The round DC plug on a 12V plug pack is a barrel connector, and nearly all of them in New Zealand are 5.5mm across. The size you need to check is the centre pin, which is either 2.1mm or 2.5mm. The polarity matters too, and so does which wire goes where when you fit your own plug or socket. This guide covers each of those, then shows how to power an LED strip, an Arduino, an ESP32 and a CCTV camera from one 12V supply.
- 5.5mmBarrel outside
- 2.1 or 2.5mmCentre pin
- Centre +Usual polarity
- 2AOur suggested limit
In this guide
5.5 x 2.1mm or 5.5 x 2.5mm
Both sizes have the same 5.5mm outer barrel, so they look identical from the outside. The difference is the metal pin in the middle of the socket: 2.1mm thick on one, 2.5mm on the other. The plug has a hole that the pin slides into, and that hole has to match.
| Plug | Socket | What happens |
|---|---|---|
| 5.5 x 2.1mm | 5.5 x 2.1mm | Pushes on firmly and stays put. This is the usual 12V size. |
| 5.5 x 2.5mm | 5.5 x 2.5mm | Pushes on firmly. Common on some laptop, soldering iron and larger 12V supplies. |
| 5.5 x 2.5mm | 5.5 x 2.1mm | Goes on, but the thin pin wobbles in the bigger hole. The device cuts out or LEDs flicker when the lead moves. |
| 5.5 x 2.1mm | 5.5 x 2.5mm | Won't go on, because the pin is too thick for the hole. Forcing it spreads the plug's contact. |
How to tell which one you have
- Read the label. Plug packs and devices usually print the size near the voltage, as 5.5 x 2.1, 5.5/2.1 or just "2.1mm".
- Try a plug you know. A 2.1mm plug that won't push onto a socket means the socket is 2.5mm. A plug that slides onto a known 2.1mm socket with almost no grip is a 2.5mm plug.
- Measure the socket pin. With vernier callipers, a pin of about 2.0mm is the 2.1mm size and about 2.4 to 2.5mm is the 2.5mm size.
A plug that "fits a bit loose" is the trap. It often works on the bench, then drops out once the project is on a shelf and the lead gets bumped.
Male plug or female socket
Barrel connectors are named the opposite way round from what you might guess by looking at them. The plug has a hole in its tip, but it's the male half because it pushes in. The socket has a pin, but it's the female half because it receives the plug. Sellers also call the socket a DC jack.
| Part | Names | Found on |
|---|---|---|
| Male plug | DC plug, barrel plug | On the end of the plug pack lead |
| Female socket | DC jack, DC socket, barrel jack | On the device: an Arduino Uno, a CCTV camera, an LED strip lead |
So if you're replacing the cut-off end of a plug pack lead, you want a male plug. If you're giving an LED strip or project box a place to plug the supply in, you want a female socket.
Centre positive and the polarity symbol
Most 12V plug packs, Arduino boards, CCTV cameras and LED strip leads are centre positive: the centre pin is + and the outer sleeve is -. The main exception you'll meet is 9V guitar effects pedals, which are usually centre negative.
Look at the label on the plug pack, next to the output voltage. If the dot's line runs to the + circle, the supply is centre positive. If it runs to the - circle, it's centre negative. A solid line over a dashed line next to the voltage means the output is DC.
Reversed polarity can kill a device instantly. Some devices have protection and simply won't turn on. Others don't: our IP67 buck converter, for example, is damaged if its input is wired backwards. Check polarity with a meter before you connect anything you care about.
Wiring a pigtail or screw terminal adapter
A pigtail is a plug or socket with a short lead already fitted. Ours have about 24cm of red and black twin cable, and the red wire goes to the centre pin, so on a centre-positive supply red is positive. A screw terminal adapter has a small green screw block on the back instead. You clamp your own wires in it, with no soldering. Ours have + and - moulded beside the screws, and the + screw goes to the centre pin. A 5V plug pack on a female screw adapter is also the simplest way to power a couple of servos, as our PCA9685 servo driver guide shows.
- Pick the half you need. A female socket accepts the plug pack's plug. A male plug feeds a device that already has a socket.
- Strip about 5mm. Twist the strands so none stick out. On a pigtail, strip the cable you're joining to by the same amount.
- Fit the wires. On a screw adapter, put the positive wire under the + screw and the negative under the - screw, tighten firmly and tug each wire. On a pigtail, join red to positive and black to negative with solder and heatshrink, a screw terminal or a lever connector.
- Plug in the supply and measure. Set a multimeter to DC volts, red probe on the + screw or red wire, black probe on -. You should see close to the supply's rated voltage with no minus sign.
- Connect the load. Unplug, connect your strip, camera or board, then plug in again.
A minus sign on the meter means swap the wires. If you see something like -12.1 instead of 12.1, the supply is centre negative or the two wires are crossed. Swap them at the screws and measure again. Don't go by colour alone on other leads: some use two black wires, or black with a white stripe for +.
