why power adapters all over-voltage?

I have accumulated many power adapters: from modems, cordless phones, scanners, Christmas lights and so on. A few were rather old, so I tested them before discarding some. I found that under no load, all of them put out substantially more than the claimed voltage. The worst was a "6 V" job that measured 11.2 V. I thought the multimeter might be faulty, so checked with another voltmeter, and got the same readings (within 0.1 V). So are these things designed to give only the correct voltage at rated load? I conjecture that there is a lot of resistance in the smoothing circuit.

"Orson Cart"

** Small mains transformers ( ie 6VA or less) have voltage regulation factors of about 25% - so the unloaded voltage drops by that factor when the rated load is applied.

When diodes and filter caps are added to make DC, the voltage drop almost doubles since the peak current charging the cap is around double the simple, resistive load case. Peak current and peak voltage coincide.

Applying this to your example, the 11.2 volts DC drops by 46% under full load to get down 6 volts DC.

Only way to improve things ( short of using a regulator IC) is to use a bigger transformer and that costs $s.

.... Phil

Non regulated types no doubt. I bet if you put 75% load on that unit you'd get ~ 6 volt reading.

Those types are just full rectifiers and your seeing the peak reading because they most likely have a reservoir cap in the unit.

Not something I would use on sensitive things.

Jamie

The old transformer-type warts have a lot of copper loss, partly for impedance limiting, partly to make them really cheap.;

The modern switcher types mostly regulate very nicely.

John

"John Larkin"

** No more than any similar size VA transformer does. ** Bollocks.

** More bollocks.

** Damn shame they are in almost every other way dangerous piles of shit.

... Phil

it.

Why are they dangerous? I have one plugged in 24/7. Been running for a couple years.

-Bill

"Complexity is the enemy of reliability".

SMPS have a far greater variety of possible (and plausible) failure modes.

"Nobody"

** Is that original ??

Not bad in any case.

** You are damn tooting.

.... Phil

"Bill Bowden" "Phil Allison"

Why are they dangerous?

** Cos any of a number of predictable failures ( particularly end of life failures) and simple accidents could easily lead to the output circuit becoming live at full supply voltage - with sufficient current to electrocute.

None of which is equally true of transformer based adaptor.

In a nut shell, the makers only pay lip service to Class 2 insulation rules and the agencies that pass them as OK are under the thumb to do so.

.... Phil

We furnish Phihong switchmode warts with a lot of our products. A universal/international plug adapter set is available, so they work most anywhere in the world, 100 volts to 240. Regulations is excellent, and I don't know of any failures out of around a thousand we've used.

John

So, how many failures have you seen or heard of since they started selling millions of them?

Thanks, Rich

"Rich Grise"

** Hardly a relevant question.

SMPS do fail and they do contain many electro caps that all have limited life spans.

The wall wart and small box kind are throw away items, never likely to be repaired by anyone.

.... Phil

That's how a Dataproducts printer sales droid answered when I asked about the life expectancy of the multiple plastic paper tray switch mechanisms in their printers. I was concerned about their life while being manhandled by users many times each day.

I never found out. The printers failed from from so many other multiple faults, that they were thrown out due to uneconomical repair costings way before any hinges broke...

So, if you put it that way, yes, I did end up with egg on my face. It still didn't stop them from being steaming piles of crap though.

--
My keyboard has an F1 key. Where is the NASCAR key?

Yabbut, I was responding to "Nobody", who wrote, "SMPS have a far greater variety of possible (and plausible) failure modes."

I was just hoping he'd list them or cite an example or something.

Cheers! Rich

shit.

Sure, but they only cost $2 at a swap meet. Who cares how complex they are as long as they don't burn the house down?

-Bill

"Bill Bowden"

Sure, but they only cost $2 at a swap meet.

** Irrelevant crapology.

... Phil

You don't have to. The fact it has more parts, by that very nature means there are more points of failure. We're generalising there, there's no need to specify numbers, just the point that in general, more parts = more failures.

But it's more complicated than that. It comes down to the design. With some designs with low component counts, the entire operation may hinge on a single component. One failure will bring it to its knees, rather than "half fail" where some things may still work. This you *can't* generalise on, because you can't generalise on designs you don't have statistics on.

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If I had anything witty to say, I wouldn't put it here.

Well, I'm no theoretical physicist or rocket scientist, but I do know that to determine failure rates of a particular design, you have to count actual failures per actual number of units, and I'm sorry, but to do that in real life you need numbers.

Hope This Helps! Rich

You're talking about more specific numbers, the OP was talking very general around-about sort of kinda you get what I mean.

Also, I have to nit-pick on your statement that you have to count failures. It defeats the purpose doesn't it? True, once you count the failures verses the live units, you get an absolute correct count (at least at that point in the life cycle). That's nice, but at this point, you're pretty much telling everyone what they already know.

The point here is to PREDICT failures. And THAT is much more difficult to do. It involves black magic and voodoo mathematics, and depending on who you ask, the phase of the moon too.

The switchmode design / component count issue having higher failure rates than an inductive transformer is a predictive model. Not an accurate one mind you, but predictive none the less.

--
You can tune a guitar, but you cant tuna fish.