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Isn't using an AC adapter with way higher current ratings harmful?

A

AC/DCdude17

Jan 1, 1970
0
X-No-Archive: Yes

The common rules for choosing a replacement AC adapter are match the
voltage and amp should be equal or greater than the original AC
adapter. This makes perfect sense in theory, although the small
transformer in AC adapters have a horrible regulation. A 12V AC adapter
often puts out 16 to 20V under no load and is designed to give 12V when
loaded to rated current. Once it's loaded to rated current, I^2R drop
brings the voltage down to 12V.

If you have a walkman that takes a 4.5V @ 250mA and use a 1A
adapter, it will probably send out 6-8V with only 250mA of load. The
walkman most likely has a linear regulator inside, so it should be ok in
a short term, but higher input voltage means higher dissipation which
may cause premature failure.
 
B

Ben Miller

Jan 1, 1970
0
Mark said:
Your answer is wrong and your premise is wrong.
It is completely safe to use an AD-DC adapter with
a higher current rating, provided that the adapters
voltage rating is accurate. If the design of the adapter
requires a load to work correctly, its a poorly designed
product. Such an adapter will not be reliable even at
the rated current and voltage. Most adapters that
put out higher voltage with no load also do not
produce the current they are rated at. So if you
are looking for a replacement adapter look for one
that is a regulated.

Not true. Dudes observation is correct. These adapters use "impedance
limiting" to meet the UL class 2 requirements for energy limited
transformers, as opposed to fuse links or other more costly methods. They
will produce approximately rated voltage at rated current, but open circuit
voltage will be higher.

Ben Miller
 
J

John Popelish

Jan 1, 1970
0
AC/DCdude17 said:
X-No-Archive: Yes

The common rules for choosing a replacement AC adapter are match the
voltage and amp should be equal or greater than the original AC
adapter. This makes perfect sense in theory, although the small
transformer in AC adapters have a horrible regulation. A 12V AC adapter
often puts out 16 to 20V under no load and is designed to give 12V when
loaded to rated current. Once it's loaded to rated current, I^2R drop
brings the voltage down to 12V.

If you have a walkman that takes a 4.5V @ 250mA and use a 1A
adapter, it will probably send out 6-8V with only 250mA of load. The
walkman most likely has a linear regulator inside, so it should be ok in
a short term, but higher input voltage means higher dissipation which
may cause premature failure.

I have had to use a lower voltage rated device when going to a higher
current rated one, to get about the voltage I wanted under load. But
the problem gets less bad with higher current rated devices, as their
regulation gets inherently better, compared to very low current rated
ones.
 
J

jriegle

Jan 1, 1970
0
Mark, You are one who is wrong. I have numerous "wall warts" and all put out
considerable higher voltage at no load than at rated load. For example all
the 12 volt ones put out 16-19 volts no load and will be higher than 12
volts with lower than rated current.

The exception is the Radio Shack 500ma one with multi voltage selector. It
appears to have a regulator in it.

Also, some of the wall warts are switching supplies with good regulation.
These often have higher current capabilities >1 amp, but are small and
light.
John
 
J

John Larkin

Jan 1, 1970
0
No, his answer and premise are incorrect. 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. A well voltage
regulated supply can have as much available current as it
wants. 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.

I cast my vote with the OP. Many warts are not regulated and are
impedance-protected, so are very "soft". Using a high-current "12
volt" unregulated supply in place of a lower-current-rated "12 volt"
one may fry the load.

UL recognizes unfused, impedance-limited transformers as compliant
with safety standards.

John
 
B

Ben Miller

Jan 1, 1970
0
Mark said:
No, his answer and premise are incorrect. 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. A well voltage
regulated supply can have as much available current as it
wants. 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.

Mark

In order to be UL listed as a class 2 transformer, it needs to have limited
short-circuit current. The impedance limiting design accomplishes this
through series reactance & or resistance. In order to deliver the rated
current at rated voltage, the output voltage must be higher under open
circuit condition. There is one anticipated operating condition, at the full
load current rating. The voltage at other loads will be higher or lower.

These devices are operating exactly as designed. How do you know that UL has
not tested the devices that carry the listing mark?

Ben Miller
 
D

DarkMatter

Jan 1, 1970
0
These devices are operating exactly as designed. How do you know that UL has
not tested the devices that carry the listing mark?


If they carry it, they were tested. Period. Almost 100% assured.

Some makers perform in house certification, and are so capable.

UL is big on busting false monikers.
 
J

John Larkin

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
Over 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. Hence we rejected them.
However 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. I suspect many companies never qualify products
they bundle. Last 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?

UL cares only about safety, not performance.

John
 
J

John Larkin

Jan 1, 1970
0
But UL still requires that the device meet its rating.

Which UL spec are you referring to? I've read and complied with lots
of them, and I've never seen such. For transformer-equipped gadgets,
UL tests ensure that it won't get too hot no matter how improbably the
transformer is loaded.

The only time UL cares about performance is in the specs for
life-safety systems.

