Keithley 515A Service manual

Model 515A

Megohm Bridge

Instruction Manual

Contains Operating and Servicing Information

WARRANTY

We warrant each of our products to be free from defects in material and workmanship. Our obligation under this warranty is to repair or replace any instrument or part thereof which, within a year after shipment, proves defective upon examination. We will pay domestic surface freight costs.

To exercise this warranty, call your local field representative or the Cleveland factory, 440-248-0400. You will be given assistance and shipping instructions.

REPAIRS AND RECALIBRATION

Keithley Instruments maintains a complete repair service and stan-

dards laboratory in Cleveland, and has an authorized field repair facility in Los Angeles and in all countries outside the United States

having Keithley field representatives.

To insure prompt repair or recalibration service, please contact your local field representatives.

To insure prompt repair or recalibration service, please contact your

local field representative 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 515A Megohm Instruction Manual

0Keithley Instruments, Inc

Cleveland, Ohio, U.S.A.

MODEL 515A MEGOHM BRIDGE

CONTENTS

Section

1.

2.

3.

4.

5.

Page

ii

1

4

15

17

20

29

31

1072R

SPECIFICATIONS

MODEL 515A MEGOHM BRIDGE

SPECIFICATIONS

RANGE: 105 to d5 ACCURACY: (when bridge is operated es described below):

Range, ObnlS

105

co lo7 .012%

107

to 108 to 109

100 109

1o1o ;: :$ 1011 eo

1012

1012 to 1013 1013 to 1014

1014 to 1015

ohms with a 7-dial in-line readout.

Standard Deviation (lr)r* Bridge Voltage

10 " .02% .03% .06% .08%

.16%

.25%

.3%

1.5%

10 "

100 Y 500 "* 500 v*

Decade

105 to 106 107 10;

lOlO

$1

$f

INPUT: Built-in comparfmenf or optional Remote Test Chamber with Teflon-insulated tri-

axial cable. GROUNDING: One terminal of unknown is at ground potential. NULL DETECTOR: Electrometer with sen*itivity of 100 microvoles per division to 1 volt

per division in five decade steps.

Meter is non-linear past l/3 of full scale for

ease in determining null.

BRIDGE POTENTIAL:

Internal: Prom 0 to + 110 volts dc in l-volt steps. External

With Keithley Model 240A or 241 High Voltage Supply, 1000 volts maximum, positive

only.

INTERNAL CHECKS: Built-in zero check anu leatige (guard fo ground) check. Test jacks

for checking wirewound standard resistors.

Bootstrap calibration from wirewound

standards for the 10' through 1012 decades.

ENVIRONMENT:

CONNECTORS: External Operate:

Any i O.S°C span between 20 and 3OOC, 20-50% relative humidity.

Teflon-insulated triaxial panel jack, Gremar 56328.

External Bridge-Potential Input: UHF. POWER: 105-125 or 210-250 volts, SO-60 Hz, 10 "atts. DIMENSIONS, WEIGHT:

Standard 19" wide x 14" high rack mounting, 11-l/2" behind front

panel (483 x 356 x 292 m), total depth, 12-314" (324 mm); net weight, 28 pounds

(12,s kg).

13

NOTES : *External supply required above

**Based an theoretical analysis of bridge errors.

10

ohms.

See Instrucrion

Manual

for details on obtaining specified performance.

ii

1072R

GENERAL DESCRIPTION

SECTION 1.

1-l. GENERAL. The Model 515A Megohm Bridge is an instrument

for measuring resistance from lo4 ohms to

GENERAL DESCRIPTION

Source spans a range from 1 to 110 volts in l-volt

steps.

lOI5 ohms with a limit of error from .05 to 1%. Ie comprises a solid-state, guarded, electrometer null detector; an ultra-stable, highly-regulated dc voleage ~o”rce, and B Wheatstone bridge.

c.

Shielded Compartment: Connection to the bridge

is made using a guarded terminal in the shielded

compartment which minimizes noise pickup.

