Ac Adaptor Connector Sizes: The A–s Guide (with Measuring Tips)

Ac Adaptor Connector Sizes: The A–s Guide (with Measuring Tips)

The Ultimate Guide to DC Connector Sizes on AC Adaptors (Types A–S)

Choosing the correct low‑voltage DC plug is just as important as matching the voltage and current of your adaptor. This quick guide explains what the size numbers mean, how to measure your plug, and lists the most common connector sizes we stock — labelled A through S — so you can find a like‑for‑like replacement with confidence.

What do the numbers mean?

Most DC barrel plugs are described by OD × ID × length:

  • OD (Outer Diameter): the outside of the barrel (e.g., 5.5 mm)

  • ID (Inner Diameter): the hole inside the barrel (e.g., 2.1 mm)

  • Length: sometimes specified (e.g., 12 mm)

  • Centre pin: some connectors have an internal pin on the device side — your plug must be the matching “with centre pin” type.

Quick tip: 5.5 mm OD is the most common barrel family. If your original plug fits snugly into a standard headphone socket, it’s likely a 3.5 mm type.

How to identify your connector in 3 steps

  1. Read the label: Note the voltage (V), current (A/mA), and polarity (centre‑positive ⊕ or centre‑negative ⊖).

  2. Measure the plug: Use digital calipers if possible. Measure OD and ID. If you don’t have calipers, compare against drill bits or known objects (e.g., a 3.5 mm headphone plug).

  3. Check the fit: The correct plug should insert smoothly and feel snug, with no wobble. Too loose → ID too large; won’t insert → ID too small or wrong style (e.g., needs a centre pin).

Not sure? Send us two photos — the plug tip and the device’s power label — and we’ll recommend the right replacement (contact details below).

DC Connector Types (A–S)

The list below covers the most common sizes by Type with their measurements and any special notes.

Type Size & Notes
A 6.0 mm × 4.3 mm with centre pin
B 6.3 mm × 3.0 mm
C 5.5 mm × 2.5 mm
D 5.5 mm × 2.1 mm
E 5.5 mm × 1.75 mm
F 4.8 mm / 4.75 mm × 1.75 mm
G 4.0 mm × 1.7 mm (G WIRE)
H 5.0 mm × 3.3 mm with centre pin
I 3.5 mm / 3.45 mm × 1.3 mm (I WIRE)
J 2.5 mm × 0.75 mm / 0.8 mm
K 3.0 mm × 1.1 mm
L 2.3 mm × 0.7 mm
M 2.0 mm × 0.7 mm
P 5.5 mm × 2.8 mm × 12 mm long
Q 4.5 mm × 3.0 mm with centre pin
R 5.5 mm × 2.8 mm
S 7.4 mm × 5.0 mm

Common mix‑up: 5.5 × 2.1 mm (Type D) and 5.5 × 2.5 mm (Type C) look identical. If a 5.5 × 2.1 mm feels loose, you may need 5.5 × 2.5 mm.

Polarity and safety checklist

Before you plug in a replacement adaptor, confirm:

  • Voltage matches your original (within the device’s specified range).

  • Current (A/mA) is equal or higher than the original.

  • Polarity matches (most modern devices are centre‑positive). Look for the polarity diagram near the DC jack.

  • Connector type/size is correct (including “with centre pin” where required).

Getting any of the above wrong can cause intermittent power, damage, or no power at all.

Handy identification hints

  • Headphone‑fit test: snug fit in a 3.5 mm headphone socket → likely 3.5 mm OD family (Type I).

  • Wobble test: wobble usually means the ID is too large — step down to the smaller ID for the same OD family.

  • Length matters: some sockets need longer 12 mm tips (e.g., Type P). If your old plug is visibly longer, match that length.

  • Centre pin devices: types A, H, Q have a centre pin; a plain barrel won’t power these.

Still unsure? We’ll match it for you

Email us clear photos and details and we’ll reply with a recommended replacement AC adaptor:

Please include:

  • A close‑up photo of the original connector tip

  • A photo of the device’s power label showing voltage, current, and polarity

  • Brand/model of the device (if known)

FAQs

Q: My plug is 5.5 mm — which inner size do I need, 2.1 mm or 2.5 mm?
A: Try 5.5 × 2.1 mm first (Type D). If it’s loose or loses power with movement, you likely need 5.5 × 2.5 mm (Type C).

Q: Can a higher‑amp adaptor damage my device?
A: No — as long as the voltage and polarity match, a higher current rating is fine. The device only draws what it needs.

Q: The plug looks right but won’t go in.
A: You may need a centre‑pin version (Types A, H, Q) or a different ID.

Q: What if my original adaptor shows “centre‑negative”?
A: You must match the polarity exactly. If unsure, contact us with photos and we’ll advise.

If you’re replacing a lost or damaged adaptor, getting the connector size and polarity right is half the battle. When in doubt, send us a photo — we’re happy to help.

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