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I was putting together a fairly basic LBB at the weekend to control a handful of DC devices which got me thinking. Why are both pin-positive and pin-negative DC barrel connectors used? Are there any concrete, electrical advantages to either design? Or is it just for manufacturer lock-in?

Pin-positive seems to me to be the obvious one to use from a safety point of view, but a quick audit of the supplies in my spares box revealed a not insignificant number of pin-negative supplies.

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  • \$\begingroup\$ What's an LBB?? \$\endgroup\$ – Andy aka Apr 25 '16 at 9:45
  • \$\begingroup\$ Little Black Box :) \$\endgroup\$ – Mourndark Apr 25 '16 at 9:47
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    \$\begingroup\$ There's me think it was something else urbandictionary.com/define.php?term=LBB \$\endgroup\$ – Andy aka Apr 25 '16 at 9:49
  • \$\begingroup\$ Now I wasn't aware of that definition! I'll have to be more careful... \$\endgroup\$ – Mourndark Apr 25 '16 at 9:53
  • \$\begingroup\$ @Andyaka Whoa! That perfectly describes me, I have to start using that. \$\endgroup\$ – pipe Apr 25 '16 at 10:37
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Ordinary barrel connector

Image from: http://www.canadarobotix.com/image/cache/data/products/400/437-1-800x800.jpg

Power barrels

On the figure above, on the left, a connector with no plug inserted. Pins 1 and 2 make contact. On the right, with a plug inserted, pins 1 and 2 makes no contact, and the plug outer ring makes contact with pin number 1.

Some connectors have a third pin that can make contact with one of the others when no cable is inserted and is disconnected when you put a cable in. This is often used to handle dual power (mains brick and battery, for example) or to signal to a processor that a cable is plugged in or no. This third pin is usually connected to the outer ring. If you need to disconnect the positive battery pole, you will have to make your polarity negative-tip. This is one of the reasons I can think of having this kind of polarity plug.

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    \$\begingroup\$ Thanks and sorry. I though "If you need to disconnect the positive battery pole, you will have to make your polarity negative-tip." was a concrete advantage for the tip-negative question. \$\endgroup\$ – ricardomenzer Apr 25 '16 at 13:39
  • \$\begingroup\$ No, this is a very good reason why one should be used over the other. And now you come to mention it, two of the negative-tip adapters I found were for appliances that could be battery powered. I'll look out for it more now. \$\endgroup\$ – Mourndark Apr 25 '16 at 13:41
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"Pin-positive seems to me to be the obvious one to use from a safety point of view" I don't see that point regarding safety. I think either is equally safe.

The danger is in mixing up the polarties especially for devices that cannot handle inverse polarity properly (this can be fixed with a diode and a fuse).

In my opinion the center negative solutions are just WRONG. There is no reason for it. But if there is, please let me know. The center pin should be the positive and if everyone stuck with that the world would be a better place.

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  • \$\begingroup\$ It may be more safe, because often large metal parts are on GND polarity. So when you put your cable away there is chance of touching positive outer with GND (if GND is connected somehow). I know it is very minor case, because cable do not have common GND with rest of staff, but still. \$\endgroup\$ – Darko Apr 25 '16 at 8:58
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    \$\begingroup\$ That depends on the mains isolation. I would rather touch a +100 V output of a mains adapter with excellent mains isolation rather than the GND of a mains adapter with poor mains isolation. Take a device with a mains adapter and measure if that GND is really 0 Volt (AC and DC). You will often find that that GND can have 80 V AC on it. That is still safe as it has a very high impedance (the voltage collapses under load). \$\endgroup\$ – Bimpelrekkie Apr 25 '16 at 9:05
  • \$\begingroup\$ Some manufacturers insist on putting power to the barrell part. Indeed if you have a metal case, this may then necessitate a plastic ring around the DC connector. If your device is high power, a series diode can dissipate considerable energy. In these cases a PTC fuse + ESD diode may be a better bet. You get a (resettable) fuse and polarity protection in one package. \$\endgroup\$ – Barleyman Apr 25 '16 at 9:18
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    \$\begingroup\$ And everyone should drive on the same side of the road and wear the same clothes and have other physical similarities LOL. All countries MUST join the EU too. All chips must have the same pin-out and be made using packages of the same pitch. \$\endgroup\$ – Andy aka Apr 25 '16 at 9:46
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    \$\begingroup\$ @ricardomenzer gives one very good reason to use center-negative. In addition to the fact that the switching contact is usually the outer contact, it is also often true that the center pin makes contact first as the connector is inserted, which is another reason to make it the ground (negative) contact. \$\endgroup\$ – Dave Tweed Apr 25 '16 at 13:44
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Center positive is no safer than negative. Voltage is relative, not absolute. "plus" and "minus" were assigned purely arbitrary. No physical significance whatsoever.

Outer barrel is very easy to short circuit to the chassis, as it often partially sticks out. So the only thing that matters is "what has been chosen as GND for this particular LBB?". For circuits with positive GND a center-negative PSU is a no-brainer.

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    \$\begingroup\$ Ah yes, in this case it does make sense. I haven't encountered positive ground devices, only 0 and +V so I'm lazy with my terminology! \$\endgroup\$ – Mourndark Apr 25 '16 at 11:20
  • \$\begingroup\$ @Mourndark There is no such thing as absolute 0. It is only what circuit designer said "hey guys, let's call this point 0 from now on". (most DC devices are fully isolated) \$\endgroup\$ – Agent_L Apr 25 '16 at 11:57
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Most automotive outlets are 12V negative ground. Most automotive DC-DC adapters are not isolated.

If your LBB might be used in a car, and by the sort of person who would buy a non-isolated adapter, then center-positive is safer, since the exposed metal could otherwise short to exposed metal (if you can actually find any in a modern car!) and blow the fuse in the adapter.

Otherwise, there is no great advantage one way or the other, however my observation is that the vast majority of adapters manufactured in the last decade or so are centre-positive, so there is often some advantage in going with the crowed. On the other had, we were able to buy up a large supply of center-negative adapters a few years ago at a very attractive price and use them with a product that can accept either polarity.

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The solution to this problem is AC, put a diode bridge between your connector and device so that it can use either flavor of DC wallwart.

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    \$\begingroup\$ Possible, but you lose 2x diode drops then. Also ground shift on the 0V side is generally considered a bad thing unless you're subsequently going to regulate it again after the bridge. (Although anyone who doesn't put one diode in their intended-positive line should have their soldering irons confiscated for being a hazard to themselves and others!) \$\endgroup\$ – Graham Apr 25 '16 at 11:49

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