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Thread: question about amps

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  1. #1
    DIY Junior Member mcf57's Avatar
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    Question question about amps

    I got an old Westell DSL gateway (computer) modem from a friend, but it didn't have the power adapter. I looked on **** for replacement power adapters and it looks like it requires one that has an output rating of 12V and 1amp/1000mA.

    Now, I also found another power adapter laying around. Its actually a Westell branded power adapter and was previously for some other similar piece of equipment. Its plug fits this Westell gateway, but it has a power output rating of 12V and 1.25amps/1250mA.

    Logically, I'm thinking it can't be used since it has too many milliamps. Yea, it might initially work, but will ultimately burn up the equipment at some point. Is this pretty much the case or will a 250 milliamp difference not really matter?

    Also, I found another adapter laying around that while it fits this Westell gateway as well, it has a lower output rating. I think its basically 7.5V and 800milliamps. Will this rating damage as well?

  2. #2
    Nuclear Engineer nukeman's Avatar
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    The main thing is that the voltage and polarity is correct. The rated amps is secondary. The rated value is more of a capacity than a continuous output. You want the rating to be at least as high as the original adapter and could be higher. The biggest problem of going too high in the amp rating is these power adapters often don't have good voltage regulation (often don't really have any regulation). For instance, on some of these power adapters, you may have one that says 12v, but it might show say 16v or 19v when there is no load on it. If you went crazy with the rating (say 12v and 10 amp), the voltage may be too high since the modem wouldn't be enough of a load for the power adapter to correctly load it.

    The modem itself likely has some regulation as well as protection for overvoltage and possibly for reverse polarity, but it is hard to know for sure.

    In short, the 12v, 1.25A adapter would be fine as long as it has the right plug and right polarity. I wouldn't use the 7.5v one. I suspect that the modem uses mostly 5v internally (the voltage is regulated from 12v to 5v), but there may be portions of it that needs say 12v. You are best off keeping the voltage correct.

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    DIY Junior Member mcf57's Avatar
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    Sounds good. I'll try to use the 12V/1250 milliamp power adapter for now. I just remembered that I also found an extra adapter that was rated at 12V and 2amps/2000milliamps. Just out of curiosity, would this one work too since it has the same 12V rating? Or is the 2000milliamps way too high even for this situation?
    Last edited by mcf57; 01-16-2013 at 06:35 AM.

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    Moderator & Master Plumber hj's Avatar
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    The amps rating is the MAXIMUM it will produce. As long as your requirement is equal to, or less than that, it will work properly. It does NOT "push" its amperage rating through your equipment, your equipment draws as many amps as it needs FROM it. It is like the old days in computers. Cheap computers had power supplies with amperage equal to the connected load, and any additional equipment would require it to be replaced. Better units had power supplies which had "excess amperage" available to accomodate added hard drives and other accessories which might be added.
    Last edited by hj; 01-16-2013 at 07:18 AM.
    Licensed residential and commercial plumber

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    DIY Junior Member mcf57's Avatar
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    Cool. Good to know and thanks for the info. I'll probably just use this 12V/1.25amp power adapter for now.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, the battery on your car may be capable of over 100A, but that doesn't mean it won't light a dashboard small lamp just fine...it's a DC power supply as well. Your house's 15A branch circuit will light a 5W bulb as easily as a 250W one. Where it would have problems is lighting a 2,400 W bulb (20A), as that would be more than the circuit was designed to supply.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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