Reducing noise on AC circuit with capacitor?

Discussion in 'The Projects Forum' started by JiffJaff, Jan 26, 2011.

  1. JiffJaff

    Thread Starter New Member

    Sep 2, 2010
    5
    0
    Hi, I have a audio/video recorder that is desinged to operate off of battery. It takes a 5v input and is rated at 300ma. When the circuit is recording on battery power - the audio is very clear. When is attached to a mains source there is a low pitch rumble.

    Am I correct to believe that this is due to the fluctuations in the AC source? I have read that these problems can be eliminated by placing capacitors in the circuit which will serve to level out the ac voltage.

    Because I am an electronics begginer, I have cobbled my current setup together from three different circuits. I have a 12v adaptor that takes power from the mains, this then attaches to a car usb adaptor which takes the 12v down to 5. I then connect this 5v to my main circuit.

    Could anyone please advise where and how I should connect my capacitors?

    Would I be correct in thinking that I should cut the positive 12v wire and join it again with a capacitor? Maybe do the same for the positive wire between the 12v and 5v - and even again between the 5 and the actual circuit? How many do you think I would need to significantly reduce the nosie I'm experiencing?

    Finally, what type of capacitors should I buy. If you can provide an exact model or link that would really help.

    Many thanks for any help!
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    Don't cut any wires. The caps you need - assuming that really will be a cure - are called electrolytic, and they have a polarity clearly marked on them. You'll need one rated for a voltage of at least ~50% over the voltage you're smoothing. For capacity, more is better, but if the concept is going to work, you should see an improvement using 500-1000 microfarads, mF on the label of the cap. It is placed across (in parallel) to the DC supply and load, like a battery.

    Any old electronics you have lying around might have such a cap in it, if you want to avoid buying one. Just look for cylindrical components, and the distinctive polarity marking and capacity label.

    Trying this strategy out before making a final hookup can be as simple as touching the cap's leads to the DC current you want to smooth. Just be careful to not short anything.

    EDIT Thinking about this a bit more, I think your 5v USB supply is probably a little too small for the camera load. It shouldn't really be passing any AC noise unless it is under too much load. That alone might not be a problem except that you've got noise also coming through the 12v adapter, which I assume is just an AC to DC wall wart? If its output is AC, then that's your problem! But if it's putting out DC, that's the place to apply your capacitor to catch as much noise as possible before it even gets to the 5v adapter. Again, be sure to use a cap rated for ~20v or more.
     
    Last edited: Jan 26, 2011
  3. JiffJaff

    Thread Starter New Member

    Sep 2, 2010
    5
    0
    Thanks for the info on the capacitor placement, I'll be sure to do it as described. I think I'll probably need to buy one as annoyingly I only recently chucked out a load of old electronic stuff.

    Would this be suitable? A 10uf (1000mg right?) 25v electrolytic capacitor?
    http://www.maplin.co.uk/axial-electrolytic-capacitors-7011?ordercode=AT02C

    The usb supply I am using was shipped with the camera - the camera is designed for use as a driving recorder - so I would have hoped they would have shipped it with a suitable adaptor... but who knows?? The internal battery that it uses when not connected to another source is 3.7v if that makes any difference to your assesment?

    Yes, the 12v supply is a wall wart and outputs DC. It doesn't state that it is a 'switching adaptor' but would I be right in thinking that such adaptors can also generate noise?

    Perhaps the 12v - 5v circuit is also a switching adaptor and could also be a source of the noise?

    Thanks for all your help, it's really appreciated :)
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,140
    3,054
    Chucked it out? You can chuck out that stuff? Never occurred to me. :D

    I'd go bigger, maybe 1000µF or more if the cost isn't too bad. If 10 doesn't work, you won't know if it was too small or the wrong strategy.
    That's good, and pretty well rules it out it if was meant to run from the noisy DC of a car.
    Actually I think the switching supplies switch so quickly that you'd never hear it. It's more likely the good ole heavy transformer kind. The rectified signal from that will be very bumpy at 60Hz, and the 12-5v adapter wasn't designed to filter that out, since it's not a huge component of noise in a car system. So filtering (or replacing) your 12v supply is the target.

    Ummm... You know that a modern 5v switching power supply can be found for almost free? They're widely used to charge cell phones and the like, anything that plugs into USB. If you can find one with enough rated current (0.5 to 1A is not unusual), you could just lop the connector off and replace it with what you need for your camera. Or use the proper adapter cable.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    You should replace the defective capacitors with the equvalent values. If necessary, you can go higher in uF and volts.

    Are you wanting to repair the equipment?
     
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