For joining pigtail leads to thicker cable with butt connectors or ferrules, our crimp terminal guide covers sizes and how to make a crimp that holds.
How much current they carry
We suggest keeping our pigtails and screw adapters to 2A. At 12V that's 24W, which covers most single cameras, short LED strips and microcontroller projects. To work out what your load draws, divide its watts by the supply voltage.
| 12V load | Amps | On 2A |
|---|---|---|
| 1m of LED strip marked 14.4W per metre | 1.2A | Fine |
| 5m of the same strip (72W) | 6A | Too much. Use a heavier connection, or feed the strip in sections. |
| IP67 buck converter, 3A version at full 5V load | 1.4A | Fine |
| IP67 buck converter, 5A version at full 5V load | 2.3A | Over. Wire its input straight to the supply with heavier cable. |
| Arduino Uno with a few sensors | Well under 0.5A | Fine |
A connector working too hard gets warm to touch, and the voltage at the far end drops, so LEDs dim towards the end of the strip. Size the plug pack's amp rating above what the load draws. A bigger amp rating than you need is fine, because the device only draws what it uses.
LED strips, Arduino, ESP32 and CCTV from 12V
12V LED strip
A female screw adapter or female pigtail on the strip's lead lets you plug the plug pack straight in. Red goes to the strip pad marked +12V or +, black to GND or -. Measure first, as in the steps above, before the strip goes on.
Arduino Uno
The Uno R3 has a 5.5 x 2.1mm centre-positive socket that takes 7 to 12V, so a 9V or 12V plug pack plugs straight in with no adapter. Stay near 9V if the board is driving much, because the regulator turns the extra voltage into heat.
ESP32
ESP32 boards have no barrel socket and run from 5V, so 12V has to come down first. Plug the supply into a female pigtail, join red to red and black to black on the IP67 buck converter's input leads, then run its yellow 5V lead to the board's 5V pin and its black lead to GND. Our 38-pin ESP32 board takes 4.75 to 5.25V on that pin. Never connect 12V or the 5V lead to the 3V3 pin. Our guide to powering an Arduino or ESP32 from 12V covers which pin to feed and how much current to allow. If you're new to the board, the ESP32 getting-started guide covers the first upload.
CCTV camera
Most 12V cameras have a short lead ending in a female socket. If that lead gets cut, a male screw adapter or male pigtail on the cable from your supply gives you a plug again. Check the camera's label for the amps it needs and keep a single run within 2A.
The plug pack does the mains side. Everything in this guide is on the low-voltage DC side of the plug pack. Don't cut into a mains lead or open a plug pack. Buy a supply with the right voltage and plug size instead.
Troubleshooting
| Symptom | Cause | Fix |
|---|---|---|
| Power cuts in and out when the lead moves | A 2.5mm plug on a 2.1mm socket, or a loose screw | Match the pin size. Retighten the screws and tug each wire. |
| Plug won't push onto the socket | 2.1mm plug on a 2.5mm socket | Use a 2.5mm plug. Don't force it. |
| Meter reads a minus voltage | Centre-negative supply or crossed wires | Swap the wires at the screws and measure again. |
| Nothing turns on, meter reads 0V | Wire not clamped, or the screw tightened on the insulation | Strip a little more, refit the wire under the screw plate and tighten. |
| LED strip dims towards the far end | Too much current for the length | Feed the strip from both ends or in shorter sections. |
| Connector warm to touch | Load above 2A or a poor contact | Check the load's watts. Move heavy loads to thicker cable. |
| ESP32 resets when WiFi starts | Supply sagging on the 5V line | Check the 5V reading under load and keep the 5V leads short. |
Parts used in this guide
Questions
Are all DC plugs the same?
No. Besides the 2.1 and 2.5mm pins on the common 5.5mm barrel, there are smaller barrels such as 3.5mm and 4.0mm on routers and small gadgets. Polarity and voltage vary too, so match the size, the polarity symbol and the voltage on the device's label.
Is a "2.1mm DC plug" the same as 5.5 x 2.1mm?
Almost always. Shops often drop the 5.5mm because it's the usual outer size, so a "2.1mm plug" is a 5.5mm barrel with a hole for a 2.1mm pin.
Can I use a 12V plug pack on a 9V device?
Only if the device's label gives a range that includes 12V, such as the Uno's 7 to 12V. Otherwise use a supply with the voltage the device asks for. The amp rating can be higher than the device needs.
Can I run these from a 12V battery?
Yes, for loads within 2A. Fit a male pigtail or male screw adapter to the battery lead so it plugs into the device like a plug pack would. Put an inline fuse of 2A or less in the red wire, close to the battery.
Do I need a 5.5 x 2.1 or a 5.5 x 2.5 screw adapter for my LED strip?
Match the plug pack's plug. Most 12V LED and CCTV supplies use 5.5 x 2.1mm. If the label doesn't say, use the try-a-known-plug check in the sizes section above.
Not sure which plug you need?
Message us with the voltage and size printed on your plug pack or device, and we'll tell you which connector fits.
