7.3.16 Electrical Test Equipment Flashcards
Why is it essential to measure electrical circuits in aircraft maintenance?
To ensure serviceability and troubleshoot issues.
What are the most important electrical measurements?
Current, voltage, resistance, and in some cases, power.
What principle is most commonly used for electrical measurements?
Electromagnetism.
What are the two fundamental assumptions of electromagnetism?
- The strength of an electromagnetic field is proportional to the amount of current that flows in the coil. 2. Voltage, resistance, and power relate to a flow of current, and if the amount of current is known, the other values can be found.
What devices use the principle of electromagnetism?
Ammeter, voltmeter, and ohmmeter.
What is the most widely used meter type?
Permanent Magnet Moving Coil (PMMC) meter.
How does a PMMC meter operate?
Its pointer deflects an amount proportional to the current flowing through its moving coil across a permanent magnet.
Who designed the DC ammeter that the PMMC meter is based on?
Jacques-Arsène d’Arsonval.
What creates a reference magnetic field in a PMMC meter?
A horseshoe-shaped permanent magnet.
What material are the hairsprings in a PMMC meter made from?
Phosphor bronze.
What is full-scale current?
The amount of current that must flow through the meter coil to cause a full-scale deflection.
What is meter sensitivity?
The reciprocal value of the full-scale current, representing the total amount of resistance for each volt needed to produce a full-scale current.
What is the typical sensitivity of many multimeters?
20,000 ohms per volt.
What is the function of a shunt in an ammeter?
To increase the amount of current it can measure by bypassing excess current.
What happens if an ammeter is connected in parallel?
It gives an incorrect measurement and can damage the ammeter.
What is a multi-range ammeter?
An ammeter that can measure several different maximum current readings or ranges.
How is a voltmeter connected in a circuit?
In parallel with the load to be measured.
What is a multiplier resistor?
A resistor placed in series with the ammeter movement to multiply a meter’s basic range.
What is the purpose of an ohmmeter?
To measure the resistance of a load or external resistance.
What must be done before using an ohmmeter?
Calibrate the device.
What happens when the test leads of an ohmmeter are separated?
The pointer of the meter will return to the left side of the scale, indicating infinity.
What happens when the test leads of an ohmmeter are separated?
The pointer of the meter will return to the left side of the scale (infinity) due to the interruption of current and spring tension.
Why is the ohmmeter indication considered ‘backwards’?
Because maximum indication (infinity) is on the left of the scale, while voltage and current meters have zero on the left.
What must be done before connecting an ohmmeter to a circuit?
The power switch of the circuit to be measured must always be in the Off position to prevent damage to the meter.
How is the current flow through the ohmmeter affected?
The amount of current that flows through the meter coil depends on the total resistance of resistors R1 and R2 and the resistance of the meter.
What happens if resistors R1 or R2 are replaced with larger values?
The current flow decreases further, leading to a lower pointer deflection and a higher scale indication of circuit resistance.
What is the nature of the ohmmeter scale?
The ohmmeter scale is non-linear and reversed compared to voltmeters and ammeters, due to the current-operated nature of the device.
What is the purpose of multi-range ohmmeters?
They allow for different operational ranges, indicated by R × 1, R × 10, R × 100, etc., with readings multiplied by the range setting.
What distinguishes a series ohmmeter from a shunt ohmmeter?
In a series ohmmeter, the resistance to be measured is in series with the meter movement, while in a shunt ohmmeter, it is in parallel.
What is a safety ohmmeter designed for?
Safety ohmmeters are specifically designed for testing resistance in volatile and explosive devices, using a very small current.
How does a moving coil meter function in AC circuits?
It requires a rectifier to convert AC to DC, as the meter movement is restricted to direct current.
What is a multimeter?
A multimeter can function as a voltmeter, ammeter, or ohmmeter by connecting it to different external resistor networks.
What is the advantage of digital multimeters over moving coil meters?
Digital multimeters are more common and preferred due to their compact design and ease of use.
What safety precautions should be taken when using a multimeter?
De-energise the circuit before connecting or disconnecting, and never apply power while measuring resistance.
What is a current clamp?
A current clamp allows measurement of current in a conductor without physical contact, using the concept of a current transformer.
What are the general safety precautions for electrical test meters?
Use calibrated meters, inspect test leads for damage, and ensure proper function and range for measurements.
