# How do I figure out the correct polarity of a DC barrel connector

I'm in the U.S. and I bought a digital wall clock from the U.K. which came with an AC adapter with input 230V/50Hz/90mA and output 9V/800mA. Now I could get a transformer to convert 120V to 230V but that means connecting the already bulky adapter to another transformer and it will look hideous. Here is how far I've gotten but I'm stuck now...

The connector in the clock is a barrel type connector. First, I got a universal AC power supply for the U.S. with 120V input and an voltage output control (which includes the 9V that I need) and 1200mA output. After I received this, it looked fine and one of the barrels seemed to fit, but it required setting the correct polarity.

Next, I bought a multimeter which I got today. I set it to DC and tried both combinations of touching the black and red lines to the middle pin in the clock and to the round outer wall. When I put the black to the outside wall and the red to the inner pin, I get 0mV, but when I put black to the middle pin and red to the outer wall, I get about 1.0 mV.

Does this mean that the clock requires positive polarity or negative polarity?

Edit: Neither the power supply that shipped with the clock nor the clock instructions or case show the clock's expected polarity :( I emailed the manufacturer and they didn't answer this question.

• Hi us2012, when you say the power supply is broken, I'm not sure what you mean - the clock is not powered on Aug 7, 2013 at 21:48
• Maybe you have it set to AC? Did the multimeter cost more than the clock? Aug 7, 2013 at 21:49
• The multimeter is definitely set to DC. I tried the multimeter on a battery and it worked Aug 7, 2013 at 21:50
• @us2012, Andy: "in the clock" Aug 7, 2013 at 21:50
• @Andy it's a \$200 clock :) Aug 7, 2013 at 21:53

You said the specification says "Output: 9V~800mA". The ~ means it is AC output. So try setting the multimeter in AC mode. That is probably why it doesn't show the polarization.

• That's surprising, I thought just about every device like a clock takes DC input? Aug 7, 2013 at 22:13
• Great point. If it's and AC supply, there will be no polarity to read! Aug 7, 2013 at 22:19
• So if polarity doesn't matter for AC, then all I need is a universal barrel adaptor that inputs 110/120V and outputs 9V/800mA in AC and not DC? I'm not sure what to search for (e.g. "ac to ac") to buy one Aug 7, 2013 at 22:28
• just search for 9V AC Adapter, and be sure to check if it really outputs AC and has at least 800mA. Aug 7, 2013 at 22:35
• something like this ebay.com/itm/… Aug 7, 2013 at 22:41

Easiest way: Buy whatever transformer you need to in order to plug the original adapter into your wall socket, and measure the polarity on the connector of the adapter when it is plugged in. Many such power adapters are good for 240V and 120V, so a passive travel adapter may be all you need. Even if the adapter doesn't work right at 120V, it shouldn't be damaged, and the output might be good enough for you to understand the polarity.

• That makes sense, so I would just plug the original adapter in, through a transformer, but not connect it to the clock, and then test the adapter's output polarity? Aug 7, 2013 at 22:13
• yup, that's how you'd do it. If you use the right transformer, guaranteed correct answer. Just pay attention to the polarity of your meter probes! Aug 7, 2013 at 22:13
• Thanks, now again, I'm a noob, so how do I interpret the results - let's say I put the black wire inside the plug and red outside and it reads positive, what does that mean? And do I need to test both combinations or will they just be negative inverses of each other? Aug 7, 2013 at 22:15
• If the black wire is connected to something that says "Ground", "GND", "Common", "CMN", or something like that, that would mean that the outside is positive compared to the inside, and you have a "Center negative" power supply. Chances are you will find a "center positive" so for maximum clarity, you want to put the black wire on the outside, and the red wire on the inside. If it reads positive, you have "center positive", and if negative, "center negative" Aug 7, 2013 at 22:18
• Yes, the black wire was connected to COM in the multimeter. I will confirm the readings... Aug 7, 2013 at 22:20

On the power pack, you're looking for a symbol like the link either on a label on in the plastic. This tells you that the center pin is positive and the outside is negative. This is more likely but it is possible to have it the other way round where the plus an minus are swapped. The clock may also have it somewhere on a label or near the socket.

http://www.accesscomms.com.au/images/reference/powerplug-positive.jpg

Edit from comments: From the part numbers, it's all AC.

A tutorial on AC/DC labelling... http://www.harmonycentral.com/t5/Technology-Computers/External-Power-Supply-Essentials/ba-p/34655117

• I should have mentioned it in the original question, but the problem is that the power pack that shipped with the clock doesn't show its polarity and I emailed the manufacturer and they didn't respond Aug 7, 2013 at 21:56
• Do you have a part number of the clock or the power adapter?
– SLaG
Aug 7, 2013 at 22:01
• Here's everything I can read off it: Nextime Model No 2897 9V~ 800mA, RoHS AC Adaptor, Model YHA0900800G-33, Input 230V~50Hz 90mA, Output: 9V~800mA. This is the part connected to the barrel connector. But then this adaptor goes into some sort of fuse adaptor: PowerConnections Harlow England, Fit 3/5 A Fuse, 250V~3A, BS1363-5 Aug 7, 2013 at 22:05
• I wonder, can I test the polarity of the original wall clock 230V AC adaptor instead of testing the polarity of the clock? Aug 7, 2013 at 22:07
• As @user2613971 said, the whole thing is 9V AC.
– SLaG
Aug 7, 2013 at 22:17

Unfortunately it doesn't mean anything. You will need to open the clock and trace one of the connections to an IC, electrolytic, or some other polarized, non-reversible device. (Most barrel connections are center-positive, but don't take that as a universal absolute.)

• Argh, I was hoping it wouldn't get that complicated because I'm not sure what I'm going to be looking at when I open it. I had assumed that polarity could be understood just by touching the two metal parts of the clock input. Aug 7, 2013 at 21:52
• What does the polarity indicator on the current wall wart say? Aug 7, 2013 at 21:53
• That's the root problem - the wall wart that shipped with the clock doesn't have the universal polarity indicator. I emailed the manufacturer but they didn't respond Aug 7, 2013 at 21:53
• Follow the traces either to an IC, or to a device with a polarity indicator (thick line, positive/negative sign, etc.) and find the datasheet for it. If it's a diode or LED, ignore it and keep looking. Aug 7, 2013 at 21:56