An autotransformer is an economical voltage converter that uses one common winding for input and output and provides no galvanic isolation. An isolation transformer has two separate windings; it separates input from output electrically, providing safety and noise filtering.
Structural difference
- Autotransformer: there is one winding. The output is taken from an intermediate point on it. Part of the winding is common to both input and output.
- Isolation transformer: there are two insulated windings, primary and secondary. Energy is transferred only through the magnetic field.
Comparison table
| Feature | Autotransformer | Isolation transformer |
|---|---|---|
| Winding structure | One common winding | Two separate windings |
| Galvanic isolation | None | Yes |
| Reduces electric-shock risk | No | Yes |
| Noise filtering | Limited | Good (better with a screen) |
| Size and weight | Small, light | Larger, heavier |
| Cost | Lower | Higher |
| Efficiency | Generally higher | Slightly lower |
| Typical purpose | Voltage matching (380↔220 V) | Safety, isolation, noise reduction |
When to use an autotransformer
- When machine voltage and grid voltage differ (e.g. 380/220 V, 220/110 V)
- In industrial supplies where isolation is not required for safety
- Where space, weight and cost are constraints
When to use an isolation transformer
- In wet environments and areas with electric-shock risk
- In medical locations and sensitive clinical equipment
- To reduce noise in servo drives, CNC and automation systems
- When an ungrounded (IT) system is wanted
- On marine vessels and special applications
Safety note
Because input and output of an autotransformer are electrically connected, faults such as a broken neutral can produce unexpected voltages on the output side. Autotransformers should not be preferred where shock risk is high. The EN/IEC 61558 series is the principal reference for the design of safety isolating transformers.
What if voltage fluctuates?
Neither transformer stabilizes voltage. If grid voltage fluctuates you need a servo or static regulator. If both isolation and stabilization are needed, combined solutions can be designed.
See our autotransformer and isolation transformer pages, or request a quote for project-specific sizing.
Frequently Asked Questions
01Does an autotransformer provide isolation?
No. In an autotransformer input and output share the same winding, so they are electrically connected; there is no galvanic isolation.
02Does an isolation transformer change voltage?
It depends on its ratio. A 1:1 isolation transformer does not change voltage; with a different winding ratio it can step voltage up or down. It does not, however, correct voltage fluctuation.
03Which is cheaper?
For the same rating an autotransformer is generally cheaper, smaller and lighter because it uses less material.