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A DC Voltage/amperage question

  1. Oct 7, 2013 #1
    Ok I need help here.....

    I am attempting to light LEDS off the phantom power of an amplifier/mixer.

    traditionally the average mixer/amp used for DJ shows has a "Phantom Power" option. When activated it sends about 48v to each microphone....some mics need it...some don't....
    I intend on using that 48v to power some LEDS.....I have purchased some 12 volt LED clusters ( here is the link to the item .....http://www.oznium.com/super-bright-12v-led-module......) and was told to wire them in series to get all for to light using the 48v system......thus not needing resistors or any special circuits and for the most part it works..they all light...HOWEVER they all light dimmer then when I try to light each one from a 12v battery.....Thus leading me to believe it is not a voltage problem but an amperage issue...I know nothing about electronics/ohms/amps/volts/or resistance
    I do not have any other source to use to power these....I must get them to light up brighter with this being the only power source I can use....
    I do not have specific details on how much amperage I am getting from the mixer ....I will hopefully get my hands on a multimeter and find that out as soon as I can figure out how to use one...
    In the mean time ...is there a way to get these to light brighter....I have come so far with this project and I can't be defeated now.......
     
  2. jcsd
  3. Oct 7, 2013 #2

    meBigGuy

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    The phantom supply generally has very little current capability (it has a series resistance). It is drooping significantly under the load. I doubt it can even drive 1 or two of those modules, but I'm not sure. What is the model/part number of the phantom power source.
     
  4. Oct 8, 2013 #3
    It's a Mackie ProFX12 mixer
     
  5. Oct 8, 2013 #4
    I just tested the voltage......it IS putting out 48 volts

    I do not know how to check the amperage......
     
    Last edited: Oct 8, 2013
  6. Oct 8, 2013 #5

    vk6kro

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    You could put a 1000 ohm resistor from the microphone input to ground.
    Then measure the voltage across the resistor.

    This will let you calculate the value of the internal series resistor since the two resistors will act as a voltage divider.

    For example, if you got 3 volts across the 1000 ohm resistor, there must be 3 mA flowing in it and in the internal resistor.
    The internal resistor has 3 mA flowing in it and 45 volts across it (48 volts - 3 volts = 45 volts) so the resistor must be 15000 ohms.

    So you could calculate the maximum current available. In this case it would be 48 volts / 15000 ohms or 3.2 mA.
     
  7. Oct 8, 2013 #6

    meBigGuy

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    You are saying that it is putting out 48 volts when you have all four led modules connected?

    If that is the case then there may be a problem with the design of the module that makes series connections a problem. I posted a question about that at their web page.

    As for amperage, multimeters have ammeter mode (thus multimeter, not voltmeter). You put the meter into ammeter mode (highest scale please) and put it in series with the modules. Switch the scale if you need to. In series means +40 to meter +, meter - to module +, module - to ground.
     
  8. Oct 9, 2013 #7

    vk6kro

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    Unless you know there IS a current limiting resistor and roughly what it's resistance is, it is better to put a safe load on the output and observe the output voltage.

    Otherwise you can risk damaging the equipment that you are trying to test if you just short out the output and try to measure the short circuit current.
     
  9. Oct 9, 2013 #8

    meBigGuy

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    No argument that measuring the impedance if the power supply is a good thing to do. A 1K resistor will dissipate 2.3 watts across 48V, so don't burn your finger.
     
  10. Oct 9, 2013 #9

    vk6kro

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    Good point.

    Probably a 1 watt resistor would be good enough for a quick test.
    5 watt wire wound resistors are expensive and sometimes hard to find.

    Just read the meter through the smoke. :)
     
  11. Oct 9, 2013 #10
    I wanna thank you guys for your assistance and at the same time I want to apologize for being so stupid....
    With absolutely no knowledge of electrical engineering and not being able to tell the difference between 1k and 1 watt I don't want you to think you're wasting your time helping....it is appreciated but I have no clue as to what you are telling me to do.......
    I don't know i I should just throw in the towel or take a step back and maybe find a different light source.....that requires less amps.......

    the manufacturer replied with a suggestion of using another power source and that's not possible ........I've been modifying the item I am putting this in for a month now to make these modules fit so maybe I'm just stressed at this setback......don't disappear but I need to think about this.....stand by....lol
     
  12. Oct 9, 2013 #11

    meBigGuy

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    Look at the FAQ on the module page. The top entry is the answer to my question as to whether you can connect 4 in series to 48 volts. They answered yes.

    1. Did you measure 48V on the supply while the modules were connected to it. It is important that the modules be connected and lit when you measure. Then also measure the voltage at each module so we can see if the drops are matched.

    2. Did you measure the 12V you were applying to the module to test it? Some 12V power sources actually put out 13.8 volts. That extra little bit can make a big difference.
     
  13. Oct 9, 2013 #12

    AlephZero

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    The IEC standard for phantom power says the maximum current required by a mic is 10 milliamps per microphone. In fact many mics only need 1 or 2 mA to operate. It's unlikely that your mixer can output more than 10mA, and it may be designed to limit the max current, in case a microphone was faulty.

    The technical spec tab on your web page about the LEDs gives currents from 31 to 74 ma depending on the color, so it's not surprising you don't get the maximum brightness.

    Sorry, but the best way to fix this would be use a separate power supply for the lamps, e.g. a 12V "wall wart".
     
  14. Oct 10, 2013 #13

    meBigGuy

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    The Mackie ProFX12 mixer spec sheet http://www.mackie.com/products/profxseries/pdf/profx_ss.pdf [Broken] has a diagram on page 5 showing a series resistor to each mic line that has phantom power. That means you are probably out of luck using the mic inputs as a 48V supply (I assume that is what you are doing).

    You have not posted an answer about what the voltage does when connected to 4 lit modules.

    Is this a 1 shot custom mod? The 48V is available raw at the phantom power switch. Someone with some wiring skills could open the mixer, add a connector, and wire to that. You run a risk of burning out the supply (I don't know what max current what it is rated for). Or you could short one of the phantom power resistors. Again, there is some risk.

    Your bet bet is a wall wart and adapter as recommended on the module site.
     
    Last edited by a moderator: May 6, 2017
  15. Oct 10, 2013 #14
    yes I was trying to use the phantom power from the mic inputs to lite these clusters of LEDs.... I have 3 other mics that someone else built which use 8 individual 5 mm LEDS and work but I was interested in making mine brighter and a more even illumination. I chose these clusters for their brightness and the abilty to wire them without worrying about using resistors..... I guess I will just have to try to reverse engineer his design and go the individual LED way if there is no way to add anything that will increase the current. Thanks for all your research and I will be back when I get some different LEDs...I will need to know what resistors to use and how to wire them......your not off the hook yet...thanks again back to the drawing board



    This is the one with 8 leds
    [url]http://www.goldcoastkaraoke.com/mics/super55-side-lit.jpg[/url]

    This is what mine would have looked like
    http://www.goldcoastkaraoke.com/mics/image 3.jpg
     
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