Keithley 179A User manual

Instruction Manual
Model 179A
TRMS Multimeter
01982, Keithley Instruments, Inc.
Cleveland, Ohio, U.S.A.
Document Number 32430
Advanced Test Equipment Rentals
www.atecorp.com 800-404-ATEC (2832)
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SPECIFICATIONS
DC AND TRMS AC AMPS
20 A A 0.5%+2d* l%+lEdt 0.5” ,Ornll
*Add 0.1 %rdg above 15A fo, self-testing.
tlktir max.
MAXIMUM INPUT: 24 250” DC o, rms (fuse protected, except ‘or 20A
range: l5A continuous. 20A for 1 minute lunfusedl.
TEMPERATTURE COEFFlClENT OF-18°C Et 28°-!XioC,:
DC f,O.Ol% +O.Zd,,T.
AC * lO.O7% +2dlPC.
CREST FACTOR Iratio of peak value to rms valuel: 31.
SETTLING TIME: DC: 1 second to within 1 digit of final reading. AC: 2
seconds to within 15 digits of final reading.
MAXIMUM OPEN CIRCUIT VOLTAGE: 5”
GENERAL
DISPLAY: Five 0.5” LED digits. appropriate decimal position and polarity
indication.
CONVERSION PERIOD: 4OOms.
OVERRANGE INDICATION: Display blinks all zmoes above 19999 counts.
MAXIMVM COMMON MODE VOLTAGE: ,400” oeak.
EWI,R,ONMENT: Operating: 0’
0 ‘-55°C. 0% to 80% relative humidity up
c. storage: -25T to +l35oc.
POWER : 105-125” 0, 210-250” bvitch selectedl. go-,lO” available.
50~60 Hz, 5.5w. Optional B-hour batmy pack. Model 1788.
DIMENSIONS. \n
IEIGHT: 85mm high x 235mm wide x 275mm deco
13%” x 9%” x WY,“,. Netweigirt l.Skg 14Ix).
ACCESSORIES SUPPLIED: ,nst,wtio” Manual md Mode, ,691 Tesf
Leads.
ACCESSORIES AVAILASLE:
Model 1010: Single Rack Mounting Kit
Model 1017: Dual Rack Mounting Kit
Model 1301: Temperature Probe
Model 1600A: High Voltage Probe ,4Ok”,
Model 1651: 50.Ampere Current Shunt
Model 1581: Clip-On Test Lead Set
Model ISSZA: RF Probe
Model 1683: Universal Test Lead Kit
Model 1584: Herd She,, Carwing Case
Model ,885: Clamp-On AC Probe
Model 1591: General Purpass Test Lead Set
Model 1788: Rechargeable Smtery Pack
Model 1792: Isolated SCD Output
Model 1793: isolated IEEE-488 O”f,,uf
Model 7008-3: IEEE~488 Cable (3 ft.1
Model 7008-6: IEEE-488 Cable 18tt.,

TABLE OF CONTENTS
PARAGRAPH TITLE PAGE
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
SECTION 1 GENERAL INFORMATION
Introduction. ............ ....................
Features .....................................
Warranty Information ... ... ..................
ManualAddenda.........~ ....................
Safety Symbols and Terms .....................
Unpacking and Inspection .. .. ~, ... .. .. ...
Optional Accessories ..........................
Specifications ....... ~, ..... ... ......... ..
4.4
4.5
4.5.1
4.5.2
4.6
4.6.1
4.6.2
4.6.3
4.6.4
4.7
4.8
4.9
4.10
2.1
2.1.1
2.1.2
2.1.3
2.2
2.2.1
2.2.2
2.3
2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
4.1
4.2
4.2.1
4.2.2
4.2.3
4.3
4.3.1
4.3.2
SECTION 2 OPERATION
Preparation for Use., .............. .. .........
LinePower........~.~...........~ ...........
Battery Pack Power ................ .........
Battery Charging .. ... ..... ...............
Operating lnsfructi”ns ............... ~, .... ...
Environmental Conditions ...... ........ ~, ...
Front Panel Familiarization ..... ...... ....
DMMMeasurements ...........................
DC Voltage Measurement ...... .. ... ..
TRMS AC Voltage Measurement .. .. ..... ..
Resistance (111Measurement ...... ............
Current Measurement IDC or TRMS ACI .. ...
AC and DC Measurement ... .. ...... .. ..
SECTION 3 PERFORMANCE VERIFICATION
Introduction. ........ ...... ..... ... .....
Environmental Conditions ..... ......... .. ...
Recommended Test Equipment .... ..... ......
PerformanceVerification Procedure ....... .. ...
Initial Conditions .. ~, .......... .. .. .. ...
DCVoltsVerification ...........................
AC Volts Verification ..... ..... .. ........
Resistance Verification. ................... .....
DC Current Verification .. ~, .. .. .........
AC Current Verification .. ... .. ... .........
SECTION 4 THEORY OF OPERATION
Introduction., .......... .... ~, ....... ......
Overall Operation ... ......... .............
Signal Conditianing ..... .. .. ... .. .......
Ohms Cowersian ......... .. ... ... .......
A/D Converter .... ...... .. ... ...........
Attenuation ...................................
DC”o,ts........~~ .........................
AC”“,ts.............~...~ ..................
2~1
2~1
2-l
2-1
2~1
2-1
2~1
2-3
2-3
2~4
2-4
2~4
2~5
3-1
3~1
3-1
3~1
3~1
3~1
3-2
3-2
3-2
3-2
4-1
4~1
4~1
4-1
4~1
4~2
4~2
4~2
~. 4~2
4~2
4-2
4~2
4~3
4-3
4~3
4-3
4-3
4~4
4-4
5.1
5.2
5.2.1
5.2.2
5.2.3
5.2.4
5.3
5.4
5.4.1
5.5
5.6
5.7
6.1
6.2
6.3
6.4
6.5
6.6
SECTION 5 MAINTENANCE
Introduction., ~,
Calibration Procedure
Rccommendcd Tesr Equipment
Environmental Canditions
Calibration Shield Installation
Calibration Adjustments
Batten/ Pack iModel 17881 installation
Troubleshooting ~. ~.
Troubleshooting Procedure
Special Handling of Static Sen&e Devices
Batten/ Charge Voltage Adjustments
Fuse Replacement ~.
SECTION 6 REPLACEABLE PARTS
Introduction ~~~ ,.,, .~,.~ ~~,,,
Replaceable Parts ~. ~, ~. ~, ~, ~,
Ordering Information ~.
Factory Service ~, ~, ~. ~,
Component Location Drawings
SchematicDiagram ~, ~, ~.
5~1
51
5~1
5~1
5-1
5-l
52
5-2
53
5-3
5~3
5-4
6-l
6- 1
6-l
6~1
6-l
6~1

