Instrumentos Básicos para Mediciones Eléctricas

Analog Electrical Measuring Instruments

The video explains the operating principles and basic circuit models of analog instruments used to measure direct-current (DC) current, voltage, resistance, and power. It emphasizes how these instruments are built from a galvanometer and how their internal resistance can affect measurements.

1. Galvanometer: the Basic Measuring Element

A galvanometer contains a movable coil in a magnetic field. Current through the coil produces a needle deflection proportional to the current, and the needle and scale provide the reading.

Ammeters, voltmeters, and ohmmeters use a galvanometer as their fundamental measuring element, with additional components to adapt it to different measurements.

2. Analog Ammeter

An ammeter measures current. It combines a galvanometer with a shunt resistor connected in parallel to divert most of the current around the sensitive galvanometer coil.

• The shunt extends the galvanometer’s current range.
• Its value is chosen so the galvanometer receives only its full-scale current when the instrument measures the desired maximum current.
• The ammeter’s effective internal resistance is the parallel combination of the galvanometer and shunt resistances.
• An ideal ammeter has zero resistance, or as little resistance as possible, so inserting it in series does not change the circuit current.

For a galvanometer with resistance (R_g), full-scale current (I_g), and desired ammeter range (I), the shunt resistance is:

[ R_s=\frac{I_gR_g}{I-I_g} ]

The video illustrates this with a galvanometer rated at 1 mA and 50 Ω, adapted to 2 A and 5 A ranges. Higher current ranges require a lower shunt resistance.

3. Analog Voltmeter

A voltmeter measures the potential difference between two points. It combines a galvanometer with a multiplier resistor in series.

• The series resistor limits the current through the galvanometer while allowing the instrument to measure a larger voltage.
• The voltmeter’s internal resistance is the sum of the galvanometer and multiplier resistances.
• An ideal voltmeter has infinite resistance, or as high a resistance as possible, so it draws negligible current and minimally loads the circuit.

For a desired full-scale voltage (V), the multiplier resistance is:

[ R_m=\frac{V}{I_g}-R_g ]

Using the same 1 mA, 50 Ω galvanometer, the video works through 10 V and 20 V ranges. The corresponding multiplier resistances are approximately 9.95 kΩ and 19.95 kΩ. Increasing the voltage range requires a larger multiplier resistance.

4. Measurement Errors Caused by Instrument Resistance

Real instruments have finite internal resistance, so connecting them can alter the circuit being measured.

• Ammeter error: An ammeter’s internal resistance adds resistance to the series circuit, slightly reducing the current. The lower the ammeter’s internal resistance, the smaller this effect.
• Voltmeter error: A voltmeter is connected in parallel with the component being measured. Its finite resistance creates an additional current path and changes the effective circuit resistance. The higher the voltmeter’s resistance, the smaller this effect.

The video defines percentage error as the difference between the true and measured values, divided by the true value, then multiplied by 100:

[ \%\text{ error}= \frac{|\text{true value}-\text{measured value}|}{\text{true value}} \times 100 ]

The examples compare ideal circuit values with values calculated after including the instrument’s internal resistance. The key takeaway is that instrument loading is reduced by making ammeter resistance very low and voltmeter resistance very high.

5. Analog Ohmmeter

An ohmmeter measures resistance using an internal battery and a galvanometer. Its circuit includes the galvanometer resistance, a zero-adjustment resistor, and the unknown resistance (R_x), all in series.

Basic procedure:

1. Short the meter’s test leads together, making (R_x=0).
2. Adjust the zero-control resistor until the needle reaches full scale. This calibrates the instrument’s zero-resistance point.
3. Connect the unknown resistance across the leads.
4. Read the resistance from the scale. A larger (R_x) reduces circuit current and therefore reduces needle deflection.

Because resistance is inferred from current, the ohmmeter scale is reversed and nonlinear: zero resistance corresponds to maximum deflection, while increasing resistance moves the needle toward the other end of the scale.

The example uses a 9 V battery, a 1 mA galvanometer, and 75 Ω galvanometer resistance. It calculates a zero-adjustment resistance of about 8.925 kΩ. With a 1 kΩ test resistance, the galvanometer current is about 0.9 mA.

6. Analog Multimeter

An analog multimeter combines current, voltage, and resistance measurement functions in one instrument.

• A selector switch chooses the measurement type and range.
• Current ranges use shunt resistors.
• Voltage ranges use series multiplier resistors.
• Resistance ranges use the internal battery and zero-adjustment circuit.
• The instrument has shared and function-specific terminals; the correct terminals and range must be selected for the measurement.
• The video’s circuit diagram illustrates how the selector routes the galvanometer through the appropriate components for each function.

7. Wattmeter for Power Measurement

A wattmeter measures electrical power by combining voltage- and current-sensing elements.

• It uses an electrodynamic arrangement: fixed coils create a magnetic field, and a moving coil interacts with that field.
• The current coil is connected in series with the load and carries the load current.
• The voltage coil is connected in parallel with the load and responds to the voltage across it.
• The interaction of the two magnetic fields produces a deflection related to power.
• Different current-coil and voltage-coil connections provide different power ranges. The video illustrates ranges formed from 2.5 A or 5 A current-coil options and 150 V or 300 V voltage-coil options.

Speakers and Sources Featured

• One presenter/narrator explains the instruments and works through circuit diagrams and examples.
• No other speakers or quoted external sources are featured in the subtitles.