Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers construction of a precision decade resistance box and fixed resistor standards for calibrating ohmmeters, resistance bridges, and ADC reference circuits. Resistors are available with ±0.01% tolerance and 2 ppm/°C TC from Vishay and similar vendors.
CHAPTER 2 — THEORY OF OPERATION
2-1. RESISTOR TYPES
For precision standards, use bulk metal foil (BMF) or metal film resistors. Wire-wound resistors are excellent for DC but introduce inductance at AC. Carbon film and carbon composition types are inadequate for calibration use due to high TC and poor long-term stability.
| Type | Tolerance | TC (ppm/°C) | Stability | Notes |
|---|---|---|---|---|
| Bulk metal foil (BMF) | 0.01% | 0.2–2 | Excellent | Vishay Z201, best choice |
| Metal film | 0.1% | 10–25 | Good | Yageo MFR, Multicomp |
| Wirewound | 0.1% | 5–10 | Excellent DC | Inductive at AC |
| Thick film (SMD) | 1% | 100–200 | Fair | Not suitable |
| Carbon film | 5% | 200–500 | Poor | Do not use |
2-2. 4-WIRE KELVIN CONNECTIONS
For resistances below 100 Ω, contact resistance in connectors and switch contacts introduces significant error. Use 4-wire (Kelvin) connections: separate current-force and voltage-sense terminals for each resistor position. See TM-CAL-010 for full Kelvin procedure.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. DECADE BOX BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 4 | Rotary switch | 12-position, make-before-break, low contact resistance |
| 40 | Metal film resistors | 1 Ω through 10kΩ in decade steps, 0.1% |
| 4 | Binding posts or BNC | Input/output terminals |
| 1 | Aluminum enclosure | Hammond 1590BB or similar |
| 1 | PC board | For resistor mounting |
CHAPTER 4 — ASSEMBLY PROCEDURES
- Mount resistors in each decade: 1Ω–9Ω (nine 1Ω), 10Ω–90Ω (nine 10Ω), etc.
- Use low-thermal-EMF solder (SAC305 or rosin-flux silver solder). Avoid no-clean flux which may cause thermal EMF at solder joints.
- Keep lead lengths short and symmetrical within each decade.
- Wire switch wipers in series: decade 4 wiper → decade 3 input → decade 2 input → decade 1 input → output terminal.
- Verify each decade with ohmmeter before final assembly.
- Mount in enclosure with clear dial markings.
CHAPTER 5 — CALIBRATION PROCEDURE
- Measure each individual resistor value before assembly. Record actual values.
- After assembly, measure at each switch position using Fluke 15B+ or better DMM.
- For positions <10 Ω, use 4-wire Kelvin measurement (TM-CAL-010).
- Record actual vs. nominal value at each position in calibration table.
- Apply correction factors in use: Vactual = switch setting × (Rmeas / Rnominal).
CHAPTER 6 — TUNING AND ADJUSTMENT
CHAPTER 7 — VERIFICATION
- Verify 1 Ω, 10 Ω, 100 Ω, 1k Ω, 10k Ω positions against NIST-certified reference resistors if available.
- For everyday use: compare to Wheatstone bridge null measurement (TM-CAL-009).
- Log calibration values; re-check annually.
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Switch Setting (Ω) | Measured (Ω) | Error (%) | Notes |
|---|---|---|---|
| 1 | 1.002 | +0.2% | Contact R dominant |
| 10 | 10.05 | +0.5% | Within 0.1% resistor spec + switch |
| 100 | 100.08 | +0.08% | Good |
| 1000 | 999.7 | −0.03% | Excellent |
| 9999 | 9998.2 | −0.018% | Excellent |