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Help with project please

  1. May 31, 2010 #1

    I need help with a project.

    I have a 16v 55Farad capacitor that I am using to run a small 50 watt DC projector and 10 watt guitar amp. The audio is provided directly from the projector. Everything is DC. Both the projector and amp are connected directly to the capacitor.

    The problem with the setup is this: when I switch on the projector I get really bad interference, a sort of buzzing in the guitar amp. When I turn the projector off, the buzzing stops. I tried running the amp on a separate power supply in case the capacitor (or other components) was causing EMI; but it appears not. I’ve also tried a different source for the audio input – when the projector is off it’s clear and when the projector is turned on it starts buzzing.

    It’s possibly something to do with the voltage change once the projector starts drawing power? A friend suggested that I need some sort of voltage regulation between the capacitor and amp.

    Grateful for your help.

    Last edited: May 31, 2010
  2. jcsd
  3. May 31, 2010 #2


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    Science Advisor

    Does the projector have a DC motor driving a cooling fan in it?

    DC motors can easily generate interference due to sparking at the brushes.

    If so, you could try some smaller capacitors across the DC supply line to the projector.
    Maybe 10 uF tantalum.

    Just curious, have you really got a 55 Farad capacitor?
    How are you charging it?
  4. May 31, 2010 #3
    Yes there is a cooling fan.

    and yes I really do have a 55 farad cap. Charging is with a bicycle generator, although this is nothing to do with the interference as I've tried it disconnected.

    To give you an idea:


  5. Jun 1, 2010 #4
    I wonder if it's an earth rail problem, which might be cured by choosing a star earth point on the (presumabably) negative terminal of the capacitor and running separate earths to the amp and projector.
  6. Jun 1, 2010 #5


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    Science Advisor

    Since everything is running on DC, the fan motor looks like a possible source of noise.

    If possible, briefly remove the power supply to this fan and see if the noise vanishes.
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