why power adapters all over-voltage?

Rich Grise wrote:

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.
Click to expand...
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.
 
On Sun, 14 Aug 2011 20:29:12 +1000, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:


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

---
Parrot 1: "What's that stink?"

Parrot 2: "That perch you're standing on."

--
JF
 
John Fields wrote:
On Sun, 14 Aug 2011 20:29:12 +1000, John Tserkezis

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

You can cream asparagus, but you can't pea soup.
Click to expand...

Parrot 1: "What's that stink?"

Parrot 2: "That perch you're standing on."

Huh? What's this supposed to mean?
Click to expand...

I like Jake's quickie from 2.5 Men: "Two muffins are baking in
the oven. One muffin says, "Boy, sure is hot in here." The other
muffin says, "Oh, my God! A talking muffin!""

Wanna hear the world's worst joke?

Q: Why do cows wear bells?
A: Because their horns don't work.

Cheers!
Rich
 
"Rich Grise"

Wanna hear the world's worst joke?

Q: Why do cows wear bells?
A: Because their horns don't work.
Click to expand...
** Did you hear about the cow they thought was a genius ?

Well, she was out standing in her field ...



..... Phil
 
On Tue, 16 Aug 2011 15:22:52 -0700, Rich Grise
<richg@example.net.invalid> wrote:

John Fields wrote:
On Sun, 14 Aug 2011 20:29:12 +1000, John Tserkezis

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

You can cream asparagus, but you can't pea soup.

Parrot 1: "What's that stink?"

Parrot 2: "That perch you're standing on."

Huh? What's this supposed to mean?
Click to expand...
---
Two parrots are sitting on a perch.

One says: "Do you smell fish?

--
JF
 
On Wed, 17 Aug 2011 08:30:24 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"Rich Grise"

Wanna hear the world's worst joke?

Q: Why do cows wear bells?
A: Because their horns don't work.


** Did you hear about the cow they thought was a genius ?

Well, she was out standing in her field ...

---
Click to expand...
Did you hear about the woman who ate a doorknob?

It turned her stomach.

--
JF
 
On Aug 16, 3:22 pm, Rich Grise <ri...@example.net.invalid> wrote:
John Fields wrote:
On Sun, 14 Aug 2011 20:29:12 +1000, John Tserkezis

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

You can cream asparagus, but you can't pea soup.

Parrot 1: "What's that stink?"

Parrot 2: "That perch you're standing on."

Huh? What's this supposed to mean?

I like Jake's quickie from 2.5 Men: "Two muffins are baking in
the oven. One muffin says, "Boy, sure is hot in here." The other
muffin says, "Oh, my God! A talking muffin!""

Wanna hear the world's worst joke?

Q: Why do cows wear bells?
A: Because their horns don't work.

Cheers!
Rich
Click to expand...
I went to a museum and asked how old the dinosaur bones were, and the
attendant said they were 65,000,013 years old. So, I asked how
technology could measure ages to such accuracy of 13 years, and he
said:

"Well, I've been working here for 13 years, and when I started they
told me those bones were 65 million years old."

-Bill
 
On Sun, 14 Aug 2011 10:55:56 +1000, John Tserkezis
<jt@techniciansyndrome.org.invalid> wrote:

My keyboard has an F1 key. Where is the NASCAR key?
Your keyboard is obviously a racing keyboard. That's why there is no
Click to expand...
NASCAR key.
Eric
 
On 2011-08-07, Orson Cart <ex-privat@parts.org> wrote:
So are these things designed to give only the correct voltage at
rated load?
Click to expand...
No, these things have capacitors which charge to a peak voltage, which is what
you're measuring with your high-impedance multimeter.

You're forgetting about peak versus RMS voltages.

For instance, to make a 12V wall bug, we will take a 10:1 transformer, which
gives us 12V AC RMS. This will rectify to a fluctuating DC, which is also 12V
RMS.

That fluctuating DC will charge the internal capacitor to about 17V: its peak
voltage (12 / 0.707).

I conjecture that there is a lot of resistance in the
smoothing circuit.
Click to expand...
The wall bug would have this behavior even if it were made up out of
superconductive components. It contains reactances; it is not a battery with an
internal resistance. That model does not apply.

The internal resistance of a battery-like voltage source will reduce voltage in
proportion to the current demand (Ohm's law).

The wall bug's voltage drop is different. With increasing loads, the average
of its increasingly rippling voltage will go down toward the RMS value, but
near the RMS value, it should stiffen up and show a different voltage drop
behavior from there toward zero ohms.
 
On Tue, 30 Aug 2011 04:04:54 +0000 (UTC), Kaz Kylheku
<kaz@kylheku.com> wrote:

On 2011-08-07, Orson Cart <ex-privat@parts.org> wrote:
So are these things designed to give only the correct voltage at
rated load?

No, these things have capacitors which charge to a peak voltage, which is what
you're measuring with your high-impedance multimeter.

You're forgetting about peak versus RMS voltages.

For instance, to make a 12V wall bug, we will take a 10:1 transformer, which
gives us 12V AC RMS. This will rectify to a fluctuating DC, which is also 12V
RMS.

That fluctuating DC will charge the internal capacitor to about 17V: its peak
voltage (12 / 0.707).

I conjecture that there is a lot of resistance in the
smoothing circuit.

The wall bug would have this behavior even if it were made up out of
superconductive components. It contains reactances; it is not a battery with an
internal resistance. That model does not apply.

The internal resistance of a battery-like voltage source will reduce voltage in
proportion to the current demand (Ohm's law).

The wall bug's voltage drop is different. With increasing loads, the average
of its increasingly rippling voltage will go down toward the RMS value, but
near the RMS value, it should stiffen up and show a different voltage drop
behavior from there toward zero ohms.
Click to expand...
---
Even after having said all that, the fact remains that because of its
internal resistance, an unregulated wall-wart's output voltage is
specified for a given load.

--
JF
 
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