1-2. FEATURES

d. Standardize Mode: This mode can be selected

a. Accuracy Verification: Accuracy is traceable

to the National Bureau of Standards by use of the

Model 5155 resistance standards available as an

for quick calibration of bridge elements eo correct for slight changes in the standard high megohm

resistors.

optional accessory.

e. Guard Leakage Check: A quick self check of the

b. Selectable Bridge Volcage: An internal voltage guard to ground resistance can be made using test

jacks on the front panel (inside the measuring cow pWCme”t.

1070

1

GENERAL DESCRIPTION MODEL 515A MEGOHM BRIDGE

TABLE 1-l.

Front Panel Controls.

Control

Functional Description Paragraph

BRIDGE VOLTS

Power Switch (S311)

Controls power to bridge; Selects INT or EXT. Xl Switch (S203) Sets voltage in l-volt steps. X10 Switch (5202) sets vo1rage in IO-volt steps.

NULL DETECTOR

Sensitivity Switch (S103) FINE ZERO Control (R120)

CO*RSE ZERO Control (5102)

READ/ZERO CHECK (S201)

FUNCTION Switch (S301)

RESISTANCE

x100 (5304) X10 (5305)

Xl X.1

X.01 (5308)

x.001

(5306) (5307)

(S309)

x.0001 (S310)

Selects null detector sensitivity, .I to'lOOOmV/div. Adjusts meter zero (inner knob). Adjusts meter zero (outer knob).

Selects READ or ZERO CHECK operation.

Selects mode of operation; 4 positions.

Adjusts bridge balance in steps of 100. Adjusts bridge balance in steps of 10. Adjusts bridge balance in steps of 1. Adjusts bridge balance in steps of .I. Adjusts bridge balance in steps of .Ol. Adjusts bridge balance in steps of ,001. Adjusts bridge balance in steps of .OOfll.

Multiplier Switch (S302) Sets multiplier ratio from lo5 to 1012,

2-2,

al

2-2, a2

2-2, a3

2-2, a4

2-2, a5

2-2, a6

2-2, a, 2-2, as

2-2, a9 2-2, a10 2-2, all 2-2, al2 2-2, a13 2-2, a14 2-2, a15

2-2, a16

Controls and Connections,

TABLE 1-2.

Front Panel Measuring comparement.

Control Functional Description Paragraph

CALIBRATE Controls

106 (R319) 107 (R320) 108 (R321) 109 (R322)

1010

1011 (R324)

(R323)

Safety Switch (~3303)

Adjusts bridge elements in CALIBRATE mode‘.

Provides a safety interlock; rernow~ bridge power when the

2-2,

compartment door is open.

INPUT Terminal (5302) Provides a guarded connection to INPUT high.

EXT. INPUT Terminal (5303)

Provides a guarded connection using a triaxial cable

2-1, a 2-1, a

for external inpllts.

Low Terminals Provides a connection to INPUT low when using the guarded 2-1, a

INPUT high terminal.

Tesr Jacks X01-210 Provide various circuit test paints for checkout.

bl

b2

2

1070

MODEL 515A MEGOHM BRIDGE

GENERAL DESCRIPTION

Power Swftch

Sensitivity

“RIO”,, VOLTS Switch Zero

Control

I Y1

I

ZERO CHECK FUNCTION Switch

S”:rch

Multiplier

Switch

FIGURE 2. FronC Panel Controls

FIGURE 3. Shielded Measuring Compartment

OPERATION

MODEL 515A MEGOHM BRIDGE

SECTION 2.

2-1. MEASUREMENT CONSIDERATIONS.

a. Connections.

