Fluke 8842a User Manual Page 120
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Theory of Operation
OHMS PROTECTION
5
5-13
The second stage (U404, precision resistor network Z401, and analog switches U402 and
U403) is a current amplifier whose gain is controlled by the In-Guard µC. The In-Guard
µC sets the output current for each range by controlling U402 and U403. (See switch
state table in Figure 5-7.)
5-16. OHMS PROTECTION
The Ohms Protection circuit (Q402, Q403, Q404, Q405, Q406, and Q407) clamps the
open circuit voltage of the Ohms Current Source and provides protection for the Ohms
Current Source.
The circuit protects the Ohms Current Source from up to +/-300V across the INPUT
terminals. The circuit also clamps voltage transients larger than 1560V with four MOVs
(RV401, RV402, RV403, and RV404). In addition, a 1 kΩ, 2W fusible wire-wound
resistor (R410) in series with the output current path fails open-circuit under extreme
overvoltage conditions.
Large positive input voltages are blocked by CR402. Large negative input voltages are
dropped equally across three high-voltage transistors (Q402, Q403, and Q404). If -300V
is present at the collector of Q404, the voltage drops equally across Z402 so that large
negative voltages never reach the current source.
The circuitry associated with Q408 (R406, R407, R408, R409, Q406, Q408, and CR403)
clamps the open-circuit voltage of the Ohms Current Source below +6.5V in the lower
four ranges and below +13V dc in the higher two ranges. The in-guard uC turns Q408 on
or off depending on range. In the lower four ohms ranges, Q408 is on, effectively
shorting R409; R406 and R409 then form a voltage divider which clamps the output of
the ohms current source below +6.5V. In the higher two ohms ranges, Q408 is off,
including R409 in the voltage divider and clamping the output below +13V.
5-17. OHMS FUNCTIONS
5-18. 2-Wire Ohms
In the 2-wire ohms function, the Ohms Current Source is connected to the INPUT HI
terminal by ohms relay K401 (Figure 5-8). The Ohms Current Source applies a known
current to the resistance under test, and the resulting voltage drop across the resistor is
measured ("sensed") as in the VDC function.
- Instruction Manual 1
- Table of Contents 3
- List of Tables 9
- List of Figures 11
- Contents 13
- Chapter 1 15
- 1-1. INTRODUCTION 16
- 1-3. OPTIONS AND ACCESSORIES 16
- 1-4. SPECIFICATIONS 17
- SPECIFICATIONS 19
- Operating Instructions 26
- 2-1. INTRODUCTION 27
- 2-2. INSTALLATION 27
- 2-5. Adjusting the Handle 28
- 2-6. Rack Mounting Kits 28
- 2-7. OPERATING FEATURES 29
- OPERATING FEATURES 30
- Function 31
- 2-10. Display Features 33
- 2-11. Error Messages 33
- 2-12. Overrange Indication 35
- 2-13. Diagnostic Self-Tests 35
- 2-14. Ranging 35
- 2-15. AUTORANGE 35
- 2-16. MANUAL RANGE 36
- 2-17. Triggering 36
- 2-19. EXTERNAL TRIGGER MODE 36
- 2-24. MAKING MEASUREMENTS 38
- 2-29. Offset Measurements 39
- 2-30. EXTERNAL CLEANING 40
- Remote Programming 42
- 3-1. INTRODUCTION 44
- 3-2. CAPABILITIES 44
- 3-3. BUS SET-UP PROCEDURE 44
- 3-5. A NOTE ABOUT EXAMPLES 47
- B1 Offset On 48
- PUT COMMAND FORMAT 49
- N <value> P0 49
- 3-7. Bn (Offset Commands) 50
- 3-9. Dn (Display Commands) 50
- 3-10. Fn (Function Commands) 51
- 3-11. Get Commands 51
- gain calibration 52
- 3-13. G1 (Get SRQ Mask) 53
- DEVICE-DEPENDENT COMMAND SET 54
- 3-17. G5 (Get IAB Status) 55
- 3-18. G6 (Get YW Status) 55
- 3-19. G7 (Get Error Status) 55
- 3-22. Put Commands 56
- 3-24. P1 (Put SRQ Mask) 57
- 3-27. Rn (Range Commands) 58
- 3-32. Yn (Suffix Commands) 60
- 3-33. Z0 (Self-Test Command) 60
- 3-36. INPUT SYNTAX 61
- 3-37. Definitions 62
