Wall Wart - Center Positive vs Center Negative

[made2hack]

Hello,

Couldn't quite find this Googleing, so I figured I'd ask here.

What do you think is more common? Center-positive or center-negative wall wart?

This is not a product I'm building where I choose the wall wart for the package. This is just a lamp for someone overseas and I want to know which way to wire the jack so that they can supply their own wall wart.

Do you figure it's 50/50? Or biased one way or another? It's for 12V (if it matters).

Regards,

[8086]

I have found centre positive to be more common.

But, a LOT of audio/guitar effect etc equipment uses centre negative, so it's quite common too.

If I was designing something I would use centre positive - it seems pretty standard (well, moreso than the other type) for non-audio devices.

Make sure you put some reverse polarity protection on it if necessary. There's always someone that doesn't read the instructions.

[Richard Crowley]

If I had a choice, I would also use center-positive as it seems somewhat more common.
OTOH, this is for a "lamp"?  If this is incandescent, then it is polarity insensitive.
If it is LED (or something polarity-dependent) you could always put a bridge rectifier between the power inlet and the circuit.
That would make it polarity-agnostic.  For that matter, it would allow operating from AC as well.

[ciccio]

Not all DC plugs are the same. The dimension (external diameter, internal diameter, internal pin diameter, length ),  are different.
I've mostly seen this : center positive for the bigger ones, center negative for the smaller.
This, obviously,  is not a rule.
I've seen a "quasi-standard" on the 12 V DC wall warts, with 5.5 mm outer diameter and 2.5 mm internal diameter. almost all were wired with positive center.
http://en.wikipedia.org/wiki/Coaxial_power_connector
Best regards.

[8086]

Speaking of sizes, the usual size I have seen is 2.1mm.

[pickle9000]

2.1 mm center positive @ 12 volts is very common and used widely for security cameras. Very easy to get.

[mikeselectricstuff]

Centre positive is the most common nowadays by far.
Music gear is the only place I've ever seen centre negative. I'm guessing there are historical reasons, may be to do with the fact that the outer breaks on cut-out sockets

[Richard Crowley]

2.1 mm center positive @ 12 volts is very common and used widely for security cameras. Very easy to get.

I agree.  2.1mm, center-positive, 12V is a very common standard that I am using on equipment I build from scratch.

[AG6QR]

Centre positive is the most common nowadays by far.
Music gear is the only place I've ever seen centre negative. I'm guessing there are historical reasons, may be to do with the fact that the outer breaks on cut-out sockets

I could be wrong, but I always thought the reason was all of those devices for automotive use that would use this style of plug.  If they're tolerant of automotive 12-14.5V or so, they can be powered by a cigarette lighter cord, with no electronics in it.  If the center is positive, then no harm comes if the outside of the connector touches bare metal in a negative ground car.  But if the center is negative, that would mean the outer shell has +12V compared to the frame of the car, a potential cause of short circuits and blown fuses.

Regardless of the reason, virtually everything I encounter is center positive.  Except an old HP palmtop computer (100LX).

[Rick Law]

....
Regardless of the reason, virtually everything I encounter is center positive.  Except an old HP palmtop computer (100LX).

HP200LX, HP600 and HP800 are 12V center negative also.  I don't recall the 100LX, but I love the 200LX.  I had two of them.  I ran double DOS on it and use microsoft mail to keep connected, RBase5000 to keep my records...

Amazing how much functionality you can get with 640K ram.

[RyanAMT]

I've always wondered this too so I did some research.  It seems the reason for center negative used largely in guitar stomp pedals and effects has to do with the fact that most pedals are designed to work with a battery and when you insert the DC plug into the pedal it switches off the positive line from battery and the switch is on barrel plugs is on the sleeve.  I got this from: http://stinkfoot.se/archives/1121

I think some companies do it because they figure that most people have a box of center positive supplies in storage so when the original supply dies they have little choice but to go back to the manufacturer for a new supply - most people don't know about searching Amazon or eBay for these things.

[WarSim]

Don't follow what the audio gear uses.  Never follow any industry that thinks wire has a polarity.  .

[Shock]

Signal wires are used as tip/center positive as well this has influenced it.
Really it comes down to the history, design, function, safety and the most important thing, not frying your device when you hook it up.

[lapm]

Neat trick for this is to use diode bridge inside your device and it docent matter anymore witch one is on center... of course if this is for comercial product you still need to put something in label 

On my experience whats on center depends entirely on person who designed sayd device... I have seen both, a lot.... My own logic would say put negative on outside, but thats just me...

[PedroDaGr8]

Neat trick for this is to use diode bridge inside your device and it docent matter anymore witch one is on center... of course if this is for comercial product you still need to put something in label 

On my experience whats on center depends entirely on person who designed sayd device... I have seen both, a lot.... My own logic would say put negative on outside, but thats just me...

Yeah that's a common and very useful trick for situations like this. You can use a bridge rectifier to ensure that no matter the polarity of the device you get the right power flow. This isn't good for high current though because of the voltage drop.

[Mechatrommer]

centre positive. not just its common, but it also "common sense" and "naturally selected". outer positive is an abomination that should be extinct by today. think about a wall wart supplying sinusiodal 90V (yes i have one around for photo strobe, except its uses different connector luckily, not centre vs exposed outer metal connector), if its outer positive, things can get nasty when you touch it. also look at coax cable, rca cable etc, do you see outer positive? no! this kind of thread should not be discussed by today i'm surprised it still is.

[mikeselectricstuff]

Whatever the polarity, by far the most important thing is that it's clearly labelled, and dosen't smoke if the polatrity is wrong.

[Psi]

ive only come across maybe 5 center negative DC jacks in my life.
vs 1000's of center positive ones.

[GeorgeOfTheJungle]

Yes GND outside, like the RCAs, BNCs, USBs, and nearly everything else. What's a fridge rectifier?

[RyanAMT]

I came across a center negative connector on a Polycom EagleEye camera the other day but it doesn't surprise me knowing Polycom.

[made2hack]

Thanks everyone, I will indeed be going with center positive 2.1mm internal diameter.

Since this is a one off, I'm not too concerned with reverse polarity protection, however, for the future, what topology would you use?

This is for a lamp made with LEDs, that will draw at most approximately 150mA, but assume 300mA as worst case (leave some room).

It will also draw 3W of power, plus whatever inefficiency loss in the circuit, so let's assume again 5W worst case scenario.

Can I get away with a simple Schottky diode as protection? (I don't want too large voltage drop on whatever diode I choose). Or should I use one of the PNP transistor protection schemes? Or better yet a p-channel fet? Again, not really major current here.

[Richard Crowley]

Aren't you going to be dropping voltage through your current-limiting resistors anyway?
If you factor in the voltage drop(s) for polarity-protection, doesn't it simply change the value of the series resistor slightly?  Why is that even a problem?

A low-loss and simple polarity protection is an input series fuse, and a beefy diode connected so that it shorts out reverse-polarity and blows the fuse.

[Zbig]

From my limited personal experience, based on what I used and/or owned, center-positive seems to be the de-facto standard except for the Japanese gear which tends to be center-negative.