Don Y said:
Any guidelines on selection criteria?
There is a Japanese standard for these, that Sony and Nintendo equipment
seems to follow.
http://en.wikipedia.org/wiki/EIAJ_connector The
diameter goes up with voltage. The standard doesn't seem to require it,
but it seems popular for the defined voltage to not be an even number.
The insulator at the tips of these plugs is usually yellow, at least on
Sony and Nintendo gear.
For "regular" plugs, the most common OD is probably 5.5 mm, with either
a 2.1 mm or 2.5 mm ID. The voltage and current can be anything at all,
up to maybe 50 W, +/- 50 W.
If you want your customers to be able to improvise a supply, pick a
common size. If you want to sell them power supplies and/or cause them
to go to the competition, pick a weird size.
I don't know what brands are good or bad, but I'd probably look for a
"name brand" like Switchcraft or Hirose first, and then look for
something cheaper when the bean counters scream.
A lot of smaller devices seem to have standardized on USB connectors,
which are almost always good for 5 V, 0.5 A; higher currents are
available but you can't always count on this. The main advantage is
that you can be pretty sure that the voltage will be between 5.0 and
5.5 V, and that the polarity will be right.
Any worries about failure modes?
I've actually had more failures of the wire right behind the plug than
I have had of the plugs or the jacks. I would tend to trust a through-
hole jack more than a surface mount one, especially if the plug will
be disconnected and reconnected often.
These are more of a design thing, but: one "failure mode" might be that
if the plug even remotely sort of fits, people will try to cram it into
the socket, so you might get all kinds of unexpected voltages coming in.
A fuse with a "backwards" diode after it guards against wrong polarity;
guarding against too-high voltage is a little trickier. If you can
stand the voltage drop, board space, and budget, put in a bridge
rectifier and a capacitor, so the user can use any AC or DC power supply
of some minimum voltage.
Also, some of the jacks have switches that open or close on plug
insertion. Some devices use these to switch between internal or
external power. Some users, though, might prefer to leave the plug
connected all the time, and remove the AC power from the power supply
instead; your device might not be able to rely on the switch contact
in the jack.
Do the "inverse" variety (now popular on laptops, LCD's, etc.) offer
any special advantages over the traditional style?
Are you talking about the ones that have a pin inside the barrel? As
far as I know, this is to offer an extra "data" channel to the power
supply; these are really 3-circuit connectors. The outside surface of
the barrel is one side of the power supply (usually negative), the
inside surface of the barrel is the other (usually positive), and the
pin is "data". The "data" can be as simple as a resistor to one side
of the power supply, which the connected device measures and interprets,
or as complex as a serial bus to a microcontroller or ROM inside the
power supply.
The "data" can be used for good or for evil. A good use is that the
cheap 20 volt 3 amp supply has (say) a 1K resistor to ground, and the
more expensive 20 volt 5 amp supply has (say) a 2K resistor to ground.
The connected device has an A/D that tells it what resistor is there,
and then it can adjust its internal switching power supply appropriately
to only draw as much current as the external supply is capable of.
(Laptops often do this; they *have* to power the CPU and then the
battery gets whatever is "left over".) An evil use is to require the
power supply to authenticate to the connected device, in order to sell
power supplies at inflated prices^W^W^W^W^W^Wprotect the consumer.
Matt Roberds
