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Thread: Will this Power Supply work?

  1. #1

    Default Will this Power Supply work?

    Hi all,

    So I'm running a 25 watt TX out mobile radio at the standard 12 volts. Since it has a cigarette plug female adapter built in I was think about getting one of these:

    https://www.walmart.ca/en/ip/celestr.../6000192309675

    I'm just curious as to if it will work for me to run emergency battery power out in the field. 25W/12V is a 2.08A draw which should work. Any feedback would be appreciated thanks!


    73,
    VA3BVH

  2. #2
    Super Moderator pmh's Avatar
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    Default

    Alas, you cannot simply divide the output wattage with the input voltage.

    I have a voltage and current monitor in my mobile set up, which reads 2.43A at 14.0V when my radio is transmitting and set to 10W. A quick interpolation calculation shows you can expect around 6A at 25W.

    Given the device you show is rated at 7AH, then you could expect just over an hour continues operation, which should be ok as an emergency backup. However, the device doesn’t advise on maximum peak current flow.

    Kind regards,



    Phil

  3. #3

    Default

    one thing to remember is that no radio (to my knowledge) is 100% efficient. No radio (to my knowledge) puts out all the power it draws.
    The radio uses some energy in just working...your rated output will never be the figure you will use to determine your power requirement.
    Your manual should tell you the minimum rated power supply to use.

  4. #4

    Join Date
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    Elgin, Illinois
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    Default

    Hello Bryan,

    One of my former job responsibilities while working in a Motorola R & D Lab required me to build a Portable 12 V DC Battery Pack. These Battery Packs were made up of three +12 V DC at 20 Ampere Batteries and oh man they were heavy something like 60 pounds! These Battery Packs were used in conjunction with our 25 W FM Portable Repeaters built into an attaché case that weighed 28 pounds by itself. When I would test these Repeaters, I would use the internal Power Supply and I could monitor the DC Current being drawn. As I remember the 25 W Repeaters drew 8~9 Amperes on Transmit at +13.6 V DC which was within the Specifications for the DC requirements. So when taking the +13.6 V DC times the 8 Ampere DC Current this would equal 108.8 W DC Input Power. When switched to the External +12 V DC at 60 Ampere Battery Pack, the 25 W FM Repeater would pull +12 V DC at 9 Ampere which equals 108 W DC Input. I would have to agree with Phil M0KPH that the Wal-Mart Portable Battery Pack is hardly worth the investment. Those individual +12 V DC at 20 Ampere Batteries were made by Gel Cell and could be charged by using a high current Automobile Battery Charger.

    Dan
    WA9WVX
    Last edited by WA9WVX; Mon 11th Dec 2017 at 20:50.

  5. #5

    Default

    Quote Originally Posted by hoochrulez55 View Post
    Hi all,

    So I'm running a 25 watt TX out mobile radio at the standard 12 volts. Since it has a cigarette plug female adapter built in I was think about getting one of these:

    https://www.walmart.ca/en/ip/celestr.../6000192309675

    I'm just curious as to if it will work for me to run emergency battery power out in the field. 25W/12V is a 2.08A draw which should work. Any feedback would be appreciated thanks!


    73,
    VA3BVH
    The current draw on transmit should be reflected in the specifications (in the manual) for the particular radio. The 25W RF output is something entirely different from the amount of power consumed during transmit. If the radio has variable transmit power settings, then the specifications should reflect the current draw at each setting.

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