FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsEasy
A technician installs an ammeter to monitor a 60-amp generator circuit on a 28-volt DC aircraft. Instead of routing full generator current through the meter movement, the technician wires a low-resistance device in series with the load and connects the meter leads across it. What is this device called, and how does it work?
- AA bus tie relay; it splits current evenly between two separate ammeters
- BA voltage regulator; it limits current to a safe value before it reaches the meter
- CA shunt; the meter measures the small voltage drop across it, which is proportional to total current flow
- DA current limiter; it caps the maximum current the generator can produce
Show answer & explanationAnswer & explanation
Correct answer: C. A shunt; the meter measures the small voltage drop across it, which is proportional to total current flow
A shunt is a precision low-resistance resistor placed in series with the load; nearly all current flows through it while the sensitive meter movement is connected across it to measure the small millivolt drop, which is calibrated to read as amperes.
Why the other options are wrong
- A. Bus tie relays connect/isolate buses; they are unrelated to ammeter measurement.
- B. A voltage regulator controls generator output voltage, not current measurement.
- D. A current limiter is a protective device, not a measuring device.
Shunt-Type Ammeter
An ammeter arrangement using a precision low-resistance shunt in series with the load so the meter measures a proportional voltage drop instead of carrying full circuit current.
- Shunt carries nearly all the load current
- Meter measures millivolt drop across shunt
- Allows accurate measurement of high currents without oversized meter movements
Memory trick: Shunt shares the load so the meter stays small.