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Hi all, interference?

Discussion in 'General Electronics Discussion' started by Chasingwires, Apr 4, 2014.

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  1. Chasingwires

    Chasingwires

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    Apr 4, 2014
    I'm new to electronics, currently going to school for electronic engineering tech and I do it relative to cars and hybrid systems at work.

    My question involves interference in cables in a vehicle. We switched to shielded cable and it barely made a dent in it and recently, I learned about rl (resistive inductive) filters in electronics theory class. You can block out low or high frequencies and only pass the other, I am guessing it is commonly used in radios. This may be a ridiculous question, but why can't that be included for cables or wires with bad interference? Or why can't rl filters be made to block out high and low frequencies at the same time? I also wondered if there were other methods to help this situation.

    I am very new to wiring and looking into experiments to learn and try things out. I appreciate this site and I hope this question isn't just nonsense.
     
  2. duke37

    duke37

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    Jan 9, 2011
    Welcome to the forum.

    Shielded cables work well, shielded cable is used on microphones and bigger coaxial cables are used for radio frequencies. The cables will output what is input.
    A car will produce lots of interference from the generator, ignition and a multitude of DC motors. The only way to reduce the interference is to suppress at source.

    A low pass filter will pass low frequencies.
    A high pass filter will pass high frequencies.
    A band pass filter will pass a band of frequencies between high and low. Radios use a lot of band pass filters to select the channels required.

    If the interference is on the wanted frequency, there is little you can do about it, for example, fluorescent lights can cause severe interference on radios and switch mode power supplies are dreadful.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    just a small correction .... and bigger cables are used for higher power.

    there are some very small diameter RF cables out there eg UT 141 (0.141 inch) and smaller ;)

    To the OP...
    you need to determine the type of interfering signal before you can determine what is needed to filter it.
    That is ... is it just noise which will possibly consist of a broad spectrum of RF ?
    or is it RF signals on specific frequencies ?
    Maybe a mixture of both ?

    you will have to figure that out before deciding on a cure

    Dave
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Screend cables only work well in the far field which is normally e-field which are produced by switching circuits in automotive systems and also many other bit of equipment. As duke said if its getting in on the input you have to filter it out.
    Adam
     
  5. Chasingwires

    Chasingwires

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    Apr 4, 2014
    I appreciate the responses, but I guess I don't know enough to understand the solution just yet. I was under the assumption that the interference was caused by the electromagnetic field alone. I can't really explain in great detail, but basically the shield cables are coming from the chassis battery to the rear end and going to a top charger. The positive hits a solenoid and two large inductors and they are charging a 48 volt hybrid system. So they run about 90 amps back and I am told that the interference is so bad shielded cables hardly put a dent in it. Basically, I'm trying to offer up a potential solution to help the engineers make our product better, even though I get no benefit from doing so haha
     
  6. duke37

    duke37

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    Jan 9, 2011
    What is a top charger?

    At 90A you are talking serious. Find out what device is causing the interference and fit inductors and capacitors to keep the rubbish inside a screened container.

    What problem is the interference causing?
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    You have two types of fields associated with interference and that is the electric field and the magnetic field. They combine at a certain distance away from the source to produce an EM wave. These two distances are called near field and far field.
    If you are within 1 wavelength of the source frequency then you are classed as in the near filed. If you are more than 2 wavelengths away you are classed in the far field.

    The point at where the two meet and are neither in the far field or near field is called the transition point and is greater than 1 wavelength but less than 2. The waves then combine and became in phase and produce a pure EM wave then. But the electric field and magnetic field are in different planes, the electric field is in the vertical plane and the magnetic field is in the horizontal plane and so is separated by 90 degrees. This is why if you place a pickup wire 90 degrees to a wire transmitting a large magnetic field you won’t pick up much. But turn the wire to be parallel with the other wire and you will pick up a larger proportion of the magnetic field.

    If you are in the near field then this is normally magnetic field dominant but the both types of field can exist on their own and if say you had a small piece of wire and not a coil causing the interference then the E field would dominate. But generally near field is magnetic and far field is electric.

    This is because the field strength of a magnetic field decays by the cubed of the distance from the source and the electric field decays by the square of the distance.
    I think I got that right.

    Adam
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, you're talking large currents and inductors.

    That means voltage spikes. I'm presuming that 90A isn't going through the solenoids or inductors, but whatever the current, you normally turn solenoids on and off and that will cause massive voltage spikes.

    It is always better to remove or reduce the interference at the source than trying to suppress it at the point it causes problems.

    I'd be looking at diodes, capacitors, or snubber networks across the solenoids if they're the cause.

    What is the cause? That will lead you to the best fix.
     
  9. Chasingwires

    Chasingwires

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    Apr 4, 2014
    It sounds like I'd need a degree in electrical engineering to find out the solution. You guys are brilliant though and give me a lot to think/learn about.

    To show you my level of knowledge, we just now covered capacitors in class.. I mainly read schematics and do what they tell me to but I also play with arduino in my free time.

    I am sure most of you could see a schematic of what I'm referring to and improve the system significantly in many ways. Since it is work related I can't share everything, but I figured there may be a usual fix.

    I appreciate the responses. As I learn more I'll be sure to come up with better questions in the future.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
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    Jan 21, 2010
    In many cases it's something that can be achieved by following some simple rules.

    The big one is:

    Don't try to make the voltage across an inductor drop suddenly. If this might happen, take precautions to ensure that you don't generate voltage spikes.

    There are other ways to generate noise, but look for this one first.
     
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