Understanding polarity symbols

Thread Starter

elmariachi

Joined Aug 1, 2022
3
Hi all, first post as I'm looking for a little help to a question which I'm sure will be really fundamental to many of you.

I have a plug for an exercise bike I bought and it's of the European type with two pins. I need to switch this for a UK 3-pin alternative but I'm having trouble understanding the polarity, whether it's negative or positive.

As far as I can tell, the terminal which connects openly into the centre within the polarity diagram determines the polarity.

However, some articles I've read focus more on the placement of the '+' and '-' symbols - whether they're on the right or the left. Most show the '+' on the right if it's positive and the '-' on the right if it's negative.

My adapter has the '+' connecting into the centre circle but it's also on the left, so I'm a little conflicted.

Can anyone help?
 

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Alec_t

Joined Sep 17, 2013
13,075
Welcome to AAC!
The two pins shown in the pic are for the AC Live and Neutral input to the adapter, so have alternating polarity.
The connector on the DC output cable of the adapter (not shown) has an inner positive pin and an outer negative shield.
 

Thread Starter

elmariachi

Joined Aug 1, 2022
3
Thanks for the quick replies, guys - connector close-up attached.

Out of interest, how do I determine exactly what size inner and outer it is? Seems like quite fine margins and hard to tell with a ruler and the naked eye..
 

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Jon Chandler

Joined Jun 12, 2008
302
If it fits the socket it's the right size. If it doesn't fit it's the wrong size. Captain Obvious.
Not always! I've had a few cases where the outer diameter is fine, and the connector seems to be a perfect fit, but the inner diameter of the plug is so big and the pin diameter so small there's no connection! Can lead to very frustrating troubleshooting.
 

dl324

Joined Mar 30, 2015
14,923
However, some articles I've read focus more on the placement of the '+' and '-' symbols - whether they're on the right or the left. Most show the '+' on the right if it's positive and the '-' on the right if it's negative.
1659371668195.png
I don't know where you found that BS about the placement of the symbols. The adapter you have is clearly center positive. If you rotated the polarity illustration 180 degrees, or any other angle, it would still be center positive.

Out of interest, how do I determine exactly what size inner and outer it is? Seems like quite fine margins and hard to tell with a ruler and the naked eye..
AFAIK, there are only two sizes that are so close in size that you might not be able to differentiate them by eye. They're 2.1mmx5.5mm and 2.5mmx5.5mm. You should be able to measure the difference with a ruler. If you look at them often enough, you'll be able to tell the difference between the two. There are also different lengths for the female plugs.

EDIT: A glance at a Jameco catalog gave: 1.3x3.5mm, 1.7x4.8mm, 2.1x5.5mm, and 2.5x5.5mm.
 
Last edited:

crutschow

Joined Mar 14, 2008
30,461
I'm afraid I don't own one, no :) Tempted though.
I recently bought this low-cost digital caliper.
It has a plastic body, but seems sturdy and works well for the occasional non-critical measurements I make.
Certainly should be good enough to determine the diameter of your plug and differentiate it from the other size plugs available.
 
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