why power adapters all over-voltage?

[Orson Cart]

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.

 

[BobW]

"Orson Cart" <ex-privat@parts.org> wrote in message
news:4e3df2ac$1@x-privat.org...

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.

Yep, you're on the right track.

Most adapters are just wimpy transformers, diodes, and some small caps.
Their source impedance (mostly from the transformer) is pretty high so they
only put out their nominal voltage when under a certain load. It's pretty
scary if your circuit's not expecting that.

The better ones have regulators and behave much more like constant voltage
supplies.

Bob
--
== All google group posts are automatically deleted due to spam ==

 

[Phil Allison]

"Orson Cart"

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.

** 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

 

[Jamie]

Orson Cart wrote:

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.
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

 

[John Larkin]

On 7 Aug 2011 04:04:28 +0100, "Orson Cart" <ex-privat@parts.org>
wrote:

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.

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

 

[Phil Allison]

"John Larkin"

The old transformer-type warts have a lot of copper loss,

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

partly for impedance limiting,

** Bollocks.

partly to make them really cheap.;

** More bollocks.

The modern switcher types mostly regulate very nicely.

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




.... Phil

 

[Bill Bowden]

On Aug 7, 7:48 am, "Phil Allison" <phi...@tpg.com.au> wrote:

"John Larkin"



The old transformer-type warts have a lot of copper loss,

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

partly for impedance limiting,

** Bollocks.

partly to make them really cheap.;

** More bollocks.

The modern switcher types mostly regulate very nicely.

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

...   Phil

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

-Bill

 

[Nobody]

On Fri, 12 Aug 2011 20:22:01 -0700, Bill Bowden wrote:

The modern switcher types mostly regulate very nicely.

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

...   Phil

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

"Complexity is the enemy of reliability".

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

 

[Phil Allison]

"Nobody"

"Complexity is the enemy of reliability".

** Is that original ??


Not bad in any case.

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

** You are damn tooting.




..... Phil

 

[Phil Allison]

"Bill Bowden"
"Phil Allison"

The modern switcher types mostly regulate very nicely.

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

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

 

[John Larkin]

On Fri, 12 Aug 2011 20:22:01 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

On Aug 7, 7:48 am, "Phil Allison" <phi...@tpg.com.au> wrote:
"John Larkin"



The old transformer-type warts have a lot of copper loss,

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

partly for impedance limiting,

** Bollocks.

partly to make them really cheap.;

** More bollocks.

The modern switcher types mostly regulate very nicely.

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

...   Phil

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

-Bill

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

 

[Rich Grise]

Nobody wrote:

On Fri, 12 Aug 2011 20:22:01 -0700, Bill Bowden wrote:

The modern switcher types mostly regulate very nicely.

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

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

"Complexity is the enemy of reliability".

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

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

Thanks,
Rich

 

[Phil Allison]

"Rich Grise"

Bill Bowden wrote:

"Complexity is the enemy of reliability".

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

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

** 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

 

[John Tserkezis]

Rich Grise wrote:

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

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?

 

[Rich Grise]

John Tserkezis wrote:

Rich Grise wrote:

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

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.

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

 

[Phil Allison]

"Rich Grise"

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.

** You fucking thick or sumtin' ?

Every part in a SMPS adaptor is likely to fail - some likely to produce a
loss of isolation from the AC supply.

The comparison is with a simple, DC adaptor that only has three parts.

FYI

1. A 50/60Hz fail safe transformer.

2. A bridge rectifier.

3. A filter electro.



..... Phil

 

[Phil Allison]

"Bill Bowden"

"Complexity is the enemy of reliability".

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

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


** Irrelevant crapology.


.... Phil

 

[Bill Bowden]

On Aug 12, 10:58 pm, Nobody <nob...@nowhere.com> wrote:

On Fri, 12 Aug 2011 20:22:01 -0700, Bill Bowden wrote:
The modern switcher types mostly regulate very nicely.

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

...   Phil

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

"Complexity is the enemy of reliability".

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

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

 

[John Tserkezis]

Rich Grise wrote:

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.

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

 

[Rich Grise]

John Tserkezis wrote:

Rich Grise wrote:

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.

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.

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