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INSTRUCTION MANUAL
MODEL 502
MILLIOHMHETER
Kelthley Instruments. Inc.
29776 Aurora Road/Cleveland, Ohio 44139/(216) 249-0400
INSTRUCTION MANUAL
MODEL 502
MILLIOHMMETER
OCOPYRIGRT 1975, ~ITRLEY INSTRUMENTS, INC.
PRINTED JULY 1977, CLEVELAND, OHIO, U. S. A.
CONTENTS
SECTION
.,,...*.........,.................................
IN'IRODUCTION I
SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...*.. II
OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. III
A. Connections
(1) Low resistances
(2) Higher resistances
B. Procedure
C. Battery Test
D. Precaution5
CIRCUIT DESCRIPTION..,............,.,.....,................... IV
,...........................,......................
MAINTENANCE v
A. Batteries
B. Trouble Shooting
C. Schematic, DR 12189 C
D. Tube Voltage Diagram, DR 12256 C
E. Replacement Parts List
0762
MODEL
502 MILLIOHMMlKTHH INl'FiODUCTION
SECTION I - INTRODUCTION
The Keithley Model 502 is a battery operated, portable milliohmmeter
for measuring resistances from 0.00003 to 1000 ohms. The instrument
employs an AC testing method, eliminating zero drift and permitting
resistance readings in the presence of DC currents. The reading is
presented on a linear scale panel meter.
Typical applications of the instrument include measurement of con-
tact resistance, conductivity of semi-conductor samples, fuse and
squib testing, and electrolyte conductivity.
Maximum power dissipation in the sample is 2 microwatts, permitting
the measurement of detonator fuses without danger of detonation.
In the measurement of contact resistance, the model 502 may be
considered a "dry circuit" tester. However, the instrument may be
used in the presence of DC biasing currents to measure the change
in resistance caused by these currents.
FIGCFZ 1. Keithley Model 502 MilJiohmmeter with
carrying case open.
l-1
MODEL 502 MILLIOHMMETER SPECIFICATIONS
Ye
::::
SECTION II - SPECIFICATIONS
RANGES: The test current, the input vpltage drop, and the power dissipation
(Pd = 2&E,,) for full-scale readings on each range are given below:
tinge, Average Average Maximum
Ohms Applied Current, Voltage Drop, Dissipation
Milliamperes Microvolts in Sample,
Microwatts
0.001 10 10 0.2
0.003 10 30 0.6
0.01 10.00 100 2.00
0.03 3.33 100 0.66
0.1 1.00 100 0.2
0.3 0.33 100 0.066
1 0.1 100 0.02
3 0.2 600 0.24
10 0.06 600 0.072
30 0.02 600 0.024
100 0.006 600 0.0072
300 0.002 600 0.0024
1000 0.0006 600 0.00072
ACCURACY: 3% of full scale on all ranges except the O.OOl-ohm range, where
it is 5% of full scale. Less than 2% error is added in measuring samples with
a series reactance of 4% of sample resistance.
OUTPUT: Meter only.
SAFETY AND RELIABILITY: Maximum power dissipation in the sample with improper
range setting is three milliwatts. Maximum dissipation caused by instrument
component failure and improper range setting is six milliwatts.
INPUT ZERO: Lever switch prevents off-scale meter indications while changing
samples.
SPEED OF RESPONSE: Five seconds to 90% of final reading on all ranges.
ZERO DRIFT: None.
REPEATABILITY: Within D
2/,.
WARMUP
TIME: Within 30 seconds.
0963R 2-1
SPECIFICATIONS MODEL 502 MILLIOHMMETER
:.
::
BATTERY LIFE: 360 hours minimum. :
BATTERY TEST: An internal resistance standard is measured in the Battery
Test Position to provide a complestrument operation.
BATTERY COMPLEMENT: Two RM 401R, two RM 42R, one 412, one 413.
TUBE COMPLEi%ENT: One 6418, four 6419.
TRANSISTOR COMPLEMENT: Four 2N1381.
ACCESSORIESFURNISHED: Model 5021 Current and Voltage Leads; one set of al-
ligator clips; one set Klipson adapters; mating connectors.
CONNECTORS: Amphenol 80C and 80-PC2F receptacles.
DIMENSIONS:~ 9 inches high x 6 inches wide x 7 inches deep.