LIST OF TABLES
TABLE TITLE PAGE
3-2
3~3
3~4
3-5
4~1
4-2
4-3
4-4
5~1
5~2
53
5~4
5-5
5-6
6-l
6~2
6~3
6-4
Model 179A Maximum Allowable Inputs 2-4
Recommended Test Equipment for
Performanceverification ........................ 3-1
DC Voltage Performance Check .................. 3~2
AC Voltage Performance Check .................. 3-2
Resistance Peformance Check ................... 3-2
DC Current Performance Check .................. 3-2
Full Scale A/D Inputs ........................... 4-2
DC Attenuation and Gain Setting Components ..... 4-2
AC Attenuation Gain Setting Components ......... 4-3
Resistance Range Setting Components ............ 4-3
Recommended Test Equipment for Calibration ..... 5-l
Calibration Adjustments ......................... 5-2
Voltage Levels ................................. 5-3
Signal Tracing Levels ........................... 5-4
Model l79A Static Sensitive Devices .............. 5-4
Fuse Replacement ............................. 5-5
Model l79A Mother Board PC-492, Parts List ...... 6.2
Model 179A Display Board PC-485, Parts List ...... 6.4
Model 1788 Battery Pack PC-451. Parts List ........ 6.5
Recommended spare parts ...................... 6~5
LIST OF FIGURES
FIGURE TITLE PAGE
2-1
2-2
2-3
2-4
2-5
2-6
2-7
4-1
4-2
4-3
6-l
6-2
6-3
6-4
6-5
Rear View Showing Line Switch ... ............
Front Panel Familiarization ..... ... ......
DC Voltage Measurement .....................
AC Voltage Measurement .....................
Resistance(R) Measurement. .................
Current Measurement Up to 2000mA ...........
Current Measurement Between 2000mA and 20A.
Simplified Signal Flow Block Diagram,
Model 179A DMM
Attenuation and Ohms Conversion
AID Converter Function
Miscellaneous Parts
Model 179A Display Board PC-485, Component
Location Drawing 29663
Model 1788 Battery Pack PC-451. Component
Location Drawing i9007 :. .‘.
Model 179A Display Board PC-492, Component
Location Drawing 32045
Model 179A and 1788, Schematic Diagram 32046.
2~2
2~2
2~3
2-4
2~4
2-5
2~5
4~1
4~1
4-4
6~1
6~6
6-6

SECTION 1
GENERAL INFORMATION
1.1 INTRODUCTION
The Model 179A is a precision 4% digit TRMS multimeter
useful for measurement of AC and DC voltage, AC and DC
Current and resistance. Ranges and accuracies are listed in
the detailed specifications which precede this section,
Ranges and functions are zselected with front panel push
buttons (see Figure Z-2). The decimal point is also po$
tioned by the selected range push button. Polarity of the
measured signal is automatically displayed.
1.2 FEATURES
The Model 179A includes the following features:
l TAMS AC measurement capability gives waveform-
insensitive measurement accuracy to applications such as
solid-state regulator design, measurement of powcr
transformer input currents and capacitor ripple currents.
. AC and DC CURRENT ranges allow continuous
measurements of up to 15A or periodic measurement up
to ZOA.
l 1OpV AC and DC sensitivity
l HI-LO Ohms. In the HI mode, enough voltage can be ap-
plied to semiconductors to turn them on for a test. LO can
be used for in-circuit measurements without turning on
semiconductor junctions. Full-scale compliance voltage is
2V on Hf. 200mV on LO.
. 1kV protection on II. 1OOOV overload protection on ohms
eliminates accidental damage due to improper function
selection.
. Optional BCD output. The Model 1792 Isolated BCD Out-
put may be ordered and is field installable.
l Optional IEEE-488 data output. The Model 1793 IEEE-488
Interface can be ordered with the unit, or can be easily
field-installed with a screwdriver. It is powered internally
from the instrument, With the interface and any of the low
c”?.t controllers now on the market. it is possible to set up
an economical, automated test system that saves the time
of manually recording, transcribing and entering large
amounts of measurement data.
1.3 WARRANTY INFORMATION
Warranty information is provided on the inside front cover
of this manual. If there is a need to exercise the warranty.
contact the Keithley representative in your area to deter-
mine the proper action lo be taken. Keithley maintains come
plete repair and calibration facilities in the United States.
West Germany. Great tlritain, France, the Netherlands.
Switzerland and Austria. Information concerning the appli~
cation, operation or sewice of your instrument mav be
directed to the applications engineer at any of the previously
mentioned locations. Check thr inside front cover of this
manual for addwsst!s
1.4 MANUAL ADDENDA
Improvements or changes to tilis manuill will t,l, r~x~:la~ned
on an addendum in<:/udcd with this manuals
1.5 SAFETY SYMBOLS AND TERMS
Safety symbols used in this manual are as follows:
The symbol A on the instrument denotes that
the user should refer to the operatin:, instructions~
The symbol on the
instrument derrotcs Ihilt
1OOOV or more may bc present on the ternlir,al(si
The WARNING used in the manual exl~ia~ns rls~q,:!~ :~u!
could result in personal injury or death
The CAUTION used in illis manual axpla~n:; hazards ~hrft
could damage the instrument.
1.6 UNPACKING AND INSPECTION
The Model 179A is inspecred both mecflai~~cally ano c1e.c
tricallv before shipments Upon receiving tfw Modui 179A
unpack all items from the shipping container and check for
any obvious damage Illat may have “CCIIIZ’~ dc,rir\!j i~i~ns~t
Report any damage to the shipping agent, Rela~n anti use
the original packqing tmatcriais 11rcst,ipmen: 1srnccessarv
The following items are sflipped with all Modal 179A urtitils
- A Model 179A TRMS Multimeter
l A Model 179A Instn~ctior Manual
l A Model 1691 tane~;rl Purpose To51 Lead Set
l Optional accessories per request
1.7 OPTIONAL ACCESSORIES
A wide range of accessories are available to facilitate the use
of the Model 179A DMM. extend its range. and adapt it for
additional uses.
1. Model 1010 Single Rack Mounting Kit To mount one
bench DMM in a standard 5%” x 19” rack mounting.
2. Model 1017 Dual Rack Mounting Kit&To mount two
berlcfi DMMs in a standard 5%” I 19” rack amounting
3. Model 1301 Temperature Probe A rugged low cost
temperature probe deslgncd to allow precision
temperature rneasuremcn~s from ~55“C to 150°C
1-l

