D'Arsonval Meter Movement

• Electrical measuring instruments are essential for modern device maintenance. Technicians must measure current, voltage, resistance, and power.
• Electromagnetism is the most common principle used in electrical measurements.
• The strength of an electromagnetic field is proportional to the current flowing in the coil.
• Voltage, resistance, and power are related to current flow; knowing the current allows finding other values.
• Common electrical measuring instruments include ammeters, voltmeters, ohmmeters, and wattmeters.
• Digital and analogue meters are the two main types used in modern aircraft and related test equipment.

D'Arsonval Meter Movement

• The D'Arsonval meter movement is a basic DC current measuring device used in ammeters, voltmeters, and ohmmeters.
• The pointer's deflection is proportional to the current flowing through the coil.
• A reference magnetic field is created by a horseshoe-shaped permanent magnet.
• Current flows through the coil, creating a magnetic field that opposes the permanent magnet's field.
• The coil rotates on low-friction bearings until the hairspring balances the magnetic force.
• D'Arsonval meter movements are used in ammeters, voltmeters, and ohmmeters.
• A slotted screw on the front allows 'zero adjustment' of the pointer.

Permanent Magnet Moving Coil Meter (PMMC)

• The PMMC meter is based on the D'Arsonval meter movement design.
• A coil of wire is wound on an aluminium frame or bobbin supported by jeweled bearings.
• Hairsprings attach to each end of the coil, returning it to its original position when there is no current.
• Hairsprings also serve as electrical connections to the coil.
• The springs resist the coil's movement when current is present.
• As current increases, the magnetic field around the coil strengthens, causing greater movement.
• A pointer attached to the coil moves across a scale, indicating the amount of current.
• An iron core inside the coil concentrates magnetic fields, increasing accuracy.
• Curved pole pieces attached to the permanent magnet ensure a steady increase in turning force as current increases.

Meter Ratings and Terms

• Full-scale current is the current needed for full-scale deflection.
• Meter sensitivity is the reciprocal of the full-scale current, representing the resistance needed per volt for full-scale current.
• A meter requiring 1 milliamp for full-scale deflection has a sensitivity of 1,000 ohms per volt.
• Multimeters usually have a sensitivity of 20,000 ohms per volt, requiring 50 microamps for full scale.
• Highly sensitive meters are used for precise measurements of small electrical quantities.
• Total meter resistance, also known as meter resistance, must be considered in computations.
• The moving coil and hairsprings have resistance; some meters include a temperature compensating resistor.
• This resistor's resistance decreases with increasing temperature, offsetting the coil's resistance change, keeping meter resistance constant.
• The lower the full-scale current rating, the more sensitive the movement.

DC Ammeter

• An ammeter measures current, named after amperes (A).
• Smaller currents are measured using a milliammeter or microammeter.
• DC ammeters are sensitive to current direction and must be connected with the correct polarity.
• The + terminal (usually red) connects to the positive side of the circuit, and the - terminal (usually black or blue) connects to the negative side.
• Ammeters are always connected in series with the circuit path.
• Connecting an ammeter in parallel can damage it due to excessive current flow.

Ammeter Shunts

• When measuring current exceeding the meter's full-scale range, a shunt is connected in parallel.
• A shunt is a resistor type connected in parallel to increase the amount of current the meter can measure.
• The load current flowing through the shunt produces a voltage drop proportional to the current.
• Shunts are designed to carry a fixed, high proportion of the current, like 99% or 99.9%, while a small portion goes through the meter coil.

Ammeter Shunt Resistance

• A standard D'Arsonval meter may have a 1mA current sensitivity and 50Ω resistance.
• If measuring more than 1mA, a shunt resistor bypasses excess current beyond the meter's 1mA limit.
• If a 1mA meter needs to measure 10mA, the shunt resistor should carry 9mA, allowing 1mA through the meter.
• With a shunt resistance and 50Ω meter resistance in parallel, the voltage across both is equal.
• Shunts can be located inside or outside the ammeter case.
• External shunts are used in high-current cables, such as generator feeder lines.

