KEI515A 电路图.pdf

Model 515A Megohm Bridge Instruction Manual Contains Operating and Servicing Information RadioFans.CN 收音机爱 好者资料库 WARRANTY We warrant each of our products to be free from defects in materi- al 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 domes- tic 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 trace- able to the National Bureau of Standards are available upon request. RadioFans.CN 收音机爱 好者资料库 Model 515A Megohm Instruction Manual 0Keithley Instruments, Inc All rights reserved. Cleveland, Ohio, U.S.A. RadioFans.CN 收音机爱 好者资料库 MODEL 515A MEGOHM BRIDGE CONTENTS CONTENTS Section Page 1. 2. 3. 4. 5. ii 1 4 15 17 20 29 31 1072R SPECIFICATIONS MODEL 515A MEGOHM BRIDGE SPECIFICATIONS RANGE: 105 to d5 ohms with a 7-dial in-line readout. ACCURACY: (when bridge is operated es described below): Range, ObnlS Standard Deviation (lr)r* Bridge Voltage Decade 105 co lo7 .012% 10 105 to 106 107 to 108 to 109 .02% 10 107 100 109 1o1o ;: :$ .03% 10 .06% 10 10; .08% 10 lOlO 1011 eo 1012 10 $1 1012 to 1013 .16% .25% 100 Y 1013 to 1014 500 * 1014 to 1015 .3% 1.5% 500 v* $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: Any i O.SC span between 20 and 3OOC, 20-50% relative humidity. CONNECTORS: External Operate: 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). NOTES : *External supply required above 10 13 ohms. *Based an theoretical analysis of bridge errors. See Instrucrion Manual for details on obtaining specified performance. ii 1072R GENERAL DESCRIPTION SECTION 1. GENERAL DESCRIPTION 1-l. GENERAL. The Model 515A Megohm Bridge is an instrument for measuring resistance from lo4 ohms to 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 vole- age o”rce, and B Wheatstone bridge. 1-2. FEATURES Source spans a range from 1 to 110 volts in l-volt steps. c. Shielded Compartment: Connection to the bridge is made using a guarded terminal in the shielded compartment which minimizes noise pickup. a. Accuracy Verification: Accuracy is traceable to the National Bureau of Standards by use of the Model 5155 resistance standards available as an d. Standardize Mode: This mode can be selected 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. 1 1070 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. 2-2, al Xl Switch (S203) Sets voltage in l-volt steps. 2-2, a2 X10 Switch (5202) sets vo1rage in IO-volt steps. 2-2, a3 NULL DETECTOR Sensitivity Switch (S103) Selects null detector sensitivity, .I tolOOOmV/div. 2-2, a4 FINE ZERO Control (R120) Adjusts meter zero (inner knob). 2-2, a5 CO*RSE ZERO Control (5102) Adjusts meter zero (outer knob). 2-2, a6 READ/ZERO CHECK (S201) Selects READ or ZERO CHECK operation. 2-2, a, FUNCTION Switch (S301) Selects mode of operation; 4 positions. 2-2, as RESISTANCE x100 (5304) Adjusts bridge balance in steps of 100. 2-2, a9 X10 (5305) Adjusts bridge balance in steps of 10. 2-2, a10 Xl (5306) Adjusts bridge balance in steps of 1. 2-2, all X.1 (5307) Adjusts bridge balance in steps of .I. 2-2, al2 X.01 (5308) Adjusts bridge balance in steps of .Ol. 2-2, a13 x.001 (S309) Adjusts bridge balance in steps of ,001. 2-2, a14 x.0001 (S310) Adjusts bridge balance in steps of .OOfll. 2-2, a15 Multiplier Switch (S302) Sets multiplier ratio from lo5 to 1012, 2-2, a16 TABLE 1-2. Controls and Connections, Front Panel Measuring comparement. Control Functional Description Paragraph CALIBRATE Controls 106 (R319) 107 (R320) 108 (R321) 109 (R322) 1010 (R323) 1011 (R324) Safety Switch (3303) Adjusts bridge elements in CALIBRATE mode. 2-2, bl Provides a safety interlock; rernow bridge power when the 2-2, b2 compartment door is open. INPUT Terminal (5302) Provides a guarded connection to INPUT high. 2-1, a EXT. INPUT Terminal (5303) Provides a guarded connection using a triaxial cable 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. 2 1070 MODEL 515A MEGOHM BRIDGE GENERAL DESCRIPTION Sensitivity ZERO Power CHECK FUNCTION Swftch “RIO”, VOLTS Switch Zero I Y1 I Control 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. 1. Shielded Test Compartment. The Test Compart- 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. Discrete 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 ban- ana 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 recep- tacle 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 ex- ternal 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). OPERATION 2. External Voltage Source. An external voltage 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). b. Guarding. A driven guard is used extensively in the bridge circuit to minimize the effects of spurious leakage currents. 1. Theory. In megohm bridge design,care must be taken to guard the high-resistance arm so that in- sulation leakage currents will not affect the bal- ance point. Guarding in the Model 51% is shown in Figure 4. The guard enclosure is indicated by the dotted-line surrounding the high megohm STANDARD 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. The potential of ehe Driven Guard is maintained at very nearly 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. FIGURE 5. Null Detector 1070 OPERATION 3. se of the Guard. a.) connections to guard. The INPUT terminal (5302) is a guarded receptacle (Gremar, Type 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 con- nector construction is shown in Figure 6. b.) Applications. The driven guard can be used 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 Read- out Resistance of 10 megohms or lo7 ohms with a 0.02% tolerance. If the Guard to Ground fe.sist- axe were 1011 ohms, an additional error of 0.01 % would result. FIGURE 6. Triaxial Receptacle 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 mate- rials 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 meas- urement 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. 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 MODEL 515A 3. Technique for Measuring 10 10 to 1015 Ohm Resisrances. Set Canrrols as Indicated: BRIDGE VOLTS: 0 FUNCTION SWITCH: OPERATE OR EXTERNAL OPERATE MULTIPLIER: 1010, 1011, or 1012 RESISTANCE DECADE DIAL X100: lo READ/ZERO SWITCH: ZERO Insert unknwn resistor. 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, etc. It may be noted fhat the meter indication has 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 decer- mine 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 se