NET WEIGHT: 7-l/2 pounds.
'02648
MODEL 502 MILLIOHMMETER OPERATION
-T
: ::
':sECTION III - o~mm10N
.A. CONNECTIONS
(1) Lower resistances (less than 3 ohms).
Each test lead has two clips, one with a red insulator and the other with
a black instilator. Use both test leads to make a connection, making sure
like-color clips are on the same side of the sample (see Figure 2). Connect
the current supply leads to the sample at any point which assures current
flow through the entire sample. This connection may include the leads
on the sample.
CURRENT
SOURCE Connect the voltmeter leads across only
Black Red that part of the sample which is to be
I RX measured, as shown in Figure 2. If the
terminal or test leads of the resistance
Red being measured are included within the volt-
Black the reading will include the
I-+ VOLTMETER meter clips,
resistance of these leads.
FIGURE 2.
NOTE
Do not connect the red clips to the black clips.
The clip-to-sample resistance of both voltmeter
clips are added in series with the unknown, and
large errors can result.
(2) Higher resistances (3 ohms and greater).
For ranges over 3 ohms full scale, the clip contact resistance is in-
significant compared with the full scale value, so voltage and current
leads may be paired and the standard two-terminal ohmmeter method em-
ployed. Of course, the four-terminal method above may be used, but the
two terminal method is faster for repeated testing.
B. PROCEDURE
Turn the power ON while the READ-SHORTSwitch is at SHORT. The instrument
will stabilize in about 15 seconds.
Connect the leads to the sample as outlined above. Then IIIOW the RRAD-
SHORTswitch to RF,AD. Rotate the OHMSFULL SCALE switch until a suitable
on-scale reading is obtained.
The READ-SHORTswitch shorts the voltage input in the SHORTposition.
It keeps the meter pointer on scale while the test leads are being connected
to an unknown. If an overload is applied to the input by inadvertently
opening the READ-SHORTswitch, no damage will result. However, approxi-
mately 30 seconds will be required for the amplifier to recover.
0264~ 3-l
O-ON MODEL 5tX MILlJon :
C. BATTERY TEST
:.
Before a reading is made, it may be desirable to check overall cir-
cuit operation.
TEST and switching
This is d&e by rotating the range switch to BATI?, -
the BEAD-SHORT switch to.BEAD. The meter should l
reati within 1% divisions of the red line on the meter face marked
BATTERY TEST. Failure of-the unit to read within these limits indicates
a circuit failure which, in most cases, will be battery failure.
Consult SEmION V - MAIWPEB4NC.E for battery changes or troubleshoot-
ing information.
PRECAUTIONS
On full scale ranges of one ohm and below, the amplifier has sensi-
tivities in the microvolt region and a pass band which includes 60
cps. Thus, a loop in the voltage leads which encloses any 60 cps
magnetic field may give a meter indication. Care should be exercised
to avoid such magnetic loops. .Care should also be taken in shielding
critical circuits to avoid 60 cps pickup from electric fields; in
general, however, the electrostatic pickup is not serious at the im-
pedance involved. One way to test for pickup is to remove. the cur-
rent supply leads with the voltage leads connected to the sample.
If no reading appears on the meter, no pickup is present. If some
reading does occur due to the presence of 60 cps magnetic field, it
may be reduced to a minimum by rotating the meter for minimum pickup.
In any case, the instrument reads correctly above any residual read-
ing. For example, if the 502 reads, say, .002 ohms due to the pres-
ence of stray fields with the voltmeter leads shorted, a .003 ohm
resistance being tested will still read ,003 ohms. This is because
the 60 cps signal is superimposed on the 100 cps square wave test
signal. Thus the 60 cps signal is not rectified and the meter reads
only the rectified square wave.
Because of the AC technique employed, inductive and capacitive com-
ponents in the test impedance may cause some waveform distortion and
erroneous readings. Series inductive impedance (at 100 cps) less
than 20% of the resistance cause the reading to be less than 2% high.
Shunt capacitive admittance (at 100 cps) less than 6% of the.hiduct-
ante cause the reading to be less than 2% low, Listed below are the
limiting values of inductance and capacitance to cause 2% error at
full scale on any range:
RANGE MAX SERIES INDUCTANCE h