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Question 1: what is the name for this DC connector?

Question 2: Why are there 3 connections for a DC connector, and what are they for.

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    \$\begingroup\$ It's a coaxial power connector and the second lead on the bottom is for types which have a detection for the plug. For standard types, they are just the same terminal and only present for mechanical stability. \$\endgroup\$ – Janka Jun 8 '17 at 2:01
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    \$\begingroup\$ The third connection is usually for a switch - it may be used to disconnect one side of an internal battery when the plug is inserted. \$\endgroup\$ – Peter Bennett Jun 8 '17 at 2:22
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    \$\begingroup\$ searching ebay for "2.1mm dc" hits a ton of listings with parts that look like that. \$\endgroup\$ – dandavis Jun 8 '17 at 6:34
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    \$\begingroup\$ @Passerby: it looks like one (2mm pin spacing almost same size as hole) and listings of 2.1mm outnumber the lesser sizes 10:1 \$\endgroup\$ – dandavis Jun 9 '17 at 7:38
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    \$\begingroup\$ @Passerby: sure, but listings are a good indicator of the market; and it happens that 2.1mm is the most common in my personal experience as well. i play the odds. \$\endgroup\$ – dandavis Jun 9 '17 at 7:41
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Rev A

It's called a DC POWER JACK but in the category of barrel-power-connectors . which normally have a sleeve switch "sleeve=outer conductor" with output on the side pin. Normally today 0V return path is on outside and +V is on centre pin. So we might call this application a low side SPST or sleeve switch with 3mm center pin and 5.5mm diameter sleeve male receptacle. Although we do not use the sexist term male and female for (floating) plugs and (fixed) jacks anymore, we prefer to call them Plugs and receptacles for mechanical reasons. But for limiting exposed power, connectors are usually "pins" for the electrical load receptor and "Sockets" for the electrical source while connectors termed "Jacks" are simply Fixed mounting connectors and Receptacles refers to the electrical connection. But unlike AC outlets, it is both an electrical and mechanical receptacle to receive a plug which takes the electrical load.

Some DC barrel connectors are explicitly defined in sort columns such as at Digikey under Internal Switch as "Does not contain switch" but these are the exception.

However, Digikey and others carry no stock.
credits to @Passerby for the nudge on "all"

Other Mfg's describe it as JACK, 2.5MM PIN, SHORT BUSHING

Connector names should be abbreviated in BOM's with description in reverse order of significance, thus a common abbreviation is simply.

CONN PWR JACK 2X5.5MM SWITCHED SOLDER

However many major connector factories find define any connector with (M) or (F) which I support as being easier to understand especially when users have the acronym as effective series inductance. (ESL)

UPDATE

Not all pins are the same diameter, and this is critical for good contact. ALthough I said 3mm and 2.5 this was a generic size THere are many others.

prompted by comments @DanDavis and @Passerby (keep up the good work)

I looked up the website in the photo Now that you know how, dont ask again. enter image description here The part number and description is DC-005 5.5-2.1MM 2.0 Power supply DC but generically it is a CONN, Barrel, DC 5.5mm-2.0mm ( I suspect the difference is the pin and socket diameter of the center conductor enter image description here http://www.xinlgo.com/product_detail-1850.htm

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  1. DC barrel jack of unknown size.

A coaxial power connector is an electrical power connector used for attaching extra-low voltage devices such as consumer electronics to external electricity. Also known as barrel connectors, concentric barrel connectors or tip connectors, these small cylindrical connectors come in an enormous variety of sizes.

  1. It is a switched jack. The third pin will connect either to the center pin or outside pin while no plug is inserted, and will break open when a plug is inserted.
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  • \$\begingroup\$ unknown, but looks a lot like 5.5mm outer and 2.1mm inner - this is the most common size \$\endgroup\$ – Jasen Jun 8 '17 at 6:04
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    \$\begingroup\$ @Jasen what about 2.5mm inner? Or 5mm outer? There are many many sizes of dc barrel jacks. \$\endgroup\$ – Passerby Jun 8 '17 at 6:06
  • \$\begingroup\$ nevertheless 5.5x2.1 is the most common. Furthermore in the image the with the square-ruler hole is 6mm and the pin is closer to 2mm than 2.5. \$\endgroup\$ – Jasen Jun 26 '17 at 4:46
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The exact name requires a part number or stamp. In general, that kind of connector is called a "Power Jack" by manufacturers. There are typically three connections instead of two in order that a "plug in detect" is possible. Two of the wires are shorted together when there is no plug in the socket, and they separate when a plug is inserted.

The jack you show a picture of, is very similar to a manufacturer called CUI. Perhaps you have a foreign made knock-off in the photo.

An example data sheet is here for a CUI part: http://www.mouser.com/ds/2/670/pj-002ah-516007.pdf

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    \$\begingroup\$ CUI also knock offs and brands under their name. They don't make anything except drawings. \$\endgroup\$ – Sunnyskyguy EE75 Jun 8 '17 at 2:44
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    \$\begingroup\$ the CUI part has notches in the upper external corners. \$\endgroup\$ – Jasen Jun 8 '17 at 6:07
  • \$\begingroup\$ CUI is in my back yard. They had parts on the shelf the last time I visited them, and yes there are notches on the CUI parts. I looked through all available parts on Mouser before replying. The one I linked is the closest match in how it functions; that's why I figured the part in the opening post must be a china knock off. The notch helps hold the connector to the case so that it doesn't wiggle and stress the PCB board. A lower cost part wouldn't bother with it. \$\endgroup\$ – Andrew of Scappoose Jun 9 '17 at 6:04
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They are called "DC Power Jack Socket 2.1mm Barrel-Type PCB Mount"

They are not switched.

5.5 x 2.1 dimensions.

Link: SODIAL(R) 10pcs DC Power Jack Socket 2.1mm Barrel-Type PCB Mount

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  • \$\begingroup\$ The side terminal is connected to the underside terminal when no plug is inserted, when a plug is inserted this connection is broken and the underside terminal connects to plug outer and the back terminal to plug inner. I'd call that switched. \$\endgroup\$ – Jasen Jun 26 '17 at 4:56

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