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Thread: What does that “500 mA MAX” marking by the LNB input on my receiver mean?

  1. #1
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    Default What does that “500 mA MAX” marking by the LNB input on my receiver mean?

    It means that the receiver’s power supply can supply that amount of DC current for an extended period of time at 14 to 18 volts DC, with out any undue damage to the receiver’s power supply circuit that feeds the LNB.

    What is this?

    To fully understand this, we need to go into a bit of electrical theories, mA, what is a mA?

    A mA is a unit of measurement for electrical current, 1 mA is 1/1000th of an amp, what is an amp?

    An amp is the term for the electrical force that drives anything attached to the circuit, be it a light bulb, motor, electric blanket or what we use an LNB.

    So 500 mA is ˝ of an amp, more information on an amp can be found here.
    Code:
     http://en.wikipedia.org/wiki/Ampere

    The normal FTA receiver will supply 500 mA at ether 14 or 18 volts DC, this will run most of the LNB’s and switches we use in this hobby.

    Now for some more on this 500 mA.

    The average single LNB in use today consumes anywhere around 100 to 250 mA of DC current, this is at 14 volts DC, most FTA receivers can handle this without any problems.

    Now we add a second LNB and a DiSEqC or 22 kHz switch to go between them, adding the second LNB and the switch will add to the current draw on the receiver.

    A normal DiSEqC switch will consume anywhere from 10 to 40 mA, depending on the type and manufacture.

    Lets add some of this up, if we have 2 LNB’s of the same type that use 150 mA, and a switch that uses 25 mA, we have a total of 325 mA, this is well within the receivers specifications for the LNB input, so we shouldn’t have any problems, right?

    Well close, we haven’t figured in the coaxes voltage loss. (more?)

    This is something that a lot of people miss out on; the coax has voltage loss, why?

    It’s because the coax center conductor and shield have DC resistance to them, the center conductor depending on its type will have anywhere from .64 to 1.2 ohms per 100 feet, (not much but wait) the shield will have anywhere from .29 to 3 ohms per 100 feet.

    The differences are due to the makeup of the center conductor and shield, the coax comes in several types, one type is the copper plated center conductor with an aluminum shield, (cheap stuff, high resistance) this type has more resistance then the pure copper core (low resistance) with copper/aluminum shielding. (good stuff)

    Let’s do some math; if you’re using the cheap stuff and you have 100 feet of it, you have a total DC resistance of 4.2 ohms.
    (Center and shield, there and back, yes the current has to come back some how, it’s a two way street)

    If we use ohms law, we take the current (I) .325, times the resistance (R) 4.2 ohms, equals 1.365 volts, (I x R = 1.365) this is the DC voltage drop at the LNB, if the LNB wont work below 13 volts you may have a problem.

    14 volts minus 1.365 equals 12.635 volts DC, 18 volts minus 1.365 equals 16.635 volts DC.

    This could explain why some people are experiencing LNB switching problems. There could be too high a DC current draw (the DC current will stay the same in an electrical circuit*) in their system, the higher the DC current, (more LNB’s or switches) the more DC voltage drop, the more DC resistance, the more the DC voltage drop.

    (*Except when using a motor to move the dish)

    Using good high quality coax can fix a ton of problems; the good stuff is the pure copper center conductor with quad shielding, you can check the type of center conductor in your system by using a small magnet, if it sticks to the center conductor, you have the cheap stuff.

    And be sure the coax is rated to 2.5 or 3 GHz. (more on that later)

    Ran out of coffee, good for you guys....
    Last edited by Terryl; 03-02-2012 at 12:18 AM. Reason: Fixed a brain foo foo
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    Default

    We can shorten this up, take a car light bulb, lets say it takes one third of an amp at 14 volts for full brightness. (330 mA at 14 volts)

    If you use thick wire it will be nice and bright, if you use thin wire it will be dimmer, and the thin wire may heat up. (Thick wire equals good coax, thin wire equals cheap stuff)

    And the more bulbs you add or longer wire, the bigger the voltage drop will be. (more LNB's, more switches, longer coax)


    All this adds up, add too much and something has to give, it may be the poor power supply.........Hummmmm replacing caps in a power supply, sounds familiar doesn't it?
    ♫♫♫ I’m a lumberjack and I’m OK ♫♫♫ I drink all night and sleep all day. ♫♫♫



    Coming soon to a crop circle near you.....

    There is a 66 and third % chance that I'm on the right planet...

    "I'm happier then a Jackalope in a balloon factory"

    "First rule of testing satellites"
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    "Second rule of testing satellites"
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  3. #3
    fn59 Guest

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    Left out a zero in the fifth line, should be 1/1000th.

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    Quote Originally Posted by fn59 View Post
    Left out a zero in the fifth line, should be 1/1000th.
    '
    Ack.....Old eye bones and too much coffee, thanks for the heads up.
    ♫♫♫ I’m a lumberjack and I’m OK ♫♫♫ I drink all night and sleep all day. ♫♫♫



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    "Can you HIDE it"?

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    Default

    Quote Originally Posted by fn59 View Post
    Left out a zero in the fifth line, should be 1/1000th.
    Not bad for an old fart only 1 typo in a long technical explination..
    Thanks terryl

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