Barrel connector power adapters look similar but carry the voltage at the centre pin or the outer sleeve depending on the device they were made for. Connecting an adapter with the wrong polarity to a device can damage or destroy it. This guide explains how to read the polarity diagram that appears on most devices and adapters.
The polarity symbol
Near the DC power input on a device, you will usually find a small diagram showing two concentric circles — the outer sleeve and the centre pin of the barrel connector — with a + or − sign next to each, connected by a C-shaped line (or sometimes shown as a circuit diagram with the two terminals labelled).
The most common polarity for consumer electronics is centre positive: the centre pin carries positive voltage and the outer sleeve is ground (negative). Centre positive is not universal, though. Many guitar pedals use centre negative instead, and so do some older audio devices and certain specialist equipment. Check the label on each unit before connecting an adapter.
How to read the diagram
- The inner circle represents the centre pin.
- The outer circle represents the sleeve.
- The C-shaped or arrow-shaped line connects them in the diagram.
- The + or − signs tell you which is positive and which is negative.
If the diagram shows + at the centre pin: the adapter must supply positive voltage at its centre pin. If the diagram shows − at the centre pin: the adapter must supply negative voltage at the centre pin (i.e., it is centre negative and you need a centre-negative adapter).
Checking the adapter
The adapter itself carries the same type of diagram. Compare the diagram on the adapter with the diagram on the device. Both must show the same polarity (+ or − at the centre pin and at the sleeve). If they do not match, the adapter is not compatible with the device regardless of whether the voltage and current ratings match.
Consequences of wrong polarity
Not all devices have reverse-polarity protection. Connecting a correctly-rated adapter with the wrong polarity can damage protection diodes, regulator circuits, or other components instantly. Some equipment will simply not power on; others may appear to work briefly before failing. Some equipment includes a protection diode that prevents damage. A series diode simply blocks reversed current.
When no diagram is present
If neither the device nor the adapter carries a clear polarity marking, do not guess. Check the device's manual, or ask a competent person to establish the required polarity before any adapter is connected.
Frequently asked questions
My device stopped working after I used the wrong adapter. Is it repairable?
Possibly. Whether damage is repairable depends on what failed. A protection diode is a cheap component. Other components may not be. A qualified electronics technician can diagnose the fault. Do not attempt to open or repair mains-connected equipment yourself unless you have the relevant training.
Can I use a universal power adapter?
Universal adapters with adjustable voltage and selectable polarity can work provided you set both voltage and polarity to match the device. Set polarity first, then voltage, with the adapter unplugged from mains. Check the device's nameplate current rating and ensure the adapter meets or exceeds it. Never open the adapter. If the adapter's label is missing or unreadable, do not guess: ask a competent person to establish the required polarity before using it. Make sure the adapter is unplugged from mains before connecting it to the device. Never adjust settings while the adapter is connected to the device.