A technician needs to supply 26-volt AC power to instrument panel lighting from a 115-volt, 400 Hz AC bus. Instead of using a two-winding isolation transformer, the technician selects an autotransformer with a single tapped winding. What is a key characteristic the technician must consider when using this device?
- AAn autotransformer converts AC to DC internally before stepping down the voltage
- BAn autotransformer provides complete electrical isolation between the primary and secondary circuits
- CAn autotransformer shares a single winding between primary and secondary, so there is no electrical isolation between input and output
- DAn autotransformer is heavier and less efficient than a two-winding transformer of the same rating
Show answer & explanationAnswer & explanation
Correct answer: C. An autotransformer shares a single winding between primary and secondary, so there is no electrical isolation between input and output
An autotransformer uses a single winding with a tap, so part of the winding is common to both the primary and secondary circuits. This makes it lighter and more efficient than a two-winding transformer but means there is no electrical isolation between the input and output circuits, which must be considered for fault protection.
Why the other options are wrong
- A. An autotransformer remains an AC device; it does not perform AC-to-DC conversion.
- B. Autotransformers lack isolation because primary and secondary share a common winding.
- D. Autotransformers are actually lighter and more efficient than two-winding transformers for the same voltage ratio.
Autotransformer
A transformer using a single tapped winding shared by both primary and secondary circuits, providing voltage step-up or step-down without electrical isolation, but lighter and more efficient than a two-winding transformer.
- Single winding with a tap, shared by primary and secondary
- No electrical isolation between input and output
- Lighter and more efficient than isolation transformers of same rating
Memory trick: Auto shares one coil — no wall between in and out.