I have purchased about 5 different cigarette lighter to twin USB adapters until I found one that can both handle 24V and actually deliver the 2.1A needed for iPad 3/4. Unfortunately it was not one of the flush mount adapters but a larger one that met all criteria.
I am planning to install USB sockets into my dashboard and connect them behind the dashboard to the cigarette lighter. My mechanic doesn't object ("doesn't care" is more accurate).
Thanks to all who contributed. I found a very simple device that fits the "cigar lighter" socket and allows 2 connections. Each item I use draws very little amperage so well within the rated limits, cost was £4.00 and no paperwork needed, job done! Many thanks
Yes, I believe that it was Douglas Bader who was quoted as saying: "Regulations are for the obedience of fools, and the guidance of wise men". There's a lot more truth to this than the humour at the surface would suggest.
When I read and understand the myriad of design standards and regulations, I see a lot of wisdom, and hard lessons learned buried in there. There's not one I have ever read, with which I wholly disagree. However, it's how they are interpreted and applied in the "practical world" which is very subjective. I frequently find myself applying one interpretation, based upon my experience with certain types of aircraft, where another person (okay, the government regulator) has another interpretation.
Sometimes, I learn something from the experience presented by the other person, sometimes they don't seem too....
Basically you need to clear it with the firm doing the servicing.
If they are happy with it, nobody else is likely to complain.
Life with so many regulations is very awkward at times and what appears to be a very simple change seems time consuming and costly. Thank you for the guidance and the very sound advice to say little if anything.
There is an argument that there is no legal way to install a power connector in a certified aircraft, unless the equipment connected to it is suitably defined and certified.
This also means that the only thing that may be plugged into a cigar lighter socket is the approved supplied cigar lighter.
Needless to say everybody ignores this, but this is why the less said the better 
It's the same argument with e.g. installing a connector which brings out the NMEA data coming out of a panel mounted GPS. If the STC (or the FAA approved installation manual, etc) for that GPS does not support that connector (which it won't, without defining what plugs into it) then it cannot be installed.
Needless to say everybody ignores such an anally retarded interpretation, too.
You get this with modern cars too, in Europe. No way to fit a towbar to a VW Scirocco for example, because it was never certified to have one, and it will supposedly fail the MOT if they see one fitted, or even the mounting points added.
That said, interference with avionics is a real problem. Not (IME) with NAV kit, but certainly with VHF comms. I have seen this with various car-market power adaptors, most of which contain crude switch-mode power supply circuits.
For a C of A installation, try to have your mechanic sign it off as a minor modification. If you have to go the major modification (approval) route, there is more to this than meets the eye. It's not actually relevant to safe flight, but bureaucratic silliness. Avoid if possible.
That said, go easy on your mechanic, and only ask for the current you need. 10 amps is a stretch, 5 amps is fair, but if you can breaker it for 2 amps no one should be troubled.
Also be aware that there is guidance on the operation of "PED's" which I think is FAA technospeak for personal electronic devices. I have been told by Transport Canada that the answer to each is "no" unless shown to be "yes" as an element of an approval process (which is what I'm doing in Germany this week). The less said about what you're going to do with your newly found electrical source, the better.
If you want to do an actual EMI test on the PED in the plane - do I have a test plan for you!
I don't think there is a 2-pin XLR. At least it would be most unusual. The XLR connector is widely used in professional audio apps and it is 3-pin and 5-pin.
There are other connectors, even nicer, from e.g. LEMO, but they are smaller and the connections to them needs to be done by people with some decent soldering skills. The wiring of the plug likewise needs more skills, because you don't want shorts. But the result is very neat.
Thank you both. I have seen the 2 pin before but not the 3 pin one. I think I shall speak with my maintenance org. and see what they will fit and then purchase accordingly. Many thanks
What reg? G or N?
In general, one can get a "signed off" installation of a power connector.
It needs to have a circuit breaker dedicated to it.
The connector is, ahem, supposed to come with, wait for it, a Certificate of Conformity, but in reality of you pick a decent quality connector, most freelance installers will do it.
A popular type is this sort of thing
made by Cannon or Amphenol, and available from e.g. RS and this one is the sort of connector I would look at. I had one of those installed when the plane was new, even before the CAA CofA issue so the CAA inspector was OK with it.
Some people install an XLR connector, which is OK too, cheaper, and has no locking so it can unplug
!http://www.filmcentre.co.uk/images/cam/con_xlr.gif!
but you won't get those with a CofC, generally.
