Aircraft Maintenance: Ammeter Shunts

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Questions and Answers

What is the purpose of a shunt resistor in an ammeter?

To bypass current that exceeds the limitation of the meter movement (correct)

To increase the current sensitivity of the meter movement

To increase the resistance of the meter

To decrease the voltage drop across the meter

What is the current sensitivity of a standard d'Arsonval meter movement?

100 mA

10 mA

1 mA (correct)

0.1 mA

What is the resistance of the meter movement in the example given?

500 Ω

10 Ω

50 Ω (correct)

100 Ω

What is the current that the shunt resistor should carry in the example given?

9 mA Signup and view all the answers

What is the relationship between the voltage drop across the shunt resistor and the voltage drop across the meter resistance?

Esh = Em Signup and view all the answers

What is the equation that relates the shunt resistance and the meter resistance?

ISH × RSH = IM × RM Signup and view all the answers

What is the calculated value of the shunt resistance in the example given?

5.56 Ω Signup and view all the answers

Where can shunts be located?

In the ammeter case or externally Signup and view all the answers

What is necessary for an ohmmeter to operate?

An internal source of voltage Signup and view all the answers

What is the purpose of the ranging resistors in an ohmmeter?

To allow just the right amount of current through the movement Signup and view all the answers

Why is the ohmmeter scale uneven and reversed?

Because the meter is a current-operated device Signup and view all the answers

What is the relationship between the resistance being measured and the meter deflection?

Inversely proportional Signup and view all the answers

What is the purpose of shorting the test leads of the ohmmeter together?

To calibrate the meter for proper operation on the selected range Signup and view all the answers

What is the main purpose of a multiplier in a voltmeter design?

To limit the current through the meter movement to a maximum full-scale deflection current Signup and view all the answers

What type of meter movement is commonly used in analogue voltmeters?

D'Arsonval meter movement Signup and view all the answers

What happens to the meter current when the test leads are shorted?

It becomes maximum Signup and view all the answers

What is the purpose of a multi-pole switch in a multi-range voltmeter?

To switch between different voltage ranges Signup and view all the answers

What is the position of the pointer on the ohms scale when the leads are shorted?

At the zero position Signup and view all the answers

What is the full-scale deflection current rating of a sensitive D'Arsonval meter movement?

50 µA Signup and view all the answers

What is the purpose of the moving coil in an ohmmeter?

To control the pointer deflection Signup and view all the answers

What is the main difference between analogue and digital voltmeters?

The way the voltage is displayed Signup and view all the answers

What is the internal wire resistance of a D'Arsonval meter movement?

1000 Ω Signup and view all the answers

What is the purpose of a multiplier resistor in a multi-range voltmeter?

To size the voltage range for a particular multiplier Signup and view all the answers

What is the primary purpose of a static discharger in an aircraft?

To prevent RFI by preventing sparks from jumping between the control surface and the airframe Signup and view all the answers

What is the typical resistance range of a serviceable static discharger?

1 - 100 MΩ Signup and view all the answers

What is the voltage range of a voltmeter that uses a D'Arsonval meter movement with a full-scale deflection current rating of 50 µA and an internal wire resistance of 1000 Ω?

50 mV Signup and view all the answers

What is the purpose of removing corrosion protection from the sleeve before testing a static discharger?

To obtain a low resistance connection Signup and view all the answers

What type of meter movement is commonly used in multimeters?

D'Arsonval meter movement Signup and view all the answers

What is the name 'multimeter' derived from?

MULTIple METER Signup and view all the answers

What type of measurement is possible with a multimeter that has a built-in rectifier?

Both AC and DC measurements Signup and view all the answers

What is the common abbreviation for a digital multimeter?

DMM Signup and view all the answers

What is the primary function of an aircraft technician's most common measuring instrument?

To measure multiple electrical parameters Signup and view all the answers

What type of magnetic field does a meter current transformer have?

Oscillating magnetic field Signup and view all the answers

What is the advantage of using voltage transformers in voltmeters?

To make use of voltage transformers in place of series resistors Signup and view all the answers

What is the function of the pivoted limb in the current transformer?

To open the core to admit the cable Signup and view all the answers

What is the purpose of the trigger in the "clip-on" type of ac ammeter?

To ensure adequate pressure at the butt joint of the core Signup and view all the answers

What type of instrument is used in the "clip-on" type of ac ammeter?

Permanent magnet moving coil instrument Signup and view all the answers

What is the advantage of using a current transformer in an ammeter?

Instead of shunts in ammeters Signup and view all the answers

How is the current in a conductor measured using the "clip-on" type of ac ammeter?

By enclosing the conductor in the meter Signup and view all the answers

Study Notes

Ammeter Shunt Resistance

A standard d'Arsonval meter movement has a current sensitivity of 1 mA and a resistance of 50 Ω.
To measure more than 1 mA, a simple shunt resistor is required to bypass excess current.
The shunt resistor carries the excess current, while the remaining 1 mA passes through the meter.
The shunt resistance and meter resistance are in parallel, resulting in the same voltage drop across both.

Ammeter Shunt Calculation

Using Ohm's law, the shunt resistance can be calculated as: RSH = (IM × RM) / I.SH
Example: RSH = (1 mA × 50 Ω) / 9 mA = 5.56 Ω

Shunt Location

Shunts can be located in the ammeter case or externally.
Remotely located external shunts are used in high-current cables, such as generator feeder lines.

Voltmeter Design

D'Arsonval meter movements are sensitive devices with full-scale deflection current ratings as low as 50 µA.
To build voltmeters with practical scales, a multiplier (RS) is connected in series with the meter movement.
The multiplier extends the voltage range of the meter movement and limits the current through the meter movement.

Multi-Range Voltmeter

A multi-range voltmeter uses a multi-pole switch and several multiplier resistors, each sized for a particular voltage range.

Ohmmeter Design

An ohmmeter uses an internal source of voltage to create the necessary current to operate the movement.
The ohmmeter has ranging resistors to allow the right amount of current through the movement at any given resistance.
The ohmmeter scale is uneven and reversed compared to the voltmeter and ammeter scales due to the inverse relationship of Ohm's law formula: R = E/I.

Multimeter

A multimeter is a DC ammeter, DC voltmeter, AC ammeter, AC voltmeter, and ohmmeter in one package.
Multimeters can be analogue or digital (DMM or DVOM).
Most multimeters use a d'Arsonval meter movement and have a built-in rectifier for AC measurement.

Transformer/Rectifier Meters (Clamp Meter)

A transformer/rectifier meter uses a voltage transformer to adapt moving-coil instruments for AC measurement.
The output of a transformer is a function of the input and can be used to supplement series resistors in voltmeters or current transformers in ammeters.
The "clip-on" type of AC ammeter uses a current transformer to measure current in a conductor by enclosing the conductor at any convenient point.

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Description

Learn about ammeter shunts and their role in aircraft maintenance, including the use of standard d'Arsonval meter movements and shunt resistance. This quiz is based on CASA Part 66 training materials.