Range: -55oc to 15ooc
Output: lmVI°C; compatible with any DMM with at
least 10MII input impedance
Accuracy: kZ°C from O” to 100°C; f3”C from -55’
to O°C and 100° to 150°C
Power: 9V alkaline or C-Zn (NEDA 16041 battery.
4. Model 1600A High Voltage Probe extends the DMM to
40kV.
Maximum Input: 40kV DC or peak AC to 300Hz
Input Resistance: IOOOMR
Division Ratio: 1OOO:l (into lOMa)
Ratio Accuracy (into lOM12 DMM): +2.5% from 1kV
to 40kV DC; -3dB at 300Hr AC
Operating Temperature: O” to 50°C
5. Model 1651 50.Ampere Current Shunt-The external
0.00112 + 1%. 4.terminal shunt permits current
measurements from O-50A DC and 20.50A AC.
6. Model 1681 Clip-On Test Lead Set contains two leads,
1.2m (48 inches) long terminated with banana plugs and
spring action clip-on probes.
7. Model 1682A RF Probe permits voltage measurements
from IOOkHz to 250MHr.
AC to DC transfer accuracy: +ldB from 1OOkHz
to 250MHz at IV, peak responding, calibrated
in rms of a sine wave. compatible with instruments
with lOML2 input resistance
Voltage Range: 0.25V to 15V rms
Maximum Allowable Input: 42V AC peak, ZOOV (DC
+ AC peak1
8. Model 1683 Universal Test Lead Kit consists of two test
leads. 1.2m (48 inches) long with 12 screw-in tips, 2
banana plugs, 2 spade lugs, 2 alligator clips with boots,
2 needle tips with chucks and 4 heavy duty tip plugs.
9. Model 1684 Hard Shell Carrying Case-Hard vinyl case,
1OOmm x 300mm x 350mm (4” x 13” x 14”) has a fitted
foam insert with room for the Model 179A, instruction
manual and small accessories.
10. Model 1685 Clamp-On AC Probe measures AC current
by clamping onto a single conductor. Interruption of the
current path is unnecessary. The Model 1685 detects
current by sensing the magnetic field produced by the
current flow.
Range: 2, 20 and 200A rms
Accuracy: i-4% of range at 60Hz; *6% of range at
5OHz
Temperature Coefficient: iO.O5%/‘C on 20A and
200A range; +0.3%/OC on 2A range
Maximum Allowable Current: 300A rms
Maximum Conductor Voltage: 600V rms
Conversion Ratio: O.lV/A rms
11. Model 1691 General Purpose Test Lead Set consists of
two 0.9lmm (36 inches) test leads with probe tips ter-
minated in banana plugs.
12. Model 1788 Rechageable Battery Pack provides six
hours minimum operation from full charge, recharges
within 14 hours and is field installable.
13. Model 1792 Isolated BCD Output provides parallel BCD
data output including sign, overrange and busy. Field in-
stallable.
14. Model 1793 Isolated IEEE-488 Interface-Field in-
stallable option provides isolated data output. Switch-
selectable TALK ONLY or ADDRESSABLE modes.
Mounts within and powered by the Model 179A. Model
7008 IEEE-488 cable is available.
1.8 SPECIFICATIONS
For Model 179A detailed specifications, refer to the
specifications that precede this section.
1-2

