Series Resistance and Meter Coil Movement

Multi-range Voltmeter

• A multi-range voltmeter can measure voltage in four different ranges: 1, 10, 100, and 1000 V
• The meter movement's scale must be equipped with labels appropriate for each range
• Each resistor value is determined by the same technique, using a known total voltage, movement full-scale deflection rating, and movement resistance
• The multiplier resistances for a voltmeter with ranges of 1, 10, 100, and 1000 V are:
  • R4 = 500 Ω
  • R3 = 9.5 kΩ
  • R2 = 99.5 kΩ
  • R1 = 999.5 kΩ

Voltmeter Loading Effects

• When a voltmeter is used to measure the voltage across a circuit component, the voltmeter circuit itself is in parallel with the circuit component
• The parallel combination of two resistors is less than either resistor alone, resulting in a decrease in voltage
• This effect is called voltmeter loading, and the resulting error is called loading error
• A voltmeter should have a high resistance compared to the circuit being measured to minimize the loading effect

Voltmeter Safety Precautions

• Always connect voltmeters in parallel
• Always start with the highest range of a voltmeter
• De-energize and discharge the circuit completely before connecting or disconnecting the voltmeter
• In DC voltmeters, observe the proper circuit polarity to prevent damage to the meter
• Never use a DC voltmeter to measure AC voltage
• Observe the general safety precautions for electrical and electronic devices

d'Arsonval Meter with Rectifiers

• When AC is converted to pulsating DC by a rectifier, the d'Arsonval movement reacts to the average value of the pulsating DC (the average value of half of the sine wave)
• The average value is 63.7% of peak
• Although the meter movement reacts to the average value of the AC, the value used when working with the AC sine wave is the effective value (RMS value)

Ohmmeters

• An ohmmeter measures the resistance placed between its leads
• The amount of current that flows through the meter coil depends on the total resistance of resistors R1 and R2, and the resistance of the meter
• The inclusion of R1 and R2 raises the total series resistance, decreasing the current and thus decreasing the pointer deflection
• Ohmmeters should never be connected to an energized circuit (that is, a circuit with its own source of voltage)

Multi-range Ohmmeter

• A practical ohmmeter usually has several operational ranges, typically indicated by R × 1, R × 10, R × 100, R × 1000, R × 100 000, and R × 1,000,000
• The reading on the ohmmeter scale is multiplied by the factor indicated by the range setting

Megohmmeter

• A megohmmeter is used to test static dischargers
• The resistance range between the ends of a serviceable static discharger is between 1 and 100 MΩ, which is far beyond the range of a conventional ohmmeter

Multimeters

• A multimeter is a device that combines the functions of a DC ammeter, a DC voltmeter, an AC ammeter, an AC voltmeter, and an ohmmeter
• There are two categories of multimeters: analogue and digital (often abbreviated DMM or DVOM)
• Most multimeters use a d'Arsonval meter movement and have a built-in rectifier for AC measurement