40 Questions
What is the primary function of the switch in a multimeter circuit?
To switch between different ranges of measurements
What is the purpose of the 'Zero Ω Adjust' knob in an analogue multimeter?
To set the meter to zero for different ohm ranges
What is the sensitivity of an analogue multimeter for measuring DC?
20 000 Ω/V
What is the function of the test terminals in a multimeter circuit?
To form the positive and negative leads
What is the primary advantage of using an analogue multimeter for testing?
It is useful for testing rheostats/potentiometers
What happens to the lead polarity when the switch is in position O?
It reverses
What is the sensitivity of an analogue multimeter for measuring AC?
1000 Ω/V
What is the purpose of the multiplier in a multimeter circuit?
To connect the meter in series with the multiplier
What is the primary purpose of a current transformer?
To scale large primary currents to smaller, easy-to-measure secondary currents
What is the typical secondary current rating of a current transformer?
1 A or 5 A
What happens if the leads of a current transformer are left unconnected?
The CT will likely be destroyed
Why is it difficult to produce an output from a DC magnetic field?
Because it does not oscillate
What is the purpose of connecting a current transformer in series with the load when measuring AC current?
To scale the high current to a measurable level
What is the significance of the ratio of the windings in a current transformer?
It determines the input and output currents
Why is it important to read AC measurements on the AC scale of a multimeter?
To ensure accurate measurements
What is the purpose of adjusting the '0 Ω' reading after changing resistance ranges and before making a resistance measurement?
To ensure accurate measurements
What is the unit of measurement for electric currents?
Amps
What is the d'Arsonval meter movement principle used for?
Measuring DC current
What type of meter is used to measure smaller values of current?
Microammeter
Why must DC ammeters be connected with the correct polarity?
To prevent damage to the meter
What is the purpose of the cylindrical keeper in the d'Arsonval meter movement?
To concentrate the magnetic field
What happens if an ammeter is connected in parallel with a circuit?
It damages the ammeter
What is the function of the hairspring in the d'Arsonval meter movement?
To balance the force caused by the magnetic fields
What is the purpose of an ammeter shunt?
To allow measurement of higher currents
What is the primary component of the permanent-magnet moving-coil (PMMC) meter?
A coil of wire wound on an aluminium frame
What proportion of the current to be measured flows through a shunt?
99% to 99.9%
What is the purpose of the jewelled bearings in the PMMC meter?
To decrease friction
What is the function of the slotted screw on the front of the d'Arsonval meter movement case?
To adjust the zero position of the pointer
What is the effect of the load current flowing through a shunt?
It produces a voltage drop proportional to the current
Why must an ammeter always be connected in series with the circuit path?
To ensure that all current passes through the load and the meter
What is the name of the French scientist who developed the d'Arsonval meter movement?
Arsène d'Arsonval
What is the result of the interaction between the permanent magnetic field and the magnetic field created by the current being measured?
The coil rotates on its bearings until the forces are balanced
What is the typical range of resistances measured by a modern digital insulation tester?
0.1 to 4000 MΩ
What is the power source of a modern digital insulation tester?
9-V battery
What is the purpose of the step-up transformer in an insulation tester?
To produce high DC test voltages
What happens to the output voltage of a digital insulation tester when the TEST button is not pushed?
It becomes 0 V
What is the purpose of the rectifier in an insulation tester?
To supply high-voltage DC to the test terminals
What is the advantage of using a digital insulation tester over an analogue insulation tester?
It is electronically restricted to a low short-circuit current
What is the typical test current of an analogue insulation tester?
1 mA
What is displayed in the lower right corner of the display of the insulation tester in the photograph?
The instantaneous output voltage
Study Notes
Analogue and Digital Multimeters
- Analogue meters operate on the d'Arsonval meter movement principle, which is a current-measuring device used in ammeters, voltmeters, and ohmmeters.
- The d'Arsonval meter movement principle uses a permanent magnet, a cylindrical keeper, and a coil to measure current, with the pointer deflected in proportion to the amount of current through the coil.
Permanent-Magnet Moving-Coil Meter
- The permanent-magnet moving-coil (PMMC) meter is designed based on the d'Arsonval meter movement principle.
- A coil of wire is wound on an aluminium frame, or bobbin, and the bobbin is supported by jewelled bearings, allowing it to move freely.
DC Ammeters
- DC ammeters are sensitive to current direction and must be connected with the correct polarity.
- The terminal marked + (usually red) must be connected towards the positive side of the circuit, and the terminal marked - (usually black or blue) is connected to the negative side.
- Ammeters must be connected in series with the circuit path being tested, as connecting in parallel would give incorrect measurements and damage the ammeter.
Ammeter Shunts
- If the range of current to be measured is greater than the full-scale current of a meter, a shunt must be installed in parallel with the meter.
- A shunt is a type of resistor that increases the amount of current a meter can measure.
- Shunts are designed to carry a fixed high proportion of the current to be measured, say 99% or 99.9% compared to 1% or 0.1% through the meter coil.
Insulation Testers
- An insulation tester measures and displays resistance values in the megaohm range.
- A modern digital insulation tester can measure resistances from 0.1 to 4000 MΩ.
- Insulation testers produce high DC test voltages by regulating battery voltage, inverting it to low-voltage AC, and then stepping it up through a transformer.
Multimeter Circuit Basics
- A multimeter is a single meter with its external circuit switched to enable it to measure amps, volts, and ohms.
- Switching the multimeter to the ammeter function connects a shunt in parallel to allow it to read high currents.
- Switching to the voltmeter function connects a multiplier in series to allow it to read high voltages.
- Switching to the ohmmeter function connects the meter with a battery to measure ohms, with the lead polarity reversed to maintain correct current flow direction.
Analogue Multimeter
- Analogue multimeters have a sensitivity of 20 000 Ω/V for measuring DC and 1000 Ω/V for measuring AC.
- Analogue multimeters have a 'Zero Ω Adjust' knob/dial to zero the scale for different ohm ranges and as the internal battery discharges.
- The 'Zero Adjust' is used to set the needle to 0, and it's essential to read AC measurements on the AC scale of a multimeter.
Current Transformers
- Current transformers (CTs) are devices used to scale large primary currents to smaller, easy-to-measure secondary currents.
- The ratio of the windings determines the relation between the input and output currents.
- CTs are typically available in ratios such as 50:5, 100:5, 300:5, etc.
- It is essential to connect the CT leads to an ammeter or short them together to avoid producing a high voltage that could destroy the CT.
Learn about the basics of analogue and digital multimeters, including the d'Arsonval meter movement principle, in the context of aviation maintenance practices.
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