The center contact is negative, and the outside is positive.

Thank you gwross.

Are you sure? Generally on power supplies like that, the outside is negative. This post says the same thing (although it is old): http://www.chiefdelphi.com/forums/sh...ower+a dapter

Last year we planned on running our robot in a local parade. I e-mailed IFI and this was their response:

'12V is the max. I know that radio Shack carries a 9.6V battery pack that we use here and at competitions. You will have to modify the connector to fit the OI. This battery pack is like a 4-pack of AA's. Radio Shack also carries a charger for this pack. The connector is a 2.1mm X 5.5mm.'

I do not remember +/- but we ran the robot over a mile without the OI shutting down using this battery pack.

Joe, I have a 12V, 0.5A power supply on the shelf beside me - it gives 19 V OC, and the still attached factory plug on it, which fits the OI power input, is pin-negative. As Apple's monsieur* Gasse' said, "The nice thing about standards is, there are so many of them." I label my supplies with a labeller, now.

BTW, the supply works well as is with a Radio Shack powered speaker. (All by way of saying, don't expect much adherence to "standards" when dealing with commodities from offshore.)

*Jean-Philllipe, maybe, I forget.

No, I'm NOT sure.:o I'm also not sure whether Loyd's "pin negative" is agreeing with me or not, but from the context ("The nice thing about standards is, there are so many of them."), I'm guessing that it means that the center pin of the socket on the OI is the negative terminal, and that he agrees with me.:)

I was going off of the picture on the adapter which is something like the picture below. (I don't have the adapter with me right here, right now.)

Aside from the possibility that I got it right, I admit that it is possible that:[list=1][*]I misread the picture, and so posted the wrong information.[*]I read the picture correctly, but mistyped the information.[*]I don't have the foggiest what the picture really is indicating. (Remember, I'm just a programmer.:) )[/list=1]

That picture indicates that the center pin is negative, and the outer covering is positive, if that is indeed the picture from the power suppy then that is what it is.

I just checked another (OI) power adapter, and the picture was exactly the opposite of the first adapter I looked at, and the multimeter shows both having the same polarity!

(As I said, I'm just a programmer, but I want to understand.) So which picture is correct? When I connect the red lead from my multimeter to the center contact, I read a positive voltage. This is the same as when I test a battery with the red lead on the positive end. So that shows me that the electrons come out of the center of the plug. (Do I have that right?) But now, is the picture on the adapter supposed to show the polarity of the contacts on the plug? (i.e. the direction of electron flow OUT OF the plug) or the direction of the flow of electrons returning to the adapter?

I'm thinking of something like when you have multiple batteries in series in a flashlight. You have the positive end of one battery in contact with the negative end of the next battery. Should the picture on the adapter be of the first battery, or the second?

This is a very interesting problem.

The results you got would be right under two circumstances (to my knowledge): 1) the diagram is wrong, or 2) your multimeter uses unsigned notation :p. Usually, when one wants to test a voltage (and is expecting to receive a positive reading) you'd connect the red lead on the multimeter to the positive terminal, and the black lead to the negative (ground, usually) terminal. After taking a reading, if you were to swap the multimeter leads for one another, you should get a reading that is negative the original value.

I wish I had an official OI AC adapter in front of me, right now.

Think of current going from an elevated voltage (+), going through a load (a light bulb, a motor, or the OI, for instance), and returning to Ground (-). That's exactly what you have when you are given a diagram of... almost anything that runs off DC. So, in actuality, the diagram of an AC adapter is both the positive and negative ends of a battery (neverminding the thought of two batteries being in series).

Another point of note is the difference between "electron flow" and "current flow." There's a nice little description and demonstrative piece on this (rather historic) problem here. (off topic: this webpage begins with this quote: "The nice thing about standards is that there are so many of them to choose from." attributed to "Andres S. Tannenbaum, computer science professor.")

I apologize deeply if I am wrong about any of this. I'll still make the excuse that it's 2:50am, though. :p

So that's why I'm so confused!:D Actually, I did know there was some kind of problem like that, but I've never studied it closely enough to remember whether the problem was

1. that negative electrons proceed from a terminal called positive -- or--
2. that those negative electrons (which are the only things that actually move in an electric "current") are actually flowing in the opposite direction of the supposed current flow.

Well, my multimeter does show positive and negative depending upon where I put the terminals, (but I'm assuming, from your ":p" that you knew that) so one of the diagrams is wrong. But I still don't know which one!



That's what I thought.


Yep, I knew that.


OK. I guess my illustration of the batteries wasn't such a good one.;) So to restate my question: Is the picture on the adapter of the adapter plug, or of the socket on the OI? (Actually, now that I think of it in the light of day, it wouldn't make a lot of sense to make it a picture of the socket, since it is an entirely passive device until power is supplied via the plug. BUT someone else must have been confused about it too since someone put an incorrect picture on one of those adapters!)

So then, if my surmise is correct that the diagram should be of the plug, (and not of the socket) then the diagram that I posted (which actually does match the diagram on one of the adapters) must be wrong. Am I right?

Heh. After rereading my previous post, I realized that I didn't answer the question. ;)

If you measured a positive voltage from the center to the shield of the plug on the AC adapter, and the diagram on the AC adapter itself is what you previously posted, then yes, the diagram must be wrong.

Actual measurements always take precedence over diagrams.

It really worries me, however, that a mixup like this could have happened... Someone might seriously damage their OI if they misinterpret the diagram...