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Thread: voltage difference projector and computer (PCI QM card)

  1. #1
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    Default voltage difference projector and computer (PCI QM card)

    I noticed that when I connected my newly built projector to the PCI QM that there was quite a bit of voltage difference between the two unlike when I connect to an USB DAC.

    The voltage could simply be felt on the skin as I touched the ilda cable and the back of my compu case with one hand.
    It doesn't seem right but didn't affect scanning too much. I went over Bills guide and need to pay better attention to the central grounding but that will not solve the voltage difference.

    Any thoughts on this?

  2. #2
    mixedgas's Avatar
    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Bad PC switcher input network. Allows the 300V hf from inside the PSU to leak back. Also, some PSus have a special ceramic cap from line to neutral as "protection", and if neutral/ground is miswired/partially open, the cap passes current.
    AKA X-cap.

    http://www.okaya.com/FAQ1.html

    Leakage some place. Often hard to find.

    Steve
    Last edited by mixedgas; 11-12-2010 at 17:01.

  3. #3
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    Is your computer properly earthed, to start with?
    Earthing your pc through the central ground in your projector, generally isn't a good idea.

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    Quote Originally Posted by Zoof View Post
    I noticed that when I connected my newly built projector to the PCI QM that there was quite a bit of voltage difference between the two unlike when I connect to an USB DAC.

    The voltage could simply be felt on the skin as I touched the ilda cable and the back of my compu case with one hand.
    It doesn't seem right but didn't affect scanning too much. I went over Bills guide and need to pay better attention to the central grounding but that will not solve the voltage difference.

    Any thoughts on this?
    I noticed that with my projector too, also with lot of other stuff in my Like dvd player,sat reciever ect ect, also I noticed small sparks somtimes when you plug stuff in when said peice of equipment is on, always confused me but never been a problem.
    anyone know how you should earth a laser projector properly, when the case is ground to all the different power suppys? ?
    I have never done it as I have always been worried about some other peice of equipment on same grid leaking to earth,ie been to various places before and get a small shock from the earth line

  5. #5
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    This is written from a EU (Actually a UK) perspective and some American practise differs, so be careful.

    Most hifi, many televisions and some laptops are what is termed class II or double insulated appliances which is to say that they do not have protective earth connection (And get away with it by taking special measures to ensure safety) and because of capacitive leakage will tend to have the chassis sit at half the line voltage (at a very high impedance). This is not usually a problem but if you connect a lot of such gear together then plug in a lead to a class I (earthed (Grounded)) device you can sometimes get a small spark due to the sum of all the leakage currents. For laptops the easy answer is to get a class I power supply brick.

    Desktop PCs, laser projectors and such SHOULD in general be class I devices with the case ground (including the case ground of all power supplies) tied to the safety ground wire in the mains supply cable.

    Now there is something of a tendency to do weird earthing in a laser projector in an attempt to reduce earth loop problems with single ended colour signals, but this should NEVER extend to floating anything that mains power could get to in the event of a fault.
    Interestingly with switchmode supplies you can sometimes get a little leakage across the transformer so one leg of the secondary usually benefits from being grounded (possibly via an impedance if appropriate).

    IMHO a better answer is to bond everything to protective ground and then use an input stage with a very high common mode impedance to get most of the benefit of running a properly balanced system without needing the full monty at the drive end of the cable.

    If something is giving a tingle, stop and investigate, either a safety earth is off somewhere (possibly in the wall, a simple 3 light socket tester is a good thing), or there is a fault, but something possibly dangerous is wrong.

    I would commend the use of plug in RCD (GFCI) devices on all feeds to laser show equipment.

    Regards, Dan.

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    The outlets in my `study/io' are not grounded so this is the main cause.
    However, having the projector and the computer share their AC ground solves the described issue, even if they are not connected to a grounded outlet.

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