Shielded Test Compartment. The Test Compart-

1.

ment shown in Figure 3 permits measurement of high

resistance while minimizing noise pickup and the effects of leakage paths. Input connections can be made using a triaxial receptacle (EXTERNAL INPUT

5303) or a guarded INPUT high receptacle (5302). a.) High Megohm Resistors.

can be easily measured by connecting TV the INPUT receptacle and any one of eight INPUT low resep-

tacles. The receptacles are designed for use with

test clips such as Grayhill 82-l which have a banana plug on one end and a spring clip on the other. The INPUT low receptacles are spaced one inch

apart for resistors up to 8 inches long. A typical

resistor connection is shown in Figure 3.

b.) External Connections. A teflon-insulated, guarded, triaxial receptacle (EXTERNAL INPUT 3303) is provided for external connections. The receptacle is a Gremar Type 56328 triaxial connector

which mates with a Gremar Type 7991-1 triaxial

plug (Keithley CS-69). A? optiQna1 accessory cable is available (Keithley MaJel 5153) for external connections; a 60" triaxial cable with CS-

69 connector on one end. The Keithley Model 5152

Remote Test Chamber permits external shielded measurements (with a 60" triaxial cable and CS-69 connector).

Discrete resistors

OPERATION

External Voltage Source. An external voltage

2. source can be connected to the bridge using the rear panel UHF coaxial receptacle (5211). This connector is a Gremar Type 6804 UHF receptacle

(Keithley CS-64) which mates with a Gremar Type

5127 plug (Keithley CS-49).

Guarding. A driven guard is used extensively

b.

in the bridge circuit to minimize the effects of spurious leakage currents.

1. Theory. taken to guard the high-resistance arm so that insulation leakage currents will not affect the balance point. Guarding in the Model 51% is shown in Figure 4. The guard enclosure is indicated by the dotted-line surrounding the high megohm

resistors, the electrometer null detector, and the guarded input terminal. The inpur high terminal utilizes a "guard ring" maintained approximately at the potential of the null detector law.

2. Circuitry. The guard potential is obtained

from the null detector (electrometer) power supply common as illustrated in Figure 5. of ehe "Driven Guard" is maintained at very

the Input High potential with the result that the High to Guard leakage is extremely small at bridge balance. The guard circuit is formed by a metal enclosure and plates which surround the STANDARD

resistors, CALIBRATE resistors, the null detector,

and the Input terminal.

In megohm bridge design,care must be

STANDARD

The potential

nearly

FIGURE 5.

Null Detector

1070

OPERATION

3. "se of the Guard. a.) connections to guard.

(5302) is a guarded receptacle (Gremar, Type

The INPUT terminal

6804)

with a center High contact and an outer Guard ring. The EXTERNAL INPUT terminal (5303) is a guarded rriaxial receptacle (Gremar, Type

5632A)

with a center High cmtact, an inner Guard confact and an outer Low confact. A drawing of the connector construction is shown in Figure

b.) Applications.

The driven guard can be used

6.

for external measurements when it is necessary to minimize the effects of spurious leakage currents across the insulation. A rypical external measurement can be accomplished using Keiehley Model 5152 Remote Test Chamber and teflon-insulated

triaxial cable. The “se of the Model 5152 is complerely described in Section 4, Accessories.

c.) Guard to Ground Leakage. The design of the

guard circuit in the Model 515A maintains the Guard

fo Low (ground) resistance greater than 1011 ohms. The Guard to Ground resistance should be high with

respect to the resistance from floating low to

ground so that the shunting effects across the Readout Resistance will not be significant. Far example the worst-case condition would be a Readout Resistance of 10 megohms or lo7 ohms with a

0.02%

tolerance.

If the Guard to Ground fe.sistaxe were 1011 ohms, an additional error of 0.01 % would result.

c. Leakage. The Input terminals of the Model 515A

have been designed using teflon insulation between High and Guard and Guard and Ground. In order eo maintain the high insularion resistance, the terminals must be kept clean and dry.

