of the power supply. The bottom
terminal is used for grounding the circuit. It is elec-
trically connected to the similar terminals for the
STANDARD DIVIDER and the TEST DIVIDER.

721A
Figure 2-1, RATIO SYSTEM CONFIGURATION
2-2

721A
MODEL 332B
Figure 2-2. INTERCONNECTION OF UNITS FOR RATIO COMPARISON

Figure 2-3. SCHEMATIC DIAGRAM OF EQUIPMENT CONNECTIONS
2-18. APPLICATIONS
2-19. The Model 721A Lead Compensator was designed
to compensate for lead resistance and end resistance in
ahigh-accuracy ratio system. Figure 2-1 shows the two
basic configurations of such a system. Lead compen-
sation is essential to accuracy of measurement in both.
In Figure 2-1 (A), the ratios of two resistive dividers
are compared. This is the typical setup used for divider
calibration. The coarse compensation is added in series
with the standard divider indicating that it has a higher
input resistance than the test divider. If the input re-
sistance of the test divider were higher, the coarse
compensation would be switched to the other side. In
Figure 2-1 (B), an unknown resistance is measured by
using astandard divider to compare it to astandard re-
sistor. Notice that the standard resistance and the un-
known resistance form avoltage divider. For this type
of measurement, the lead compensator is used to set
point R4equal to zero setting on the standard divider
and to set point Rj equal to 1.
0
setting on the standard
divider. The standard divider settings at points R2and
R3at null are then determined and used to compute the
unknown resistance from the formula:
^unknown 1"R2
^standard R3
2-20. LEAD RESISTANCE COMPENSATION
2-21. To use the Model 721A for lead compensation, it
must be connected into the ratio comparison system as
shown in Figure 2-2. Figure 2-3 illustrates aschematic
diagram of the equipment connections. The equipment
should be turned on and allowed to warm up for 30 min-
utes before the lead compensating adjustments are made.
After warm-up perform the following steps to compen-
sate for lead resistance.
a. Set the power supply to the desired output voltage.
b. Set the MODE switch of the Model 721A to RSTD<
RTEST or RSTD> RTEST as appropriate.
c. Set both dividers to zero and turn the VOLTAGE
switch of the Model 721A to OFF.
d. Place the null detector in the zero mode of operation
and adjust it for zero meter deflection.
e. Return the null detector to the operating mode and
set it to the desired sensitivity.
f. Note the meter deflection, tt is caused by the ther-
mal voltages in the circuit.
'Hotel
To avoid overloading the null detector am-
plifier during initial balance steps it is ad-
visable to reduce sensitivity before turning
the VOLTAGE switch to ON.
g. Turn the VOLTAGE switch to ON and adjust the
LOW BALANCE controls while increasing sensitivity
until the deflection noted in step fis obtained at the
desired sensitivity.

h. Set the HIGH BALANCE COARSE control to the
same setting as the LOW BALANCE COARSE con-
trol. Hthe meter deflection changes, readjust the
LOW BALANCE FINE control to obtain the meter-
deflection noted in step f.
i. Turn the VOLTAGE switch to OFF, reverse power
supply polarity, and turn the VOLTAGE switch to
ON.
j. Observe the meter. If meter deflection changes,
repeat steps dthrough juntil the null detector read-
ing remains constant for both polarities of applied
voltage.
k. Turn the VOLTAGE switch to OFF and set both di-
viders to full scale.
l. Place the null detector in the zero mode of operation
and adjust it for zero meter deflection.
m. Return the null detector to the operating mode and
set it to the desired sensitivity.
n. Note the meter deflection caused by the thermal
voltages in the circuit.
The thermal voltages, and meter deflection,
may be different at different points in the
circuit.
o. Turn the VOLTAGE switch to ON and adjust the
HIGH BALANCE FINE control to obtain the meter
deflection noted in step n.
p. Turn the VOLTAGE switch to OFF, reverse power
supply polarity, and turn the VOLTAGE switch to
ON.
q. Observe the meter. If meter deflection changes,
readjust the HIGH BALANCE FINE control to find
asetting which will cause the same deflection for
either supply polarity. The Model 721A now com-
pensates for the lead resistances at the high and
low ends of the circuit.
2-23. The technique used for measurement at calibra-
tion points must eompensa-fce for thermal voltages if
optimum accuracy is to be obtained. Adjustment of the
Model 721A compensates only for voltage drops at the
high and low ends of the circuit caused by end and lead
resistance; it can not compensate for thermal voltages
at calibration points because they may vary from one
calibration point to the next. To make thermal com-
pensated measurements at calibration points, proceed
as follows:
a. Turn the VOLTAGE switch to OFF and set both di-
viders to the desired calibration point.
b. Place the null detector in the zero mode of operation
and adjust it for zero meter deflection.
c. Return the null detector to the operating mode.
d. Turn the VOLTAGE switch to ON and note the meter
deflection.
e. Turn the VOLTAGE switch to OFF, reverse power
supply polarity, turn the VOLTAGE switch to ON,
and observe meter deflection.
%>tcf
The object of this procedure is to obtain the
same meter deflection with normal and re-
versed power supply polarity.
f. Adjust the setting of the standard divider to bring
the meter needle one half the distance toward the
deflection noted in step dand note the deflection.
g. Turn the VOLTAGE switch to OFF, return power
supply polarity to normal, turn the VOLTAGE
switch to ON, and note meter deflection. Hit is the
same as the deflection noted in step f, the measure-
ment is complete.
h. Continue reversing power supply polarity and ad-
justing the standard divider setting until the meter
deflection is unchanged when polarity is reversed.
The setting of the standard divider is now the true
ratio of the test divider.