Electrical Measurements: Voltage

• Electrical measurements involve quantifying electrical quantities.
• These measurements are crucial for designing, testing, and maintaining electrical and electronic systems.
• Key parameters include voltage, current, resistance, power, and frequency.

Voltage Measurement

• Voltage is the electrical potential difference between two points in a circuit.
• It's the driving force that pushes current through a circuit.
• Voltage is measured in volts (V).
• Voltmeters are used to measure voltage.
• Ideal voltmeters have very high input impedance to avoid loading the circuit.
• Voltmeters are connected in parallel across the component whose voltage is to be measured.
• Analog voltmeters use a moving coil to indicate voltage.
• Digital voltmeters (DVMs) display voltage numerically, offering higher accuracy and resolution.
• DVMs use analog-to-digital converters (ADCs) to convert analog voltage signals to digital readings.
• Common DVM features include auto-ranging, which automatically selects the appropriate voltage range.
• Overvoltage protection is essential to prevent damage to the voltmeter.

Current Measurement

• Current is the rate of flow of electric charge through a circuit.
• It is measured in amperes (A).
• Ammeters are used to measure current.
• Ideal ammeters have very low input impedance to minimize voltage drop in the circuit.
• Ammeters are connected in series with the component whose current is to be measured.
• Analog ammeters use a moving coil or iron vane movement to indicate current.
• Digital ammeters use a current-sensing resistor (shunt) to convert current to voltage, which is then measured by a DVM.
• Current clamps are used for non-contact current measurement, employing the principle of electromagnetic induction.
• High currents may require current transformers (CTs) to step down the current to a measurable level.
• Always start with the highest current range on an ammeter to prevent damage from overcurrent.

Resistance Measurement

• Resistance is the opposition to the flow of current in a circuit.
• It is measured in ohms (Ω).
• Ohmmeters or multimeters are used to measure resistance.
• Ohmmeters work by applying a known voltage to the resistor and measuring the resulting current.
• The resistance is then calculated using Ohm's Law (R = V/I).
• When measuring resistance, the component must be isolated from the circuit to avoid parallel resistances affecting the reading.
• Continuity testers are used to check for a complete circuit path (low resistance).
• Insulation testers (megohmmeters or megger) measure high resistances to assess the insulation quality of cables and equipment.
• Four-terminal sensing (Kelvin connection) is used for precise measurement of low resistances, eliminating lead resistance errors.

Power Measurement

• Power is the rate at which electrical energy is transferred or consumed in a circuit.
• It is measured in watts (W).
• Power can be calculated as the product of voltage and current (P = V * I).
• Wattmeters are used to directly measure power.
• Analog wattmeters use electrodynamic movements to measure the product of voltage and current.
• Digital wattmeters sample voltage and current simultaneously and calculate power digitally.
• Power factor (PF) is the ratio of real power to apparent power in AC circuits.
• In purely resistive circuits, PF = 1.
• In reactive circuits, PF < 1, indicating phase difference between voltage and current.
• Power analyzers measure voltage, current, power, PF, and other parameters in AC circuits.
• Three-phase power measurement requires special techniques like the two-wattmeter method or the three-wattmeter method.
• Clamp-on power meters are useful for measuring power without interrupting the circuit.

Frequency Measurement

• Frequency is the number of complete cycles of a periodic waveform per unit time.
• It is measured in hertz (Hz).
• Frequency counters are used to measure frequency accurately.
• Frequency counters work by counting the number of cycles of a signal within a known time interval.
• Oscilloscopes can also be used to measure frequency by determining the period of the waveform (frequency = 1/period).
• Heterodyne frequency meters mix an unknown frequency with a known frequency to produce a beat frequency, which is then measured.
• Spectrum analyzers display the frequency components of a signal, allowing measurement of individual frequencies and their amplitudes.
• Frequency meters are essential for characterizing oscillators, signal generators, and other frequency-dependent circuits.
• Period measurement is the inverse of frequency measurement, measuring the duration of one cycle.
• Common applications include testing audio equipment, RF circuits, and power systems.