Preferred cleaning materials include: Chemically pure alcohol, sterile cotton swabs (to prevent contamination of alcohol), and a drying agent such as nitrogen. Leakage paths across the terminal can create intermitrant errors or

difficult bridge balance. The user should also take

care to insure that the unknown resistor, holding fixtures and case are insulated properly. Glass envelopes (high.megohm resistors) can be contaminated

by oil and salts from improper handling. Paper base bakeliee insulation can be degraded by improper

handling and exposure to moisture. The humidity of the laboratory environment can also affect the measurement of very high resistances. See Specificarians.

d. Noise. Noise pickup from ac electric and mag-

netic fields is minimized by the unit construction

of rhe chassis and the use of a closed, shielded

measuring compartment. When using an external unknown

resistance, care should be taken to:

1. Use shielded cables such as Keithley Model

5153 triaxial cable.

2.

Fasten dawn the cables so that flexure noise

is minimized.

3. Maintain Guard to Ground insulation using

teflon insulation.

FIGURE 6. Triaxial Receptacle

4. Use an external shielded test box such as

Keithley Model 5152 Remote Test Chamber.

e. Accuraw.

1. Specification. The specified accuracy for

measurements on various ranges is valid for the

following conditions.

a.) Minimum Bridge Potential. This potential

is the minimum voltage required for resolution.

b.) Environment. The ambient temperature and relative humidity must be controlled within limits stated.

c.) Standardization. The Standardization procedure should be performed prior to very critical measurements.

d.) Proper Operating Technique. Care must be taken when connecting the unknown (See Measure-

ment Considerations, Paragraph 2-1) and balancing

the bridge (See Accuracy Considerations, Para-

graph Z-7).

2. Verification. The Model 515A accuracy can

be verified using the Model 5155 Megohm resistance

standards.

1072R

5

OPERATION

10

3.

Technique for Measuring 10

to 1015 Ohm

Resisrances.

Set Canrrols as Indicated:

BRIDGE VOLTS: 0 FUNCTION SWITCH: MULTIPLIER:

OPERATE OR EXTERNAL OPERATE

1010, 1011, or 1012 RESISTANCE DECADE DIAL X100: lo READ/ZERO SWITCH:

Insert unknwn resistor.

ZERO

Set READ/ZERO Switch m READ and note offset of null deeector with MULTIPLIER setring to be used in measurement. Allow approx-

imately 15 minutes for reading t” stabilize. The affser of the null detector is due to offset current from the null derecror and from stressing of Teflon insulation surrounding the Hi terminal of the bridge.

Use the offset reading as null for measuring the

unknown. Ser READ/ZERO Switch to ZERO.

Select the Bridge potential for the measurement based on desired accuracy as described in derail in the OPERATION secfion af the manual. Set READ/ZERO Switch m READ and balance bridge using Resistance decade dials.

?IETER NOISE: I” balancing the bridge when measuring

resistances greater than lolo ohms there is meter noise present due to l/f noise, alpha particle noise,

It may be noted fhat the meter indication has

etc. a base-line from which meter fluctuations diverge. The actual null detector reading is this base-line when balancing the bridge.

NOTE

Care should be taken m allow enaugh time for bridge to stabilize fc, a reading. This rime will vary from one measurement to another however, a minimum time of 15 minures is advisable to decermine final null reading.

2-2. CONTROLS AND SWITCHES.

a. Front Panel.

1. Power Switch (5311). This switch controls the power to the bridge including the bridge porenrial and null detector supplies. The INT position permits a setting of the bridge potential

from 1 to 110 volts using the Xl and X10 BRIDGE VOLTS switches.

The EXT position connects the external voltage input (5211) so that a bridge potential up to 1000 V can be applied “sing an external voltage supply such as Keithley Model

241.

2.

Xl Switch (S203). This switch permits a set-

ting af the bridge p@zential in l-volt increments

up to 10 volts.

3. X10 Switch (S202). This switch permits a

setting of the bridge potential in lo-volt incre-

nle*tS up to 100 volts.

4. Sensitivity Switch (S103). This switch se-

lects the null detector sensitivity from .l to 1000

milliu0lts per division.