John
 
M

meirman

Jan 1, 1970
0
A 12V AC adapter often puts out 16 to 20V under no load
Where R in the formula above is the internal resistance of the
adaptor. It was a transformer winding and maybe a diode, or other
parts in the output, so of course those parts have resistance, which
doesn't show when there is no output current.

In sci.electronics.basics on Fri, 18 Jul 2003 15:01:05 GMT "Ben
Miller" <[email protected]> posted:

Even if what Ben said below had never been said, or in other cases
where there were no equivalent answer to Ben's, let me add this:

So those must be the products that AC/DC is talking about. Let's talk
about them. Why is it necessary to say his premise is wrong**, when
there are such adaptors, when he said "although the small transformer
in AC adapters have a horrible regulation." So he's talking about
small tranformers with horrible regulation, even if better adaptors
exist. Lots of products are not worth buying a better adaptor for if
there is another way to keep them from burning out, or if it turns out
they won't burn out anyhow.

**As if his premise is totally wrong. His premise is partially right,
just as your premise that he is wrong is only partially right.

But in fact, loads of cheap appliances work pretty well with cheap
adaptors. When I get a device that doesn't have a spec written on it.
I start with a universal adaptor and work my way up in voltage until
it works well. Then often I look through my box of scrap adaptors,
almost all of them cheap, and if I find one that matches, I use that.
Some things I give away after that. I'm not going to go out and buy a
good adaptor for a cheap appliance that I'm going to give away, or use
once every 6 months.
Not true. Dudes observation is correct. These adapters use "impedance
limiting" to meet the UL class 2 requirements for energy limited
transformers, as opposed to fuse links or other more costly methods. They
will produce approximately rated voltage at rated current, but open circuit
voltage will be higher.

Ben Miller


Meirman

If emailing, please let me know whether
or not you are posting the same letter.

Change domain to erols.com, if necessary.
 
D

DarkMatter

Jan 1, 1970
0
The only time UL cares about performance is in the specs for
life-safety systems.


This is true. We make a product for GE Medical that we had to send
an Engineer up to UL in order to train them on the equipment. They
are very stringent on manufacturing conformance, and that to the
original approved spec. No changes without a re-cert.
 
D

DarkMatter

Jan 1, 1970
0
When you get an adapter, unless you bought it yourself from
the real manufacturer. You have no idea if the information on the
unit is valid.

We sell several, and we have them made in china, by a standard
contract manufacturer. The best way is to simply research the
manufacturer of an item you may wish to purchase. Chances are, if you
cannot even find information on the maker of the product online, it's
a hokey product. Current limited, overdesigned products work well,
because they are hard to "blow up". Don't find many out there,
though. Some fudge on ripple specs too. Watch out particularly if
you re powering devices meant for communicating with computers, or
that have microcontrollers in them. A good, clean power supply is
what proper circuit operation is all about. Low ripple figures at
rated loads are what good, clean power supplies are all about. (Well,
it actually does get much deeper than that... )
By the way... have I said **** you to you lately?
 
D

DarkMatter

Jan 1, 1970
0
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.


That is what we refer to as an unregulated, proportional supply.
 
D

DarkMatter

Jan 1, 1970
0
Over 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.

There are UL LISTED devices, and UL approved devices. There is a
difference.
Hence we rejected them.

I'm not sure you knew how to decode them. From the above statement.
However 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.

You would be surprised at what level of quality products are made
there, apparently.
I suspect many companies never qualify products
they bundle.

Now, you are talking about system integration. Many makers DO make
sure their vended adapters are conformal. It would be like shooting
one's self in the head, let alone the foot. I suppose it does happen,
though. Just not in my company.
Last 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?

No. But nor do I believe that the adapter only put out less than one
tenth its rating. Are you sure that it wasn't 750 mA out of 1000?
That is out of spec as well, yet far more believable.
 
D

DarkMatter

Jan 1, 1970
0
If the design of the adapter
requires a load to work correctly, its a poorly designed
product.

For LV power supplies, exactly.

There are HV supplies that are specifically "proportional" type
supplies and are designed to be so.

Wall warts, and inline adapters should be regulated to a specific
loading at a specific ripple figure at a given design voltage. I know
of none that are of proportional type. AC-AC adapters are mere
transformers in a box, and will of course, be limited by the
capabilities of the transformer itself.
 
G

Gym Bob

Jan 1, 1970
0
I guess it depends on your definition. Some call all units "AC adapters".

Ever think a simple question would get so hard to get an answer?....LOL
 
G

Gym Bob

Jan 1, 1970
0
Have to agree with the DC output theory but he really didn't specify in his
post.

It depends on the specific adapter and application and cannot be predicted
here.
 
E

Eric Palmer

Jan 1, 1970
0
I have 3 external modems all three are 12 VAC and will not work with a DC
supply. Also a card reader for my mother-in-laws sewing machine is also 12
VAC. We have lost one and with a 1.5 Amp rating I have not found a
replacement as most power supplies are DC. I could easy make one up but
since the internals are 56K and externals 33K I no longer use them except to
send and receive faxes as they have ring ID built in where internals do not.

All best Eric GW7MGW Ex VR2ZEP and VP8BKM looking forward to returning to
use HF again.
 
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