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Thread: Running 10.5 volt modem on 12 volt system

  1. #1
    Join Date
    Nov 2007
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    Wisconsin's Northwoods
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    Default Running 10.5 volt modem on 12 volt system

    Hi,

    My ISP sent me a broad-band modem to upgrade my service. However, it includes an AC adapter (117AC to 10.5 volt DC/900ma). Since my home is a 12 volt stand alone operation I can't see using an inverter to run this little modem. But I do have a 12 volt power converter with following step-down settings:

    Input: nominal 12 volt
    Stated output: "9 volt (900ma)"
    (meter on it reads 10.3 volts)

    And:

    Stated output: "12 volts (3 amps)"
    (meter on it reads 13.6 volts)

    Question: Can I run the modem off the power converter at the 9 volt setting without damaging it? (again, a voltage meter on it reads 10.3 volts -- darn close to 10.5 volts). Or would I do better using the 12 volt setting? (meter on it reads 13.6 volts). Or is that too high voltage for it to handle? My system voltage gets up to 14.01 volts when the batteries are fully charged. Why they just didn't make this modem 12 volt is beyond me.

    Thanks!

  2. #2
    Join Date
    Jun 2003
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    pittsburgh, pa
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    Default Re: Running 10.5 volt modem on 12 volt system

    that is an odd voltage. i say try the 9v. it's the higher voltages that would be likely to blow things out and not usually the lower voltages that when they don't work mostly just sit there. out of curiosity, is the voltage the same on it with a load as without if it's possible for you to measure it? i ask this because wall warts usually aren't big on regulation, but some are now the better switching supplies.
    NIEL

  3. #3
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    Default Re: Running 10.5 volt modem on 12 volt system

    Quote Originally Posted by niel View Post
    that is an odd voltage. i say try the 9v. it's the higher voltages that would be likely to blow things out and not usually the lower voltages that when they don't work mostly just sit there. out of curiosity, is the voltage the same on it with a load as without if it's possible for you to measure it? i ask this because wall warts usually aren't big on regulation, but some are now the better switching supplies.
    Do you mean the voltage of the wall power supply unit that came with the modem or the modem itself?

    I'm not sure how to test them under load except that I do have a small digital voltmeter.

  4. #4

    Default Re: Running 10.5 volt modem on 12 volt system

    You have to be able to somehow connect the DVM to the modem's power supply while the unit is hooked up and running. That's a bit difficult of course, as the connection between the two is no doubt some type of sealed mini-jack. Can you measure the power supply's open circuit voltage first? It's bound to be higher than the 10.5. And yeah, that's an odd voltage all right!

    But yes; start low.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  5. #5
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    Default Re: Running 10.5 volt modem on 12 volt system

    Quote Originally Posted by Cariboocoot View Post
    You have to be able to somehow connect the DVM to the modem's power supply while the unit is hooked up and running. That's a bit difficult of course, as the connection between the two is no doubt some type of sealed mini-jack. Can you measure the power supply's open circuit voltage first? It's bound to be higher than the 10.5. And yeah, that's an odd voltage all right!

    But yes; start low.
    I'll check the power supply's open circuit voltage. That's easy enough.

  6. #6
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    Default Re: Running 10.5 volt modem on 12 volt system

    Quote Originally Posted by paulstamser View Post
    I'll check the power supply's open circuit voltage. That's easy enough.
    that'll be good as it will show the unloaded voltage. if it's high like 13-14v then it is either not regulated or regulated well and putting 12vdc on it with a few series diodes (i'm guessing greater than or equal to 3-4 diodes @ 1 amp or greater) would suffice to run it. each diode will drop the voltage by about .6v give or take and you would not want the voltage configuration you are working on to ever exceed that of your supply voltage as you measured it.
    NIEL

  7. #7
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    Default Re: Running 10.5 volt modem on 12 volt system

    Ya, I bet it's 9vdc running under load, or darn close to it.

    I agree they should make a standard for all external equipment being 12v or 5v or something standard anyway. It would be nice in a computer room if there were a standard and you could have one 100w power supply feeding a bus that all the peripherals in to. Just think of all the wall warts that could replace and wasted idling transformers... Anyway...
    XW6048, 4 KC 120's, 4 KC 130's and 4 Evergreen 200's totaling 1800w of PV, MX-60 charge controller, Trimetric meter and eight AGM 8A8D's or 490 amps at 48v. 4 tons of geothermal and 3 tons of air source.

  8. #8
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    Default Re: Running 10.5 volt modem on 12 volt system

    Quote Originally Posted by niel View Post
    that'll be good as it will show the unloaded voltage. if it's high like 13-14v then it is either not regulated or regulated well and putting 12vdc on it with a few series diodes (i'm guessing greater than or equal to 3-4 diodes @ 1 amp or greater) would suffice to run it. each diode will drop the voltage by about .6v give or take and you would not want the voltage configuration you are working on to ever exceed that of your supply voltage as you measured it.
    I tried to measure the wall wart power supply output but it's DOA. That is, no juice is coming out of it when plugged into my inverter. I know the inverter and voltmeter are good because two other wall wart power supplies give an output reading just fine (one 12 volt unit reads a whopping 19 volts).

    So I called and they are sending a replacement, whenever that arrives.

  9. #9
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    Default Re: Running 10.5 volt modem on 12 volt system

    This is how I deal with trying to see what the supply voltage actually is.
    Get 2 straight sewing pins, piece the power cable cord, about 2" apart, one pin on each side, turn unit on, and measure voltage at the pins ! Power off, and remove pins. The pinholes are far enough apart, and so tiny, only imersion in water will be a problem.

    I think the straight battery voltage, when charging, will be too high, and cook something eventually. You can get a 7412 regulator, and set up a trim adjustment to get your 10.5V or , use several 2 or 3 amp diodes in series, you get a bit of voltage drop for each diode, but it will still fluctuate, but it wont be 14.8V anymore.
    http://tinyurl.com/LMR-Solar
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  10. #10

    Default Re: Running 10.5 volt modem on 12 volt system

    I'd go to Ebay and search for; 10.5v car adapter

    and change the end to what I need if It needed changing...

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