SECTION 2
OPERATION
2.1 PREPARATION FOR USE
The Model 179A is shipped ready to use. The instrument
may be powered from line voltage or from rechargeable bat-
teries (when the optional Model 1788 Rechargeable Battery
Pack is installedl.
21.1 Line Power
The Model 179A is provided with a three-wire line cord
which mates with third-wire grounded receptacles. Connect
the instrument to AC line power as follows:
1. Set the LINE VOLTAGE switch an the back of the instru-
ment to correspond to line voltage available. Ranges are
105 to 125 volts and 210 to 250 volts AC as shown in
Figure 2-l.
CAUTION
Connect only to the line voltage
selected. Application of incorrect
voltage can damage the instrument.
2. Plug the power cord into a properly grounded outlet.
WARNING
Ground the instrument through a pro-
perly grounded receptacle before oper-
ation. Failure to ground the instrument
can result in severe injury or death in the
event of short circuit or malfunction.
2.1.2 Battery Pack Power
The Model 179A may also be operated from rechargeable
sealed lead-acid batteries contained in the optional Model
1788 Battery Pack. The battery pack will operate the 179A
for up to six hours. Circuits within the battery pack will
automatically shut down the instrument when the battery
charge is insufficient to maintain accurate readings. Refer to
Section 5, paragraph 5.3 for installation procedures.
2.1.3 Battery Charging
After the Model 1788 Battery Pack is installed in the Model
179A it can be charged and recharged as follows:
1. Connect the instrument to line power as described in
paragraph 2.1.1.
2. With the power switch off, the battery charge circuitry is
automatically engerized to charge the battery at the max-
imum rate. When the battery pack is first installed, or if it
is completely discharged, allow it to charge for at least 14
hours,
NOTE
For maximum battery life, do not allow the
battery pack to remain completely discharg-
ed. Constant charging will not harm either
the battery pack or the instrument. Allowing
the batten, pack to discharge below 7.2V
and remain discharged will ruin the battery
pack.
3. When the 179A is in use on line power, the bart~?ry
charger maintains a trickle charge on the battery pack.
2.2 OPERATING INSTRUCTIONS
2.2.1 Environmental Conditions
All measurements should be made at iii, ;Imlwnt
temperature within the range of O°C to 55°C. anti with a
relative humidity of 0% to 80% up to 35OC. For instliii w:~s
above 35OC derate humidity 3% per “C up to 55”C, If tilt sn
strument has been subjected to extremes of temperature,
allow sufficient time for internal temperatures to react1 cnw
ronmental conditions. Typically, it takes orx hwIr !o
stabilize a unit that is 10°C 118”Fl out of spei:lf!ixi
temperature range.
2.2.2 Front Panel Familiarization
The following text and Figure 2~2 provide a brief description
of the front panel controls, input terminals and display.
ON/OFF Depressing (in) this push button turns the ins
strument on for either battery power lif the Model 1788 is
installed) or line power. Releasing (out1 this push button
turns the instrument off.
NOTE
In the OFF position, the Model 1788 !if in
stalled) will be charging if the instrument 1s
connected to line power.
AC/DC This switch is used along with the volts IV1
and current (A) functions. Depressing linl this push butt
ton selects AC and releasing lout) this push button
selects DC.
LO/HI This feature is used along with the !! funcllon.
The front panel push button selects the LO or HI mode
for the 20kR. 200k12 and 2000kU ranges. Depressiw the
push button lin) selects LO and releasing the push button
selects HI. On the 2kI2 range the Model 179A is in rhc LO
mode, regardless of the push button position. On the
20MR range the Model 179A is in the HI mode,
regardless of the push button position.
2-l

These adjustments are used only for calibration. They are not intended for
adjustment during operation.
Figure 2-l. Rear View Showing Line Switch
MlNUS SIGN DISPLAYED
PLVS SIGN IMPLIED OVERRANGE IS INDICATED BY A FLASHING “0000” EXCEPT ON THE 1000 VOLT RANGE
‘MENT
Figure 2-2. Model 179A Front Panel View
2-2

A. Use the HI mode for measurements in the 20k. 200k.
2000k and 20M ranges. Full range voltage drop is 2
volts and is sufficient to cause forward conduction of
semiconductor junctions. The HI terminal is positive.
B. Use the LO mode for measurements in the 2k, 20k.
200k and 2000k ohm ranges. Full range voltage drop
is 2OOmV. Maximum open circuit voltage is 5V on all
ranges.
4. Function Selection
A. R Depressing this push button selects the ohms
function.
B. V Depressing this push button selects the volts
function.
C. A Depressing this push button selects the current
function.
5. Range Selection Select the desired range by depress-
ing the appropriate push button.
6. Input
A. 20 AMP jacks (grey and black) Use this pair exclu-
sively for measuring current up to 20A.
6. INPUT jacks (red and black1 Use this pair for current
measurements up to 2000mA and all other
measurements.
7. Zero Adjustment The from panel zero adjustment nulls
input offset on the 20, 200 and 1200 DC voltage ranges
and on all resistance ranges. Typically. this adjustment
need not be performed more often than once a week
unless the instrument is operated at ambient tempera-
tures outside the range of 1WC to 28OC. Zero the instru-
ment as follows:
A. Turn on the power and select LO II and the 200k
E range. Plug in test leads and short them. Adjust the zero ad-
justment pot on front panel to obtain a reading of
0000 +3 digits.
NOTE
The zero adjustment may also be used for
lead compensation on a particular R range.
2.3 DMM MEASUREMENTS
1. Turn on and zero the instrument as described in
paragraph 2.2.2 step 7. Zero the instrument before the
first use or whenever the instrument is used outside the
temperature range of 18OC to 28OC. and weekly during
normal use.
2. TRMS The Model 179A measures the true root mea”
square of a signal within the frequency range of 45 to
20kHz The maximum crest factor for rated accuracy to
NOTE
Accuracy is specified for 2000 counts and
above. The method of calibrating the con-
verter may yield an offset up to 50 digits with
the input shorted, This does not affect the
instrument’s accuracy.
3. Crest Factor (CF) is the ratio of the peak voltage to the
r”X Vdtag‘3 CF = V
e-
Some typical crest factors:
Sine wave: CF =E
Square wave: CF = 1
Triangular wave: CF =w
Positive pulse train: CF- l/Vdufy cycle (duty cycle
forCF=3is0.11) NOTE
There will be some additional measurement
error for signals with a crest factor greater
than 3lCF> 3). CAUTION
Do not exceed the maximum allowable
inputs of the 179A or instrument
damage that is not covered by the war-
ranty, may occur. See Table 2-l for max-
imum inputs. WARNING
Exercise extreme caution when measur-
ing voltage thaf present a shock hazard
hazard to the user. The American Na-
tional Standard Institute (ANSI) etetes
that e shock hazard exists when voltage
levels greeter then 30 volts rms or 42.4V
peak are present. A good safety practice
is to expect that hazardous volfages ere
present in sny unknown circuit fo be
measured, until actual conditions are
verified.
2.3.1 DC Voltage Measurement
1. Select DC V function.
2. Select desired range.
3. Connect the unknown DC voltage to fhe INPU~r jacks of
the Model 179A as shown in Figure 2~3.
4. Note reading on display.
Figure 2.3. DC Voltage Measurement
2-3