Multirange Ammeter

• Multirange ammeters use several shunt resistors with a switch to select the desired range.
• Most modern meter movements have sensitivities from 5 microamperes to 1 milliampere.
• The ammeter has five ranges, selected by a switch; 100 microamperes, 1, 10, and 100 milliamperes, 1 ampere.
• In the 100 microampere position, all current goes through the meter movement, using none of the shunt resistors.
• In the 1 milliampere position, current goes through the meter movement and all the shunt resistors.
• A portion of the current will go through the meter movement and the rest will go through the shunt resistors.
• The meter should be switched to the appropriate range based on the desired amount of current to be measured.
• Shunt resistors are made with close tolerances, typically less than 1%.
• Since a shunt resistor is used to protect a meter movement and allow accurate measurement, it is important that the resistance of the shunt resistor is known very accurately.

Range Selection

• Proper range selection is crucial to avoid damage.
• If the current exceeds the selected range, excessive current can damage the meter.
• Always start with the highest range.
• If possible on multiple ranges, select the range for a reading near the middle of the scale.
• If the reading is too small at the highest range, switch to lower ranges.
• If the current indicated is equal to or greater than the next range of the ammeter, the meter should not be switched to the next range.

Ammeter Insertion Effects

• Ammeters affect the circuit resistance and current measurements, causing a frequently overlooked error.
• Inserting an ammeter increases circuit resistance, reducing measured current.
• The error depends on the resistance of the ammeter and the measured circuit.
• The smaller the meter movement resistance (Rm), the less it affects the circuit.

Voltmeter

• A voltmeter measures the voltage between two points in an electric circuit.
• It has a high resistance and employs a different circuit external to the meter movement.
• Analogue voltmeters have a pointer that moves across a scale proportional to the voltage.
• Digital voltmeters display a numerical voltage reading

Voltmeter Design

• Most D'Arsonval meter movements have full-scale deflection current ratings as low as 50 μA with a (internal) wire resistance of less than 1000 Ω.
• This makes for a voltmeter with a full-scale rating of only 50 mV (50 μA Χ 1000 Ω).
• To build voltmeters with practical scales, the voltage needs to be reduced to a level the movement can handle.
• The basic d'Arsonval meter movement can be converted to a DC Voltmeter by connecting a Multiplier (Rs) in series with the meter movement. The purposes of the Multiplier (Rs) in voltmeter design:
  • To extend the voltage range of the meter movements
  • To limit the current trough the d'Arsonval meter movement to a maximum full-scale deflection current

Multi-range Voltmeter

• Multiple ranges can be established for an electromechanical meter like the voltmeter, allowing it to read over a broad range effectively.
• A multi-pole switch and several multiplier resistors, each one sized for a particular voltage, accomplish range selections.
• Multiple ranges can be established for an electromechanical meter like the voltmeter, allowing it to read over a broad range effectively.
• A multi-pole switch and several multiplier resistors, each one sized for a particular voltage, accomplish range selections.

Voltmeter Loading Effects

• A voltmeter is connected in parallel with the circuit component to measure voltage.
• The parallel combination of two resistors is less than either resistor alone.
• The resistor seen by the source is less with the voltmeter connector than without.
• The voltage across the component is less when the voltmeter is connected because the voltmeter loads the circuit.
• Voltmeter loading causes a decrease in measured voltage
• The multimeter affects the voltage that you’re trying to measure.
• A voltmeter should have 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 voltage range.
• De-energize and discharge the circuit completely before connecting or disconnecting the voltmeter.
• In DC voltmeters, observe the proper circuit polarity to prevent damage.
• Never use a DC voltmeter to measure AC voltage.
• Follow general safety precautions for electrical and electronic devices.

Ohmmeter

• Mechanical ohmmeters are rarely used today
• The purpose of the ohmmeter is to measure the resistance between its leads.
• Leads are connected across an external resistance.
• The measurement is indicated through a meter movement.
• The ohmmeter has an internal voltage source to create the required current.

Ohmmeter Scale

• The ohmmeter scale is uneven and reversed compared to voltmeter and ammeter scales.
• This is because ohmmeters are current-operated devices.
• Low resistance allows high current flow and large deflection.
• High resistance means low current flow and small deflection.

Ohmmeter Design

• The ohmmeter's pointer deflection is controlled by the battery current.
• Before measurement, test leads are shorted together.
• With the leads shorted the meter is calibrated.
• In calibration the battery is at a zeroed position.
• If the leads were left connected, they could come in contact with each other and discharge the ohmmeter battery.
• Zero adjustment compensates for changes in internal battery voltage

Using the Ohmmeter

• When test leads are separated, the pointer goes to the left (infinity).
• Maximum resistance is on the left of scale.
• After adjusting, the ohmmeter is ready to be connected to a circuit to measure resistance.
• The circuit power switch on a circuit to be measured should be in the OFF position.

