Reverse Polarity = brick. What fried?

btw, when I say reverse polarity, I mean the 9v into the unit was reversed, not the 110V into the transformer.

Well, if you can open it up without wrecking it, look near the incoming power jack. Follow the PCB trace to the center post. It should go to a fuse and/or a diode going to the other post. Check the fuse for continuity and check the diode for shorts. The fuse may have blown, or the diode may have sacrificed itself and shorted.

OH! OR the 9V wall-wart may have died when plugged into the DVD player again, if the DVD player diode shorted.

I think we have this dicussion every 6 months or so.

Why don't manufacturers put a 5 cent diode across the input so it blows the fuse in the AC adapter - or destroys it, but doesn't damage the much more expensive powered equipment?

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I think it might be a good idea to look at making it a condition of gaining safety approval. At reasonable production volumes, the diode wouldn't even cost 5 cents, so there really is no excuse.

Even better would be a full bridge rectifier and smoothing cap. That way it could run off AC or DC, and polarity would not be an issue. This would probably wouldn't be viable on very cheap equipment though.

Dave

If it's a DC input, why not just a diode in series? Then it simply just doesn't work if connected to the wrong polarity. But nothing gets damaged.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I assume they wouldn't want to waste the power.

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Interesting view. ;-)

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

Because in high-volume manufacturing, five cents is a very significant cost.

Years ago, a friend worked for RCA's television operation in Indianapolis. He told me that adding twenty-five cents to the cost of a high-end TV required a manager-level review, adding fifty cents required director approval, and adding seventy-five cents caused heart palpitations in the executive suite.

Isaac

I know this is beyond ignorant, but:

I know a diode has 3 legs. to check it for shorts, I check it across which legs? If I get "0" ohms across any of the 3 possible paths I replace it?

At least I'm bright enough to know a bad fuse when I find one! :-)

Bob

'turns out I was even ingnoranter than I supposed. Disregard my last post.

Bob

I've never seen a fuse inside an AC adapter. Guess they're "designed" to be impedance limited by using really thin wire in the transformer primary. They save the cost of a fuse *and* save money on the thinner wire. What a deal.

They've sometimes got a thermal fuse incorporated in the transformer.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

if they spent the nickel they would louse the all mighty dollar.

leave the transistor alone