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External power adaptors

For consumer devices that employ external power adaptors (bricks and wall warts), when it comes to determining the polarity/pinout of the *required* adapter, is it safe to advise:

"Determine which power inlet pin(s) show continuity with known 'grounds' as accessible via other common connectors on the device"

I.e., check the inlet vs. a USB port's ground, audio I/O, etc. (recall not all power adaptors use "barrel" connectors)

Are there any "common connectors" on the device whose grounds should be avoided (e.g., speaker outputs can have floating "returns") in this analysis?

Reply to
Don Y
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The obvious gotcha is that a big capacitor across the input can seem to show continuity.

You can sidestep all that nonsense by putting a bridge rectifier on the appliance input.

NT

Reply to
tabbypurr

Or things where for reasons of designer perversity that have the supply feed and outer casing earth swapped around. It isn't for nothing that generic wallwarts come with a set of reversible fits anything plugs.

Safest way is take the top off and look for a polarised capacitor nearest the power input (and pray its installed the right way round).

I'd say if you have to guess then centre positive has a ~80% hit rate but there is a real chance of killing some stuff that way. Sometimes there is a faint icon hidden somewhere on the ratings plate. Stuff is designed to be thrown away these days if the original PSU dies.

If USB ground is same as outer case then I'd be prepared to bet on a centre positive supply without any further tests. Guitar effects seem to be one common counter example to centre positive just catch you out.

I have a selection of old portable PSUs ranging from 12v to 21v and 3-6A with almost every possible diameter of round plugs on them!

--
Regards, 
Martin Brown
Reply to
Martin Brown

Yes, but a VOM would let you determine if the sleeve is wired to "what the device considers to be GND" -- hence the point of my carefully worded "advice".

The "-" of said capacitor would be electrically connected to "what the device considers to be GND", right? I.e., a VOM could verify continuity between that cap pin and the "GND" on an EIA232 connector... or, a USB connector... or, ... Similarly, back to the power inlet (regardless of what type of connector is employed)

The non-US market seems to be the biggest red-flag re: center NOT positive (assuming we're talking only about barrel connectors, here)

Ditto -- though my range is larger (lots of sub-12V devices for odd bits of consumer kit and up to ~48V for PoE sources -- as well as "AC" outputs). I also make a point of harvesting any "plug sets" (as you mentioned in your first P) that I come across -- they usually have only a few different supply-side mating connectors. (the altoids/mintz "tins" are perfect for storing these sorts of things)

I'm trying to come up with a simple guideline to make it easy for folks to sort out *likely* mating connectors with nothing more than a voltmeter (or a little jig that's a "Voltmeter for Dummies" -- i.e., a light!). So, the guiding principle is: use this technique UNLESS the device in question is sufficiently "precious" to merit special attention, then, call for help.

Reply to
Don Y

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