MODEL 515A

5. FINE Control (Rl20).

This control is the inner knob af a dual-concentric canerol. The FINE Control permits adjustment of the meter zero.

6.

COARSE ZERO Switch (S102). This switch is the Outer knob af a dual-concentric control. The COARSE Switch permits adjustment of the meter zero in 10 steps.

7.

READ/ZERO CHECK Switch (S201). This switch selects READ or ZERO CHECK operation for the meter circuit. In ZERO CHECK position the null deeecmr High and Low ‘are shorted together.

8. FUNCTION Switch (5301). This switch selects

the mode of Operation in 4 position,

OPERATE,

STANDARDIZE, CALIBRATE, and EXTERNAL OPERATE. A complete discussion of these modes is given in paragraph 2-3.

9. Xl00 Resistance Switch (5304). This switch adjusts the “Readout” am of the bridge in steps of 100.

10. X10 Reeietance Switch (S305). This switch adjusts the “Readout” am of the bridge in seeps of 10.

11. Xl Resistance Switch (5306). This switch adjusts the “Readout” arm of the bridge in steps

of 1.

12. X.1 Resistance Switch (5307). This switch

adjusts the “Readout” arm of the bridge in steps of .l.

13. X.01 Resistance Switch (S308). This switch

adjusts the “Readaut” am of the bridge in steps of .Ol.

14.

X.001 Resistance Switch (S309). This switch adjusts the “Readout” am of the bridge in steps Of .OOl.

15. X.0001 Resistance Switch (S310). This switch adjusts the “Reabou:” arm of the bridge in steps Of .OOOl.

16.

Multiplier Switch (S302). This switch sets

the multiplier ratio from 105 to 1012.

b. Measuring Cmpartment.

1. CALIBRATE Confmls. These controls are used to adjust the bridge when the FUNCTION Switch is set to CALIBRATE. The use of these controls is de-

scribed in paragraph 2-4, d (Standardization pro-

cedure).

2.

Safety Switch (5303). This switch is a normally-open interlock which remwee bridge power when the compartment door is open. The safety in-

terlock is defeated when the FUNCTION switch is set to EXTERNAL OPERATE.

c. Rear Panel.

117-234V Line Switch (5312). This switch sets the Model 515A far either 117V or 234V rms line power,

50-60

Hz.

6

1072R

MODEL 515A

OPERATION

2-3. MODES OF OPERATION.

a. operate.

This mode of operation permits meas-

urements of high megohm resistances when connected to

the INPUT receptacle.

FUNCTION switch to OPERATE.

To select this mode, set the

Either the internal voltage source (bridge potential) or an external voltage source up to 1000 volts can be used. The safety

interlock switch prevents operation of the bridge

whenever the compartment door is open.

If the unknown resisCa”ce must he measured externally, “se the External Operate mode.

b. External operate.

This mode of operation permits resistance measurements the same as for the Operate mode.

To select this mode set the FUNCTION

switch to EXTERNAL OPERATE.

WARNING

When the FUNCTION Switch is set to EXTERNAL OPERATE the safety inter-

lock feature is defeated. Therefore the bridge voltage (up to 1000 volts) is present at ehe Guard circuit at receptacle 5303.

The user should be ca”tio”s when “sing very high bridge voltages. For maximum safety, the

Power Swifch (S311) should be set to

INT when not making a measurement.

c.’ Standardize/Calibrate. These modes of operation permit adjustment of the bridge elements to compensate for slight variations of the standard high

megohm resistors. To select either made set the FUNCTION Switch to STANDARDIZE or CALIBRATE as described in paragraph 2-4, d.

2-4.