Table 2-1. A Model 179A Maximum Allowable Inputs
2.3.2 TRMS AC Voltage Measurement
1. Select AC V function
2. Select desired range.
3. Connect the unknown AC voltage to the INPUT jacks of
the Model 179A as shown in Figure 2-4.
4. Note the reading on display.
-Maximum Inputs
450V rms continuous; 1200V peak, for
lo seconds per minute.
1200V peak.
1OOOV rms; 1400V peak; 107WHz.
2A, 250V DC or rms (fuse protected)
15A continuous, 20A for 1 minute (50% duty
cycle1
450V rms sine wave; 1OOOV peak, for
lo seconds per minute.
Figure 2-4. AC Voltage Measurement
2.3.3 Resistance III) Measurement
1. Select the 12function.
1. For current measurements up to 20OOmA:
A. Select the ACA or DCA function,
B. Select the desired range (up to 2000mA).
C. Connect the unknown current to the INPUT jacks of
2. Select the HI mode or the LO mode (see paragraph 2.2.2 the Model 179A as shown in Figure 2-6.
SteD 31. D. Note reading on display.
Figure 2-5. Resistance InI Measurement
2.3.4 Current Measurement IDC or TRMS AC)
NOTE
To prevent measurement errors, connect the
current test leads to either the 20A jacks or
the normal INPUT jacks. Disconnect all cir-
cuits from the unused jacks.
WARNING
To prevent electrical shock, remove
power from the circuit to be measured
before connecting the Model 179A.
3. Connect the unknown resistance (RI to the INPUT jacks 2. FW current measurements between 2000mA and 20A:
of the 179A as shown in Figure 2-5. A. Select the ACA or DCA function.
4. Note reading on display. B. Depress the 20A range switch.
C. Connect the unknown current to the 20 AMP jacks of
the Model 179A as shown in Figure 2.7.
D. Note the reading on display.
2-4

Figure 2-6. Current Measurements up to 2006mA
NOTE
Up to 15A may be applied continuously
without degradation of the measurement
due to self-heating effects. For currents be-
tween 15A and 20A. specified accuracy can
only be obtained when measurements are
limited to a 50% duty cycle (i.e., apply the
current for a maximum of one minute and
then allow et least one minute for cooling
before the next measurement).
NOTE
The test leads used must be capable of
handling 20A and it is recommended that
they be twisted (see Figure 2-71 to minimize
external fields which could affect the Model
179A or other equipment. Also, keep the
test leads as short es possible to mlnlmlze
voltage drop.
Figure 2-7. Current Measurements Between 2000mA
and 20A
2.3.5 AC + DC Measurement
Use the Model 179A to meawre TRMS on ;I sigrw WIWII
has both AC and DC components as lollows:
1. Measure and record the TRMS AC con~~onent as
described in paragraph 2.3.2.
2. Measure and record the DC component as descritxxl III
paragraph 2.3.1.
3. Compute the rms value from the followim] e(/uat~on
ERMs im
2-512-6

SECTION 3
PERFORMANCE VERIFICATION
3.1 INTRODUCTION
Performance verification may be done upon receipt of the
instrument to ensure that no damage or misadjustment has
occurred during transit. Verification may also be performed
whenever there is question of the instrument’s accuracy and
following calibration if desired.
NOTE
For instruments that are still under warranty
(less than 12 months since date of
shipment), whose performance falls outside
specifications at any point, contact your
Keithley representative or the factory imp
mediately.
3.2 ENVIRONMENTAL CONDITIONS
Measurements should be made at 18-28°C and at less than
80% relative humidity.
3.3 RECOMMENDED TEST EQUIPMENT
Table 3-l lists all the test equipment required for verifi-
cation. If alternate equipment is used, the alternate test
equipment’s specifications must be at least as good as the
equipment specifications listed in Table 3~1.
3.4 PERFORMANCE VERIFICATION PROCEDURE
Use this procedure to verify the Model 179A’s accuracy. If
the Model 179A is out of spec. proceed to maintenance
(calibration) Section 5, unless the Model 179A is under
warranty. NOTE
Verification should be performed by
qualified personnel using accurate and
reliable test equipment.
WARNING
Some procedures require the use of
high voltage. Take care to prevent con-
tact with live circuits which could cause
electrical shock resulting in injury or
death.
3.5 INITIAL CONDITIONS
Before beginning the verification procedure, the instrument
must meet the following conditions:
1. If the instrument has been subjected to extremes of
temperature, allow internal temperatures to stabilize for
one hour minimum at the environmental conditions
specified in paragraph 3.2.
2. Turn on the 179A DMM and allow it to warm up for ten
minutes. The instrument may be operated from either
line power or battery pack power. as Ion51 as the battery
pack has been fully charged as described m paragraph
2.1.3.
3. Zero the instrument as described in paragraph 2.2.2
stea 7.
3.6 DC VOLTS VERIFICATION
1. Select the DC V function.
2. Connect the DC Calibrator iltem A, Table 3~11 ro the
instrument.
3. Select the 200mV range. and apply positive 1OOmVDC to
the DMM. The reading must be within the limits specified
in Table 3~2.
4. Select each remaining range and apply ihe required
voltage as specified in Table 3~2. verify that the reading is
within specifications.
5. Repeat all checks with negative voltage.
Table 3-l. Recommended Test Equipment for Performance Verification
DC Calibrator
AC Calibrator
Decade Resistor
Current Calibrator
SPECIFICATION
.~~
D.lV, 1v. 1ov. 1oov. 1ooov
f 0.002% or zouv
.lV, 1v. 1ov. 1oov
* .022%
1ooov @t .044%
1.9kR. 19k12. 190kI1
1.9MR. lOM12, ,O.Ol%
lOO,‘A, lmA, lOmA. lOOmA,
1A. 10A. +0.03%
MFR MODEL
Fluke 343A
Fluke 5200A
Fluke 5215A
ESI RS725
Valhalla 2500E
3-l

