Advanced Amateur Radio Exam

Meter Modification and Calibration in Electronics

Introduction

In the field of electronics, modifying and calibrating meters for specific measurements is a common practice. This involves altering ammeters, voltmeters, and wattmeters to extend their range or change their sensitivity. This article explores how to calculate the necessary modifications for different types of meters and discusses their practical applications.

Meter Modification Calculations

Shunt Resistor for Ammeter Extension

• Scenario: Converting a 40 microampere full-scale meter with 96 ohms internal resistance to read up to 1 mA.

• Calculation:

  • First, we calculate the desired voltage using Ohm's Law: E (volts) = I (amps) x R (ohms)

  • E = (40 x 10^-6) (96) = 0.00384

  • Now we calculate the ratio between desired amps: 1 mA (.001) / 0.00004 (40 microamps) is 25 to 1.

  • Given this ratio, the shunt resistance is 1/25 of 96, or about 4 ohms.

Series Resistor for Voltmeter Extension

• Scenario: Converting a 1 mA full-scale, 0.5 ohms internal resistance milliammeter to a 20 volts full-scale voltmeter.

• Calculation:

  • Ohm's Law: E (volts) = I (amps) x R (ohms)

  • 20 = .001 x R

  • R = 20/.001

  • R = 20,000 ohms

  • Remember, we already have 0.5 ohms internal resistance, so subtract that and you get 19,999.5 Ohms

Multiplier Resistor for Voltmeter Range Extension

• Scenario: Extending a 150 volts, 150,000 ohms voltmeter to 750 volts.

• Calculation:

  • Multiplier resistance (Rmultiplier) = (Vfull-scale desired / Vfull-scale meter) × Rmeter - Rmeter

  • Rmultiplier = (750 V / 150 V) × 150,000 ohms - 150,000 ohms = 600,000 ohms
    
    

Meter Sensitivity and Range Extension

• The sensitivity of an ammeter is the current causing full-scale deflection.

• Voltmeter sensitivity, often expressed in ohms per volt, is calculated by dividing the meter's resistance by the full-scale voltage. For instance, a voltmeter with a resistance of 150,000 ohms on a 150-volt range has a sensitivity of 1000 ohms per volt.

• The range of a DC ammeter is extended by connecting an external resistance (shunt) in parallel.

• To increase the range of a voltmeter, a series resistor is added.
  
  

Practical Applications

• Switching a multimeter to a higher voltage range involves adding resistance in series with the meter to handle higher voltages safely.

• For accurate readings of transmitter output power, an RF wattmeter should be connected at the transmitter output connector.

• Most RF wattmeters operate at a line impedance of 50 ohms, which is standard in RF systems.

• Voltmeter sensitivity is usually expressed in ohms per volt. This means that a voltmeter with a sensitivity of 20 kilohms per volt would be a 50 microamperere meter.
  
  

Conclusion

Modifying meters for specific measurement tasks requires careful calculation and understanding of electronics principles. Whether it's extending the range of an ammeter or voltmeter, or ensuring accurate readings with RF wattmeters, each modification serves to enhance the meter's functionality for specific applications. These modifications enable more precise measurements and wider applicability, making them essential skills for electronics professionals and enthusiasts.