Multi-range Ohmmeter

• A practical ohmmeter usually has several operational ranges (R x 1, R x 10, R x 100, etc.).
• The ohmmeter reading is multiplied by the factor indicated by the range setting.
• Ohmmeters should never be connected to an energized circuit.

Shunt Ohmmeter

• The ohmmeter described to this point is known as a series ohmmeter.
• In the shunt ohmmeter, the resistance to be measured shunts (is in parallel with) the meter movement of the ohmmeter.
• Shunt ohmmeters have "infinity" on the right and "0" on the left.
• R1 is a rheostat used to adjust the infinity reading (full-scale deflection).
• If you use a shunt ohmmeter, be certain to switch it to the OFF position when you are finished using it.

Safety Ohmmeter

• Safety Ohmmeters are specifically designed for testing explosive devices.
• Used for testing in volatile and potentially explosive atmospheres.
• A safety ohmmeter uses a very small current for testing resistance (typical: current limited to 0.5 mA).

Megohmmeter (Megger)

• An ordinary ohmmeter cannot be used for measuring resistance of multimillions of ohms.
• To adequately test for insulation break down, it is necessary to use a much higher potential than is furnished by the battery of an ohmmeter.
• An instrument called a MEGOHMMETER (MEGGER) is used for these tests.

AC Meters

• A DC meter connected in an AC circuit will indicate zero.
• To use a DC meter an AC signal must be rectified.
• Two basic types of rectifiers: half-wave and full-wave.

D'Arsonval Meter with Rectifiers

• D'Arsonval meter can be used to measure AC current and voltage by connecting rectifier.
• The most frequently used circuit for full-wave rectification is the bridge-type rectifier.
• When AC is converted to pulsating DC by rectifier, the d'Arsonval movement will react to the average value of the pulsating DC (which is the average value of one-half of the sine wave).

Other Meter Movements

• The d'Arsonval meter movement (permanent-magnet moving-coil) is only one type of meter movement.
• The process of "smoothing out" the oscillation of the pointer is known as DAMPING.

Electrodynamic Meter Movement

• Fixed coils replace the permanent magnet.
• The three coils (two field coils and the moving coil) are connected in series across the meter terminals so that the same current flows through each.
• The wattmeter is the most important application.

Moving Iron Vane Meter

• The moving iron vane movement can be used to measure both AC current and voltage.
• The moving iron vane meter operates on the principle of magnetic repulsion between like poles.

HOT-WIRE METERS

• Hot-wire and thermocouple meter movements both use the heating effect of current flowing through a resistance to cause meter deflection.

MULTIMETER

• Multimeter is the most common measuring instrument used by the aircraft technician.
• There are two categories of multimeters, analogue multimeters and digital multimeters.

Basic Multimeter

• In its simplest form the multimeter is a single meter with its external circuit switched to enable it to measure amps, volts and ohms.

Multimeter Safety Precautions

• De-energize and discharge the circuit completely before connecting or disconnecting a multimeter.
• Never apply power to the circuit while measuring resistance with a multimeter.
• Connect the multimeter in series with the circuit for current measurements, and in parallel for voltage measurements.

CURRENT TRANSFORMERS

• Current transformers are devices used to scale large primary currents to a smaller, easy to measure, secondary currents.

WATTMETER

• Electric power is measured by means of a wattmeter.
• The actuating force of a wattmeter comes from the field of its current coil and the field of its potential coil.

METER GENERAL SAFETY PRECAUTIONS

• Meters must frequently be used in operating electric circuits. As a result, the risk of electric shock is often present. To avoid injury to personnel and damage to equipment, follow these basic safety rules when using electrical measuring instruments:
  • Use a meter that meets acceptable safety standards
  • Use a meter that is calibrated (check currency on calibration label)
  • Use a meter with fused current inputs. This will protect the user and/or equipment should leads inadvertently short
  • Inspect test leads for physical damage before use. A frayed lead could be very harmful
  • Use the meter to check continuity of the test leads
  • Use test leads with shrouded connectors and finger guards
  • Only use meters with recessed input jacks. Ensure the correct jacks are used with the correct function (Current readings)
  • Select the proper function and range for measurement. If in doubt select higher than required and move selection down