Table 3-Z. DC Voltage Performance Check Table 3-4. Resistance Performance Check
2ov
zoov
12oov
200mV lOO.OOmV 99.93 to 100.07
2v 1.oooov 0.9995 to 1.0005
1o.ooov 9.995 to 10.005
1oo.oov 99.95 to 100.05
1ooo.ov 999.5 to 1000.5
v&age
Allowable Readings
at 180 to 28OC
3.7 AC VOLTS VERIFICATION
1. Select the AC V function.
2. Connect the AC Calibrator (Item B, Table 3-l) to the
DMM. Set the calibrator frequency to 1kHz.
3. Set the DMM to the 200mV range and apply 1OOmV AC
to the DMM. The reading must be within the limits
specified in Table 3.3.
4. Select the 2. 20 and 200 volt ranges and apply the re-
quired voltages es specified in Table 3-3. Verify that the
readings are within specifications.
5. To check the 1OOOV range, connect the AC Calibrator
Amplifier (Item C, Table 3-l) to the output of the AC
Calibrator per the manufacturer’s instructions. Set it for
en output of 1OOOV AC rms and verify that the DMM
reading is within the specified limits.
Table 3-3. AC Voltage Performance Check
Range
200mV
2v
2ov
2oov
1ooov
Allowable Readings
99.15 to 100.85
0.9925 to 1.0075
9.935 to 10.065
99.35 to 100.65
393.5 to 1006.5
3.8 RESISTANCE VERFICATION
1. Select the a function.
2. Set the HI/LO push button to HI and select the 20kQ
range.
3. Connect the decade resistor (Item D, Table 3-l) to the
DMM.
4. Set the decade resistor to zero and measure the
resistance of the test leads. Subtract this reading from
the displayed reading in all of the following steps.
5. Set the decade resistor to 19.000k12. Verify that the
reading is within the limits specified in Table 3.4.
6. Select the next range and measure the next resistance es
specified in Table 3-4. Verify that each reading is within
specifications. Test the remaining ranges in the table,
switching the HI/LO push button es indicated.
Resistance
19.000kR
190.00kR
1.9000k12
19.000Mn
1.9000k0
19.000kI2
190.00k11
1900.0k$2
I
Allowable Reading!
at 18” to 28°C
18.991 to 19.009k11
189.91 to 190.09kR
:899.1 to 1900.9kR
18.980 to 19.020MI2
1.8957 to 1.9043kR
18.957 to 19.043kR
189.57 to 190.43k12
1895.7 to 1904.3k12
3.9 DC CURRENT VERIFICATION
1. Select the DC A function.
2. Connect the DC current source (Item E. Table 3-l!to the
DMM.
3. Select the 200/iA range and apply a current of lOO.OOfiA
to the DMM. The reading must be within the limits in
Table 3-5.
4. Select each range and apply the required current as
specified in Table 3-5. Verify that the reading is within
specifications.
Table 3-5. DC Current Performance Check
Allowable Readings
to 28°C
99.78 to 100.22pA
0.9978 to 1.0022mA
9.978 to 10.022mA
99.78 to 100.22mA
997.8 to 1002.2mA
9.348 to 10.052A
3.10 AC CURRENT VERIFICATION
Since AC Current uses the same circuitry as AC Volts and
DC Current already checked in paragraphs 3.6 and 3.9, no
additional accuracy checks are necessary.
3-2

SECTION 4
THEORY OF OPERATION
4.1 INTRODUCTION
This section contains circuit descriptions for the Model
179A DMM and Model 1788 Battery Pack. An overall signal
flow block diagram is provided in Figure 4-l. An overall
schematic diagram, drawing 32046, is contained at the end
of this manual.
Figure 4-1. Simplified Signal Flow Block Diagram.
Model 179A DMM
4.2 OVERALL OPERATION
The Model 179A DMM uses a 2V or 200mV full scale analog
to digital (AID) converter with a 4% digit multiplexed
display. Signal conditioning permits the AID converter 10
handle full scale AC and DC voltage and current measure-
ments over five decades, and to measure resistance over
five ranges.
4.2.1 Signal Conditioning
Signal conditioning includes DC attenuation (except on the
2V and 200mV ranges). AC attenuation, X10 amplification,
AC to DC conversion, ohms conversion and current shunts
as shown in Figure 4-2.
1. In the DCV mode, signal conditioning to the AID con-
verter is an active attenuator, except on the two lowest
ranges. The A/D input is ~V,,~,o* Rf/FiI (Rf-feedback)
resistance, RI = input resistance), except on the lowest
ranges or under overload conditions. In the DCA mode,
the voltage across the shunt resistor is applied to the AID
converter with 200mV giving a full scale reading.
2. In the ACV mode, AC inputs pass through the attenuator
on all ranges. The input is scaled to 2V rm:; full scale, ins
eluding X10 amplification for the 200mV ranges The
TRMS converter outputs a positive DC signal propor~
tional to the true root mean square AC signal. This DC
signal is the AID input. In the ACA mode, shunt voltage
is treated as a 200mV signal,
Figure 4-2. Attenuation and Ohms Conversion
4.2.2 Ohms Conversion
Resistance measurements are made by configuring the ate
tenuator as a resistance to voltage converter. Attenuator
stage voltage feedback resistors R, function as amplifier in
put resistance connected to either O.lV reference lLOt or
the l.OV reference (HI). The unknown resistance is con
netted as a feedback resistor around the atterwat~on
amplifier. The resulting voltage applied to the A/D cow
verter is proportional to the unknown resistance.
4.2.3 A/D Converter
The A/D converter is a large scale integration (LSII
ratiometric device. Converter output is a multiplexed five
digit binary coded decimal IBCD) number which is equal !(I
4-l