- 3-38. Input Processing 62
- 200 PRINT @3, "F4" 64
- 300 INPUT @3, A 64
- 200 INPUT @3, A 65
- 300 PRINT @3, "F4" 65
- 200 INPUT @3, A 65
- 3-42. Types of Output Data 66
- 3-44. MEASUREMENT DATA 66
- 3-45. OVERRANGE INDICATION 67
- 3-46. ERROR MESSAGES 67
- 3-47. Status Data 67
- 3-48. Output Priority 67
- 3-49. SERVICE REQUESTS 68
- 3-51. The SRQ Mask 69
- 3-52. INTERFACE MESSAGES 70
- 3-56. TALK-ONLY MODE 71
- 3-57. REMOTE CALIBRATION 72
- 3-58. TIMING CONSIDERATIONS 72
- 3-59. IMMEDIATE MODE COMMANDS 72
- 3-60. EXAMPLE PROGRAMS 73
- EXAMPLE PROGRAMS 74
- (interface clear) 91
- 4-1. INTRODUCTION 93
- 4-2. DC VOLTAGE MEASUREMENT 93
- DC VOLTAGE MEASUREMENT 94
- 4-5. RESISTANCE MEASUREMENT 95
- 4-8. 4-Wire Ohms 96
- 4-10. TESTING DIODES 98
- 4-13. DC CURRENT MEASUREMENT 99
- Measurement Tutorial 100
- REDUCING THERMAL VOLTAGES 100
- 4-16. True RMS Measurement 101
- 4-17. Waveform Comparison 101
- 4-18. Crest Factor 103
- 4-21. Bandwidth 104
- 4-22. Zero-Input VAC Error 104
- MEASUREMENTS 106
- Theory of Operation 108
- 5-1. INTRODUCTION 110
- 5-4. DC SCALING 111
- DC SCALING 112
- 5-5. VDC Scaling 113
- 5-6. VDC Protection 114
- 5-7. mA DC Scaling 114
- 5-8. Analog Filter 114
- 5-9. TRACK/HOLD CIRCUIT 115
- TRACK/HOLD CIRCUIT 116
- 5-10. Track Configuration 117
- 5-11. Settling Configuration 118
- 5-12. Hold Configuration 118
- 5-15. OHMS CURRENT SOURCE 119
- 5-16. OHMS PROTECTION 120
- 5-17. OHMS FUNCTIONS 120
- 5-20. A/D CONVERTER 122
- 5-21. Timing/Data Control 124
- 5-22. Precision DAC 124
- 5-25. DISPLAY 125
- 5-26. KEYBOARD 126
- 5-27. DIGITAL CONTROLLER 126
- 5-28. In-Guard Microcomputer 128
- 5-31. Calibration Correction 130
- 5-35. GUARD CROSSING 131
- 5-36. POWER SUPPLY 132
- 5-38. Out-Guard Microcomputer 133
- 5-39. Guard Crossing 133
- 5-41. Signal Conditioning 134
- 5-44. VAC Scaling 134
- 5-45. mA AC Scaling 135
- A Message From 136
- PORTIONS REPRINTED 137
- Maintenance 138
- 6-1. INTRODUCTION 140
- 6-2. PERFORMANCE TEST 141
- local control 142
- PERFORMANCE TEST 144
- 6-6. Resistance Test 145
- 2 = 4 ½ digit counts 146
- 6-9. CALIBRATION 147
- 6-11. INITIAL PROCEDURE 148
- • Autoranging 149
- CALIBRATION 150
- 6-18. VERIFYING CALIBRATION 154
- 6-20. TOLERANCE CHECK 155
- 6-23. Remote Calibration 157
- 6-24. TIMING CONSIDERATIONS 160
- 6-25. REMOTE ERASURE 161
- 6-27. DISASSEMBLY PROCEDURE 162
- 6-28. Case Removal 163
- DISASSEMBLY PROCEDURE 164
- 6-31. Main PCA Removal 167
- 6-33. REASSEMBLY PROCEDURE 169
- REASSEMBLY PROCEDURE 170
- 05 Only) 173
- 6-36. TROUBLESHOOTING 173
- TROUBLESHOOTING 174
- 6-38. Diagnostic Self-Tests 178
- 6-39. Self-Test Descriptions 179
- 6-48. DISPLAY SYSTEM 185
- 6-59. ANALOG CONTROL SIGNALS 188
- 00000 11111 000000 000000 0 1 191
- 6-69. SERVICE POSITION 201
- 6-70. DIAGNOSTIC PROGRAM 201
- 6-72. SERVICE POSITION 203
- 6-73. MAJOR PROBLEMS 204
- 6-74. MORE OBSCURE PROBLEMS 206
- 6-76. INTERNAL CLEANING 207
- INTERNAL CLEANING 208
- List of Replaceable Parts 210
- 7-1. INTRODUCTION 212
- 7-2. HOW TO OBTAIN PARTS 212
- 7-4. NEWER INSTRUMENTS 213
- 7-5. SERVICE CENTERS 213
- SERVICE CENTERS 214
- Options and Accessories 230
- 8-1. INTRODUCTION 232
- 8-2. ACCESSORIES 233
- 8-10. Current Shunt (80J-10) 234
- Option –05 IEEE-488 Interface 236
- 805-3. EXTERNAL CONTROLS 238
- 805-4. INSTALLATION 238
- 805-1. INTRODUCTION 238
- 805-2. CAPABILITIES 238
- 805-6. MAINTENANCE 239
- LIST OF REPLACEABLE PARTS 240
- Option –09 True RMS AC 244
- 809-1. INTRODUCTION 246
- 809-2. INSTALLATION 246
- 809-3. OPERATING INSTRUCTIONS 247
- 809-4. MAINTENANCE 247
- Schematic Diagrams 252
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