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.
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.
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.
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
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.
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?
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?
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?
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.
That's not been my experience with UL, they expect the unit to perform to the ratings you intend to sell the unit at. You can tell them what that rating is. Then they will make sure the unit is safe. They go a lot father then heat. They test plug life, shorting and what ever the hell they feel the unit needs. And I have never found them to be very reasonable about what they do for testing. I have always made sure what I submitted to UL would pass.
However I have seen a lot of china made adapters that are UL marked and not even safe. I don't know what UL would do if they got one of those as a real submission. I have seen adapters from china with UL markings on what could not possibly be UL approved material. So I know they were never submitted.
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. I have seen units that had UL numbers for a component on the adapter itself as if that makes it UL listed as an adapter.
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" 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.
Meirman
If emailing, please let me know whether or not you are posting the same letter.
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.
On Sat, 19 Jul 2003 15:07:07 -0700, John Larkin Gave us:
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.
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 f*ck you to you lately?
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