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The voltage problem relates to current. The adapter
has an internal impedance that reduces the voltage
when current is drawn. The amount of voltage drop is
the product of the current times the impedance. Draw
less current and the voltage drop is lower, so the output
voltage is higher.
There are UL LISTED devices, and UL approved devices. There is aOver the years I have been involved in hundreds of
products that have AC adapters. In the qualification
of that piece of the product it is tested and then researched.
UL has a nice paper trail. Its been my experience that
a high number of these adapters have ID on them that
don't belong to that product.
I'm not sure you knew how to decode them. From the above statement.Hence we rejected them.
You would be surprised at what level of quality products are madeHowever I have purchased products for comparison
qualifications and found these do not have valid numbers
on them either. You would be surprised what comes out
of china.
Now, you are talking about system integration. Many makers DO makeI suspect many companies never qualify products
they bundle.
No. But nor do I believe that the adapter only put out less than oneLast year I had a 7.5V 1A adapter from the
orient. Tested with load the device was only 75mA @ 7.5V, now
do you believe that UL would have approved that adapter?
For LV power supplies, exactly.If the design of the adapter
requires a load to work correctly, its a poorly designed
product.
The question has never been about AC output. An AC adapter outputs DC.
"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message news:utcjhv8j0go2405jn4i3rdfk7q8f9ut5g9@4ax.com...
On Sat, 19 Jul 2003 20:25:04 GMT, "Mark" <md.siegel@sbcglobal.net
wrote:
The question is " Isn't using an AC adapter with way higher current
ratings
harmful?
There is no assumption here that the supply is not regulated to put out
rated voltage. The question is strictly a current rating question.
So are you saying that a AC adapter that has a regulated 3V 500mA
rating would hard a device with a 3V 200mA pull?
Among other things, I said "many warts are not regulated". The
AC-output ones never are.
John
I don't know about you but I cant remember the last
device I saw that used an AC-AC adapter. And since
the example was a sony walkman AC-DC must be assumed
since we are talking about replacement adapters.
The question related to current not voltage. Had he stated
that a regulated adapter was not a problem his premises is fine.
However he leaves the reader with the assumption that all adapters
are created equal.
His further assumption that any two adapters with the same
voltage and current rating will produce the same result under
the same load. This again is a poor assumption unless both adapters
are regulated. His premises is that no adapters are regulated.
ehsjr@bellatlantic.net> wrote in message
news:3F1A06A9.9D0001CF@bellatlantic.net...
Mark wrote:
No, his answer and premise are incorrect.
Again, you are wrong. Go back and re-read his
original. Nowhere does he say AC-DC adapter.
Your original answer specified an AC-DC adapter.
Higher
available current by itself will not harm a device. The
device pulls what it needs. Every answer here that is
a problem relates to voltage not current.
The voltage problem relates to current. The adapter
has an internal impedance that reduces the voltage
when current is drawn. The amount of voltage drop is
the product of the current times the impedance. Draw
less current and the voltage drop is lower, so the output
voltage is higher.
A well voltage
regulated supply can have as much available current as it
wants.
That's true, but is not what the OP was talking about.
He specifically addressed badly regulated supplies - here's
what he said: "... the small transformer in AC adapters have
a horrible regulation."
I will agree there are many AC adapters on the market
that don't meet there rating. Most of those have a UL stamp
on them but have never been UL tested.
They meet their ratings at the specified current.
What you apparently do not realize is that many AC adapters
rated at say 12 volts at say .5 amp will put out much higher
voltage when loaded well below their current rating. The OP
said some of the adapters he is talking about put out between
16 and 20 volts with no load, and 12 volts when loaded at
their current rating. I'll use 12 volts at .5 amps as the
transformer specs and pick 17 volts - on the lower side of
the OPs range - as the no load output voltage, in the
following. How could that be? Our 12 volt transformer puts
out 17 volts with no load and 12 volts with a .5 amp load.
That means it has an internal impedance of 10 ohms.
Say your device uses 50 ma at 12 volts. The device impedance
is 12/.05 or 240 ohms, and it will dissipate .6 watts when
supplied by 12 volts. Suppose it is connected to the 12V
transformer whose internal impedance is 10 ohms. The total
circuit impedance is 250 ohms, so .068 amps will be
drawn. (17/250) The total dissipation is I^2R, or 1.156
watts. .04624 watts are dissipated in the transformer,
and 1.10976 watts are dissipated in the device - almost
twice what it would dissipate with a proper adapter.
Gym Bob wrote:
Have to agree with the DC output theory but he really didn't specify in
his
post.
As you said, he did not specify. Mark just assumed AC-DC.
But it doesn't matter whether it is an AC to AC, or an AC to DC
adapter, in any event. Most AC-DC adapters are not regulated,
and the OP clearly was talking about unregulated adapters.
It depends on the specific adapter and application and cannot be
predicted
here.
You can't predict specifics when you don't what the load or
the supply is. However, you can predict that an unregulated
adapter will put out higher voltage than its rating when
loaded well below its current rating. Words to that effect
can be found in some catalogs, so it is not just speculation -
those predictions have been made and are in print. For example:
(from the latest copy of the Allelectronics catalog, page 59)
"A Note on Wall Transformers: A.C. and D.C. Wall Transformers
are not regulated unless otherwise stated. They provide a
specified voltage at a specified load. If the load is lower
than specified, the voltage will be higher than specified.
For example, a 12 volt 500 ma. wall transformer may read 20
volts with no load or 9 volts with a 600 ma load."
By the way, that page shows 9 AC to AC adapters, and 21
AC to DC adapters. None of them are regulated.
Every modem I have bought for external use uses an AC/AC adaptor.