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Meaning of Auxiliary Supply
The auxiliary supply (also called aux supply or power supply) is the voltage that powers the meter's own electronics: the display, the microprocessor and any relays or communication ports. It is separate from the signal the meter measures.
Auxiliary supply vs. measuring input
| Measuring input | The voltage or current you want to read, for example a 0–300 V line or a 0–5 A CT secondary. |
|---|---|
| Auxiliary supply | The power that keeps the instrument itself running, for example 85–265 V AC/DC or 24 V DC. |
Why a separate supply?
- Reads small or zero signals. The meter still works and displays when the measured quantity is zero, for example a DC voltmeter on a battery that has failed.
- Isolation. The measured circuit and the meter's electronics can be electrically separated, which is safer and reduces noise.
- Wide-range operation. One universal aux supply (such as 85–265 V AC/DC) lets the same model run on different sites and voltages.
Self-powered meters
Some meters take their power from the measured voltage and have no separate aux terminals. They are simpler to wire but go blank if the measured voltage drops too low. Check the datasheet to see which type you have.
Common mistakes
- Connecting the aux terminals to the wrong voltage. Always match the rating printed on the label.
- Forgetting a fuse or MCB on the aux line.
- Confusing a DC-only aux rating with an AC/DC one.