PRELIMINARY PROCEDURES

1. COARSE ZERO Switch (S102) (Outer Knob). This switch has eleven positions for adjustment of meter zero.

2. FINE ZERO Control (R120).

This control provides fine (potentiometer) adjustment of the meter zero.

c. Warmup. The instrument should be allowed to

stabilize with power on (at least 30 minutes). If

the instrument has been exposed to an extreme ambient temperature change the warmup time should be extended fo

24

hours or more. Standardize Procedure. For critical measure-

d.

ments the instrument should be standardized prior to each measuremene to compensate for slight changes of the standard high megohm resistors due to temperature variations and aging with time. The Standardize procedure should be performed as described in Table 2-l. A complete discussion of the Standardization technique

is given in paragraph 2-5. 2-5. MEASUREMENT PROCEDURE

a. Connect Unknown Resistance. Determine the method of connection to the unknown as discussed in paragraph 2-l.

b. Select the Bridge Potential. The minimum Bridge Potential should be determined for rated acc”racv as

stated in the specifications. The voltage can b; applied internally (with Power Switch set to INT) or externally as described in paragraph 2-l.

c. Standardize Bridge. For measurements where the effects of variations of the bridge elements must be minimized, the Standardization procedure should be performed as described in paragraph 2-3, d.

a. Power.

1. Line Voltage. This inscrumenc can be connect-

ed to 117 volt, 50-60 Hz line power when the Line Voltage Switch (on the rear panel) is set to 117”. The fuse should be a type 3AG, 1/4A, SLO-BLO. When using

234V

power, set the Line Voltage Switch to

234V and replace fuse with a type 3AG, 1/8A, SLO-BLO.

2. Accessory Outlet. A three terminal power outlet (5301) is provided on the rear panel far operation of an accessory such as an external power

S”PPlY. and is not controlled by the Power Switch.

3.

This outlet is wired to rhe line power cord

Power Cord. A three wire power cord is supplied (6 feet long). A third prong is used far earth ground connection for the chassis. An adapter is supplied for converting to a two prong outlet, but should only be used when a solid, earth-connec-

tion is made by some other means.

b. Meter Zero. The meter circuit can be zeroed by

adjustment of COARSE ZERO and FINE ZERO Controls. The READ/METER ZERO Switch (S201) should be -et to METER ZERO.

1072R

d. Meter Zero. Adjust meter zero as necessary.

iiridge Balance.

e.

With the FUNCTION Switch set to OPERATE, proceed to balance the bridge (set the READ/ZERO Switch ea READ).

Increase the null sensi-

riviey as necessary to obtain a precise bridge balance.

f. External Operate. If the unknown resistance is connected externallv the FUNCTION Switch should be set to

EXTERNAL

2-6.

a.

OPERATE before bridge balance is attempfed.

STANDARDIZE.

Purpose of Standardization Technique. Wirewound resistors have the greatest accuracy and keep their calibrations over long periods of time. Values greater than about one megohm, however, are too large and coo expensive to be widely used. Film type resistors pravide values up to lOI ohms and higher with reasonable SUCCESS and this type resistor is used in rhe Model 515A. B”t the value of these resistors changes with time, sometimes one percenr per year.

The Keithley Model 51% Megohm Bridge has been designed so that frequent compensations can be made for variations of its high-megohm standard resistors. This process is called Standardization and is carried out as in paragraph 2-4, d.

7

OPERATION

TABLE 2-l.

Standardization.

MODEL 515A

Step

A B

c

D

E

F

G

H

I

J

K

L

M

N

8

Power On

Meter Zero

Standardize

Calibrate

Standardize

Calibrate

Standardize

Calibrate

Standardize

Calibrate

Srandardize

Calibrate

Standardize

Calibrate

Procedure Set Power Switch to INT. Set READ/ZERO Switch to ZERO and adjust FINE

control for meter zero. Set Controls as indicated. Set READ/ZERO Switch

to READ and balance bridge using RESISTANCE

decade dials. Increase sensitivity as necessary.

Set READ/ZERO Switch to ZERO and go to next step.