the ratio of input voltage to reference voltage. A separate
clock circuit supplies a 1OOkHz timing input to the integrated
circuit, which also multiplexes the BCD output. Full scale
A/D inputs for various ranges and functions are listed in
Table 4-l. Table 4-l. Full Scale A/D Inputs
DCV
DCV
ACV
DCA
ACA
11
200mV
2, 20, 200
1200v
All
All
All
HI
LO
Full Scale
A/D Input
200mV
2v
2V
200mV
2v
2v
200mV
Reference
Voltage
O.lV
l.OV
l.OV
O.lV
l.OV
l.OV
O.lV
4.3 ATTENUATION
When measuring AC and DC voltages, input signal condi-
tioning is provided by inverting amplifier UlOl and addi~
tional components as described below.
4.3.1 DC Volts
Input resistance is set by R102 and R103. During calibration,
R103 is adjusted to obtain a total input resistance of 10MIl.
Zero adjustments are provided for UlOl since an amplifier
output resolution of lO!V is required for LO resistance
measurements.
1. On the 2V and 200mV ranges. input HI is connected to
the A/D converter through protection resistors R106.
R135G and R136. Diode-connected FETs Q106 and Q107
clamp the AID input during overload.
2. On the 20, 200 and 1200 volt ranges, the amount of ate
tenuation is selected by switching feedback resistors into
the attenuator with relays KlOl, K102 and K103. Gain
setting components and attenuation values are listed in
Table 4-2.
Table 4-2. DC Attenuation and Gain Setting
Components
4.3.2 AC Volts
R&ly/
Switch
KlOl
K102
K103
Attenuation
Signal Bypasses
attenuator
0.1
0.01
0.001
Input resistance is 1MR IRlOl). Shunt capacitance is
typically less than 75pF. Additional conditioning is as
f0ll0ws:
1. For all ranges except the 200mV range. the amount of at-
tenuation is selected by switching feedback resistors into
the attentator with relays KlOl through K104. For the
200mV range, non-inverting X10 amplifier U102 boosts
the signal to a 2V full scale. Gain setting components and
attenuation values are listed in Table 4-3.
2. On the 200mV and 2V ranges. high frequency compen-
sation is adjusted with capacitor Cl1 1, as shown in Table
4.3. On the 20V range, adjustment is performed with
C112. On the 200 and 1000 volt ranges, adjustment is
performed with C106. Some low frequency rolloff is in-
troduced by input blocking capacitor C105. and AC con-
verter input capacitors Cl15 and C116.
4.4 AC CONVERSION
The AC converter is a monolithic TRMS module. Output
VDC =J---.
Avg (V 12 Potentiometer R143 provides gain adjust-
ment, and R142 establishes output zero. Settling time and
ripple are determined by Cl10 and C120. LOW frequency
rolloff is a function of C120.
4.5 OHMS CONVERSION
During calibration, the lOML2 input resistance CR102 and
R103) and all attenuator feedback resistors are adjusted for
both ratio and absolute value. Therefore. these resistors can
also serve as reference for resistance measurements. In the
11 mode, the attenuation (feedback) resistors are discam
netted from the output of the attenuation amplifier (UlOl)
and are connected instead to the A/D converw reference
voltage. Since two reference voltages and two AID cow
verter gains are available, the Model 179A DMM provides
the option of measuring resistance with the sense current
reduced by a factor of ten.
4.5.1 Range Selection
Operation of the range push buttons selects range resistors
to provide the reference current listed in Table 4-4. Opera-
tion of the HI/LO push button selects the 1V or O.lV
reference respectively on the 20k11, 200kn and 2OOOk(2
ranges. Relay K105 is always energized in the R mode.
4.5.2 R Circuit
For resistance measurements, relay K105 and terminals 4, 5
and 6 of the I2 push button connect the input HI terminal
directly to the amplifier summing node. Input LO is discon-
nected from ground and is connected to the A/D converter
input through the protection components described below.
The unknown resistance (Rx) then becomes the amplifier
feedback resistance.
1. Current flow in the unknown resistance is from input HI
to Input LO. At full scale, the voltage acr”ss R,is either
2V (HI) or 200mV (LO). Reference source loadmg does
not affect accuracy since the AID ““nverter is
ratiometric.
4-2

Table 4.3. AC Attenuation Gain Setting Components
200mV
2V
2ov
2oov
1000v
Alla. R126 KlOl
R119. R127 K102
R120: R128 / K103
R121. R122.
R129
Attenuation Freq. Camp.
Capacitors
llX10’) ClO6, Cl11
1 C106. Cl11
0.1 C106, Cl12
0.01 C106. Cl13
0.001 C106, Cl14
“Signal applied to X10 AC amplifier U102
Table 4-4. Resistance Range Setting Components
Range Resistors I Nom. lREF
1~~in LO !!
R121, R122. R129 K104 ’ lOO,<A
R120, R128 K103 lOO,,A lO,,A
R119, R127 K102 lO,<A i l,,A
R118, R126
R102. R103 KlOl
1 1000 switch,
2. The HI terminal is clamped to analog common by 0101
and Q102. The instrument protection network at the
amplifier output consists of a pulldown resistance iR104
and CR103, CR104and CR1051. R104sinksapproximaW
ly 150,tA. During in~range measurements. this current is
supplied by the reference voltage through CR105 and
voltage through the amplifier iU101) and CR104.
Overloads with input HI positive are sustained by CR105;
diodes CR103 and CR104 sustain negative overloads.
Open circuit voltage is set to less than 5V by R150 and
R151 through CR103 and CR105. AID protection in ff is
the same as in V except R105 is substituted for R106.
4.6 A/D CONVERTER
The A/D converter operates on the dual slope principle. The
timing is divided into three periods as described below.
Operation with high and low reference voltages is described
separately in paragraph 4.6.4.
4.6.1 Auto-Zero
The auto-zero period IA. Figure 4-31 is 1OOms in length,
which corresponds to 10,000 clock pulses. During this
period, reference voltage V,,, (see paragraph 4.6.4) is
stored on capacitor C124. Capacitor Cl17 stores
VREF + VOSl -vos2.
4.6.2 Signal-Integrate
The -i(rnal-integrate period (6, Figure 4-3) is 1OOms In
length, The A/D input is buffered by U104 lsee paragraph
4.6.4) and integrated by U103. Positive signals generate a
negative-going ramp at the integrator output (pit, 14,. bvi,iie
negative signals produce a positive~going ramp The level of
the integrated signal at the end of the signal-integrate period
is proportional to the average of the applied signal during
this period. Since signal integration continues for 100ms.
the A/D converter exhibits high normal mode iejectlw for
AC signals in multiples of 10Hr. paticularly the 50 and 60Hr
line frequencies.
4.6.3 Reference-Integrate
The reference~integrate period IC or D, Figure 4 31 is 200ms
or 20,000 counts in length. During this period, the ins
tegrator is returned to baseline level by applying a reference
voltage of a polarity opposite to that of the siyxl. A
posltlve~golng ramp is obtained by grounding the Ibl.ffer ins
put, while negative going ramp is produced by the integral
tion of 2XV,,, ithal is, V,,, + the voltage stored on C1241.
The time, or number of clock pusles reqGred for d~scfiarge
is proportional to the signal input. Digital output is from
latches within U106 which store the number of clock pulses
required for the integrator to return to baseline level. The
maximum count during this period is 20,000 which cork
responds to a discharge period of 200ms or full scale input.
4.6.4 Reference Voltages
Reference voltage VREF rmay be either 1V or O.lV. The
voltages are provided by a divider across a temperature
compensated rener diode. An operational amplifier on U103
provides the zener with a self~regulating bias. Use of the
O.lV reference increases converter sensitivity to 200mV full
scale, permitting accurate LO ohms operation. 1OrV resolu~
tion on DC voltage measurements, and DC amperage
4-3

