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Thread: question about amps

  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
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    DIY Junior Member mcf57's Avatar
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    OK, this mentioned adapter didn't work. Upon further examination of the Westell gateway, I just noticed at its plug area that it states "DC 12V". The 12V/1250milliamp adapter says "AC" on it so based on some other info from other forums, I think that is why it didn't work.

    I actually also have a spare "DC 12V/2000mA" adapter with a plug that fits it as well. So I briefly tried it and it seemed to initially work perfectly. All needed lights came on the Westell gateway & I was able to access it from a networked PC. I only plugged it in for a few seconds since I wasn't totally sure, but I assume the same rule applies with a DC adapter; it will simply draw the needed 1000mA from the adapter even though its rated up to 2000mA. Or are there other factors to consider with a DC 12V adapter?
    Last edited by mcf57; 01-16-2013 at 05:19 PM.

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    Plumber jimbo's Avatar
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    There is a finite relationship between volt, amps, and ohms. It is actually called Ohm's law. But you have to look at the starting point. In the current discussion, the SOURCE of the power is the VOLTS output from the adapter. ( AC or DC matters, as you discovered). Per Ohms law, with a certain applied voltage, the resistance of the LOAD will determine how much current flows. Ohms law says simply that I = E/R meaning amps will equal volts divided by ohms.

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    DIYer, not in the trades LLigetfa's Avatar
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    Use a car battery as an example. It is capable of hundreds of amps yet you can plug in a cellphone charger that uses milliamps.

    As long as it is a decent quality switching power supply putting out a true 12V DC, it will work. Some old power supplies could "float" to a much higher voltage if no load or low load was applied.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    It's critical to match the TYPE of voltage (ac vs dc) AND, with dc, the polarity to the device you're trying to power. Applying a reverse dc voltage to something often will burn it up. Not always, sometimes they have internal protection for that, but you can't depend on it. Applying ac where d/c is expected or vice-versa might destroy things, it might not, but it's not good for it. 12vac will have peak to peak voltages considerably higher than a 12vdc supply.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member mcf57's Avatar
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    Quote Originally Posted by jadnashua View Post
    It's critical to match the TYPE of voltage (ac vs dc) AND, with dc, the polarity to the device you're trying to power. Applying a reverse dc voltage to something often will burn it up.
    How can I then possibly check the polarity of this equipment to make sure they match?

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    Nuclear Engineer nukeman's Avatar
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    Look on both power adapters. There should be a symbol that will have a solid circle surrounded by a ring. This symbolizes the connector (center part vs. the outer part). Compare the symbols to see if they match or take a pic of both of them and we can tell you.

    The fact that it works when plugged in tells me that the polarity is okay. In circuits with polarity protection, they will block the flow of current if it is backwards, so the device won't work until the polarity is corrected. If it had no polarity protection and you reversed it, you would probably damage it and it won't work at all then and maybe not in the future either (you may get smoke and/or a smell from things burning up).

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    Plumber jimbo's Avatar
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    Quote Originally Posted by nukeman View Post
    (you may get smoke and/or a smell from things burning up).
    One of the first things we learned in Navy Electronics school: all electrical and electronic devices have a built-in sealed smoke reservoir. As long as you don't let any smoke leak out, things are fine. Once the smoke gets out, that is the end of the line!!!!

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    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by DonL View Post
    Chances are it is AC for a reason, polarity does not mater on a AC adapter.

    Many modems use AC adapters because they can double or triple the voltage for the Modem chips to work.

    It is easy to step-up voltage using a few caps rather than using a switching supply. The supply output is much cleaner. 24-30 Volts may be required by some electronics in a modem.


    In short, You need a 12 V AC out adapter, and polarity does not mater.

    A adapter with DC out will not work.
    Ya, Ya. As Jim D said, you gotta know if that modem requires AC or DC power input, as well as the polarity, if it is DC.

    I have had lots of old old external dial up modems in the past, but i cannot remember anymore if they were all class 2 AC low voltage inputs or DC. My cable modem is at home, so I cannot see IT at the moment
    I am definitely not a pro plumber, but I am a pro crastinator

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    To get dc to operate most electronics takes a conversion from ac to dc. The most common first step is to use a transformer, which drops the ac voltage down. On some electronics, to keep them smaller, they use an external supply that is just a transformer to drop the voltage down and then the rest is done inside the device. Then, to change the ac current into dc, it gets run through a rectifier, which now makes pulses, all going the same way, referenced to zero (before that, the ac was going above and below zero equal amounts). To get a smooth voltage, it is then run through a capacitor and often a voltage regulator to get it nice and smooth and exact. This is a quite simplified explanation, and many of the newer power supplies are what is called switching, but except for an extra step, they essentially do the same thing. They can be lighter and smaller, and with today's techniques, and copper costing what it does, often cheaper, but not everyone uses them.
    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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