With bridge at balance set FUNCTION Switch to

CALIBRATE. Adjust calibrate potentiometer for

bridge rebalance with the READ/ZERO Switch set

to READ. Set READ/ZERO Switch to ZERO,

Set Controls as indicated. Set READ/ZERO Switch

to READ and balance bridge using RESISTANCE

decade dials. Increase sensitivity as necessary.

Set READ/ZERO Switch to ZERO and go to next step.

With bridge at balance set FUNCTION Switch to

CALIBRATE. Adiust calibrate potentiometer for bridge rebalance with the R&/ZERO Switch set

to READ. Set Controls as indicated. Set READ/ZERO Switch to READ and balance bridge using RESISTANCE decade dials. Increase sensitivity as necessary. Set READ/ZERO Switch to ZERO and go to next step.

With bridge at balance set FUNCTION Switch to

CALIBRATE. Adjust calibrate potentiometer for bridge rebalance with the READ/ZERO Switch set to READ. Set Controls as indicated. Set READ/ZERO Switch to READ and balance bridge using RESISTANCE decade dials. Increase sensitivity as necessary. Set READ/ZERO Switch to ZERO and go to next step. With bridge at balance set FUNCTION Switch to CALIBRATE. Adjust calibrate potentiometer for bridge rebalance with the READ/ZERO Switch set to WAD. Set READ/ZERO Switch to ZERO.

Set controls as indicated, with zero Bridge volts and Xl00 Resistance decade dial set to 10. Set

READ/ZERO Switch to READ. Note offset of null

detector after allowing time for stabilizing after switching FUNCTION Switch (10 - 15 minutes). This offset is due to offset current. Use this offset reading as null for Standardizing 10'". Set controls as indicated. Set READ/ZERO Switch to READ and balance bridge using Resistance decade dials. Increase sensitivity as necessary. Set READ/ZERO Switch to ZERO and go to next step. Use null detector zero for null in CALIBRATE. With bridge at balance set FUNCTION Switch to CALIBRATE. Adjust calibrate potentiometer far bridge rebalance

with the READ/ZERO Switch set to READ. Set READ/

ZERO Switch to ZERO.

Set conrrals as indicated, wirh zero Bridge volts and Xl00 Resistance decade dial set to 10. set READ/ZERO Switch to READ. Note offset of null detector after allowing time for stabilizing after switching FUNCTION Switch (10 - 15 minutes). This

offset is due to offset current. Use this offset

reading as null for Standardizing 1011.

Set controls as indicated. Set READ/ZERO Switch

to READ and balance bridge using Resistance decade

dials. Increase sensitivity as necessary.

READ/ZERO Switch co ZERO and go to next step. Use null detector zero for null in CALIBRATE. With bridge at balance set FUNCTION Switch to CALIBRATE. Adjust calibrate potentiometer for bridge rebalance with the READ/ZERO Switch set to READ. Set READ/

ZERO Switch to ZERO. Set FUNCTION to OPERATE.

Set READ/ZERO Switch to ZERO.

Dial Seefing FUNCTION Switch

Set

Multiplier

(S302) Setting (S301)

10’5

106

107

108

109

lOlO

1010

lOlO

loll

STANDARDIZE

CALIBRATE

STANDARDIZE

CALIBRATE

STANDARDIZE

CALIBRATE

STANDARDIZE

CALIBRATE 109

OPERATE

STANDARDIZE

CALIBRATE

OPERATE

STAND*RDIZE

CALIBRATE

Calibrate

Control

106

107

108

lOlO

loll

1072R

MODEL 51524

b. Description of circuit and technique.

I. A simplified bridge circuit with FUNCTION

switch in ST*NDARoIZE is shown in Figure 9.

2. With Resistance Multiplier Switch eef et lo6,

then the bridge null is obtained for the condition s6,S, = B6/A, where “S,j”, s’S7’1, “B6”, and “A” are defined as follows:

= Standard resistor (lo6 ohm, .02%) selected

"'6"

“ST?<

"'6" ,,*,,

by resistance multiplier switch set at 106.

