Last night I decided to repurpose an old 9V AC Adaptor for use in my electronics projects. The connector at the end was not suitable for my purposes, so I cut it off and soldered in a standard 2.1 x 5.5 barrel connector.
When I tested the power on the “completed” job, I noticed that I was getting none of the results that I was expecting.
Firstly, I was getting a negative value where I was expecting a positive, and secondly, the 9V that I was expecting … was showing 11.98V (let’s be gentle and say 12V). The AC Adaptor clearly shows that it is supposed to be 9V output … WFT? Also, the polarity symbol on the AC Adaptor shows that the sheath is supposed to be negative while the pole is supposed to be positive. I am annoyed (and rightly annoyed … with myself).
What I should have done was to perform some tests of the barrel connector first.
With the above diagram in mind, how can I test it? Simple …
Turn the multimeter on and switch it to continuity test.
Hold the black probe against the positive tab (that should be the short tab at the top in the diagram) and place the red probe into the positive inner jack. There should be a continuity buzz coming from he multimeter showing that there is a continuous circuit. Just to be sure, change the position of the red probe to the outer sheath of the barrel. There should be no continuity, so no buzz from the multimeter.
Now hold the black probe against the negative tab (that should be the long tab with the crimp tabs) and place the red probe against the outer sheath of the barrel. This should result in a closed circuit … so … buzz. Again, test that the negative tab does not connect to the inner jack by moving the red probe to the positive jack. While the final step isn’t really necessary since you have already tested continuity of this path for the positive rail, it doesn’t hurt to check it twice.
This test should reveal the way that the tabs on the back of the plug are routed and you should now know where you are soldering to.
Testing the terminal is a bit more fiddly. The post at the back of the terminal should be the positive rail, while the post under the terminal should be negative. On a three post terminal, the post on the side should close when the plug is inserted into the terminal.
I only care about the positive and negative connections.
To test this (at the moment, I’m talking about continuity testing) you will need to test each post and connection independently. The difficulty here lies in the fact that the spring will be making contact with the post, creating a circuit. You need to slide a non-conductive strip between the post and the spring clip. Something like a toothpick should do the trick.
When this is done place the black probe against the back post of the terminal and touch the red probe to the post in the middle of the socket at the front of the terminal. You may need to use alligator clips to attach the black probe to the rear post so that you can turn the socket over and poke around inside it. When you have contact with both the rear post and the post inside the terminal, you should get a buzz. Move the red probe to the spring clip inside the plug hole. You should not get a buzz here.
Attach your black probe to the post underneath the terminal and go back to probing the post and the clip with the red probe. This time, you should get a buzz when probing the spring clip and no buzz when probing the post.
Remove the non-conductive spacer from the terminal.
Thirdly, test that the plug and socket circuits are good. To do this, plug the male connector into the female socket. With the black probe attached to the short tab of the connector, probe the rear post with the red probe. You should get a buzz. With the black probe attached to the long tab of the connector, probe the post underneath the terminal. Again … buzz. You will probably also get a buzz at this stage if you probe the side post of the terminal.
Now that you know the polarity of the socket, you can happily match that up with your circuit.
You should note that I’m talking from the perspective of normal polarity, some circuits and some installations have these reversed … and that’s OK. The important thing is that you know which pole is which when you are connecting it to your circuit.
The most common use for this kind of connector (for home users and hobbyists) is to connect a 9V battery to a circuit board.
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