Figure 4-3. A/D Converter Function
measurements with a full scale burden of 200mV. Increased
sensitivity is accomplished by switching input buffer U104
into a gain of 10 configuration by turning on Q105. Auto-
zero charging on Cl24 is to a 1OOmV reference instead of a
1V reference. Integrator and comparator voltage levels are
unaffected by buffer gain. Buffer offset voltage is zeroed.
4.7 DISPLAY
Five LED indicators are driven by U201, which is a CMOS
BCD to seven segment decoder/driver with bipolar current-
sourcing outputs. Segment currents are limited to approx-
imately 20mA peak by resistor network R202. The LED
readout is a multiplexed, common-cathode configuration
with Darlington array U202 sequentially sinking current
from each digit. Blanking of the overrange digit is ac-
complished by gates U107A and U107B. Emitter-follower
Q108 ensures that CMOS-compatible levels are maintained
on U107A. pin 1, regardless of the loading of U202. The
minus polarity readout is blanked on AC voltage and
resistance ranges by contacts on the push button switch.
Proper decimal point position is determined by the combina-
tion of function and range selected.
4.8 CURRENT MEASUREMENTS
In the A mode, the signal is switched into one of six current
shunts ahead of the attenuator section. For DC current
measurements, the shunt voltage drop is applied directly to
the AID converter input at 200mV full scale. For AC current
measurements, the shunt voltage drop is treated as a
200mV AC signal and passes through the AC attenuator
and the X10 AC amplifier. Overload clamping occcurs at
three diode voltage drops which is a level high enough to
permit high crest factor current waveforms.
4.9 AC POWER SUPPLY
When the DMM is operated from AC line power, the power
supply furnishes f5, + 15 and -15 volts from regulators
VR104, VR102 and VRlOl, respectively. Full-wave rectified
AC from bridge rectifiers CR101 and CR102 is filtered by
reservoir capacitors C108, Cl04 and Cl03 and is applied to
the linear voltage regulators.
4.10 MODEL 1788 BAlTERY PACK
When the Model 1788 Battery Pack is installed in the DMM,
S102 must be set to the BAT position to provide additional
secondary voltage for battery charging. 5102 also switches
4-4

the input to VR104 from bridge rectifier CR101 to batteries
87301. Four 2V, 2.5 ampere-hour lead-acid cells supply ap-
proximately 9.8V at full charge. After six hours of use on
battery power. the battery pack should be recharged to en-
sure long battery life.
4.10.1 Battery Charging Circuit
While the DMM is plugged into line power and the battery
pack is installed, battery charging proceeds as follows:
1. Full wave rectified voltage from CR101 is applied to the
anode of Q301. which is an SCR which regulates charg-
ing voltage. When 0301 is triggered on by a sufficient
gate-cathode voltage differential, the batteries receive
charge. Charging continues as long as the bridge output
voltage exceeds battery voltage by 1V or more. Resistor
R304 limits charging current when recharging a set of
completely discharged ceils. A filtered positive output
from CR102 lor T301l provides the necessary gate turn-
on bias through R306, and diode CR301. Resistor R303
ensures proper high temperature operation on 0301.
2. When the battery voltage reaches the preset float voltage
of 9.N. zener VR301 conducts sufficient current to turn
on 0302 and thus remove the gate trigger voltage from
Q301. Float voltage is adjusted with R301. This is a fac-
tory adjustment which normally does not need field re-
adjustment.
4.10.2 Battery Operation and Shutdown Circuit
The DMM operates as follows on battery power:
1. When the power is turned on, the batteries are connected
to the input of VR104 to supply + 5V for the logic, display
and the clock circuit. The clock output is applied to the
AID converter as described in paragraph 4-6 and also to
U301, which is a divide-by~four binary counter. The outs
puts of U301 drive a DC to DC inverter which is synchro~
nired to the AID converter to filter out invr?rter noise. The
25kHz operating frequency is optimal for the small
transformer size, and results in low switchtng losses.
Blocking capacitors C301 and C302 protect Q307 and
Q308 from damage if the drive is lost. Two half~wave reck
tifiers (CR304 and CR3051 on the secondary of 7301 pro-
vide rectified AC to filter capacitors C304 and C305
which provide power to + 15V and ~15V regulators
VFi102 and VRlOl.
2. To prevent permanent loss of battery capacity caused by
deep discharge. a shutdown circuit stops operation on
battery power when the battery voltage drops below apt
proximately 7.2V. Shutdown is performed by micropower
voltage detector U302. The open-collector output U302.
pin4) saturates low and turns off pass transistor 0309
when the input voltage (at U302, pin 3) drops below
1.15V (typical). Resistor R314 provides sufficient
hysteresis to prevent discharge from resuming when the
battery voltages rises following disconnection of the load.
4.5/4-6