= Standard resistor (lo7 ohm, 1.0%).

= 105 ohm, .02%.

= Resistance decade potentiometer adjusted

for null (106 ohms).

NOTE

This technique is used ta determine the ratio of

r?,611 to ‘37” accurately as read by resistance

decade dials.

3. A simplified bridge circuit with FUNCTION

switch in CALIBRATE is shown in Figure 10

4. In the CALIBRATE position, a resistance ratio

network of 1:lO is connected in place of “56” and

*‘S7”. The bridge null is obtained far the condition R = +/A, where “R”, “S7”, and “A” are defined as follaws:

“Ru

= Ratio network of ‘I:10 with .005% accuracy.

OPERATION

2-7.

THEORY OF OPERATION. Bridge Theory. The Megohm Bridge has been de-

a.

signed to measure very high resistances using a Wheatstone Bridge and a sensitive null detector. The Wheatstone Bridge circuit basically consists of four

arms, identified as A, 8, S, and X as shown in Figure

11. (A

thorough

discussian of bridge meaeurement is

available in Electrical Measurement, F. K. Harris, Wiley, New York, 1952). The equation for the bridge

at balance can be described by the equation:

x = s x A/B

b. Null Detector Sensitivity. The sensitivity of

the bridge can be described by the equation:

e = Sd

OL

xE

e = sensitivity in volts d = incremental unbalance (in terme of the unknown) E = bridge potential

S = standard resistance

X = unknown resistance.

C. Voltage Across the Unknown. The Voltage acr~se the unknown resistance can be determined for a given set of conditions where:

E =

bridge potential (bridge volts setting) standard resistance (multiplier setting)

s =

x =

unknown resistance (approximate value of the

“IlkFlOWn )

“87” = Calibrate potentiometer.

= Resistance decade p@zentiometer adjusted

“A”

previously in b2.

NOTE This technique is used to set the calibrate paentiometer “8,”

such that errcx in “57”

is compensated.

5. A simplified bridge circuit with FUNCTION

switch in OPERATE is shown in Figure 11.

6.

In the OPERATE position, the bridge null is obtained for the condition ST/X = 07/A. when resistance decade potentiometer “A” is properly adjusted.

7. A simplified bridge circuit with FUNCTION

Switch in OPERATE and resistance multiplier 106

is shown in Figure 8.

TEST JACKS

5201 2 3 4 5 6

oegoouooo

7 8 9 10

JUMPERS (NORMAL POSITION)

FIGURE 7.

Test Jack Identification.

“, =

voltage across x = E x

X+S

2-a. GuARo LEAKAGE CHECK. The fallowing procedure should be used to verify the

guard to ground resistance. The teet jacks are identified in Figure 7.

Procedure :

1. Connect power cord to line voltage.

2. Place

3.

iumers

3203

and j204

5205

and 5206

set 515‘4 controls

between the followine

as foliows.

test

iacks.

BRIDGE POWER - EXT

MILLI"0l.T PER DIVISION

- 1000 FUNCTION - OPERATE MULTIPLIER - 105 READ/ZERO CHECK

- ZERO CHECK Xl00 DIAL - 1 DOOR

4.

Connect Keithley Model 240A Power Supply to

- CLOSED EXTERNAL INPUT on the rear panel.

Set Model 240A to 1000 volts with

5.

6.

Set READ/ZERO CHECK switch to READ. Allow

7.

five minutes for the Made1 515A reading to

OUTPUT to +.

stabilize. Reading shall be less than 1000 mV.

NOTE.

The meter indicates the voltage drop acrcse a 108 resistox in series with the leakage path. Leakage resistance from GUARD to

GROUND

is deter-

mined as fallows.

Leakage current = (Voltage drop : lo* ohms) Leakage resistance = (1OOOV + leakage current) Typical GUARD to GROUND resistance is greater

than 1011 ohms.

9