XLR power leads

For many decades professional video equipment have used 4-pin XLR connectors for 12 volt DC power connections.  In every case I've come across they follow the conventions for wiring 3-pin XLR audio connections, of using pin 1 for the ground wiring, and having no voltages on exposed pins (the female is the power supply, the male is the power input, the same scheme being used with 48 volt phantom power on mike sockets).  And the standard wiring uses pin 4 for the positive 12 volts, with pins 2 and 3 being un-used.

There are some variations on that:

It's not always exactly 12 volts, but within the range that can be reasonably expected to come directly from a fully charged 12 volt battery, or the 12 volt accessory socket of a car (which could be up to 14 volts).  So equipment using this wiring scheme needs to be compatible with being directly powered from a battery (they usually can handle up to 15 or 17 volts—being able to handle the supply from a full battery, a car with the engine running, or a CCU at the end of a short or very long cable).

I have seen cameras using the two spare middle pins for carrying high voltages between a camera CCU cable and a regulated 12 volt supply strapped onto the camera (my 1980s JVC KY-2000 cameras do that).

I've seen a portable power supply / battery charger use one of the spare pins to control it's behaviour.  With the spare pins left open circuit it operates in charger mode, and when pin 2 is connected to pin 1 it goes into power supply mode.  And from memory, it couldn't charge through the 4-pin XLR (as it put out low-current negative 5 volts on pin 4 when in charge mode), it only charged batteries slotted into the V-Lock bracket.

I've used the two spare middle pins for carrying intercom audio on some of my equipment (the intercoms ran on 12 volts).

I've seen people wire pins 1 and 2 together, and pins 3 and 4 together, supposedly to increase the current carrying capability (it shouldn't be needed, as the XLRs are a high current capable connector).  And, supposedly, to reduce the losses caused by the resistance between plug and socket (this may or may not be significant).  But it's often done simple because they've used a cable that's just too fat to fit into the solder cups inside the XLRs.

Standard wiring scheme:

  1. Ground, zero volts, or the negative battery terminal (they all mean the same thing)
  2. not connected
  3. not connected
  4. Positive 12 volts (or near 12 volts)

Most professional power supplies don't ground their DC output.  So you have 12 volts between pins 1 and 4, floating without any reference to earth.  This prevents mains ground loops when connecting equipment together.  Some may have a resistive and/or capacitive connection to ground, so outputs don't float at high voltages, and to shunt away any electrical interference noise.

Conversely, most professional equipment will ground pin 1 to chassis.  Often right next to the connector.

In general, we'd always been able to plug any professional or industrial 12 volt camera into any professional or industrial 12 volt power supply.  It didn't matter who built them.