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A relay which causes rf interferences?

Discussion in 'General Electronics Discussion' started by MisterBeppe, Feb 27, 2015.

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  1. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    Hi, I am a new user and I have a noob question :)

    I've seen these videos [1] which shows a simple circuit based on a relay, a coil and a capacitor: when this circuit is powered, the relay starts to quickly change his on/off state and as result it causes some rf interferences (in the following video is able to block the TV signal from some meters of distance). How is that possible? Someone could explain me which kind of electrical phenomenon occurs?

    Well, since I am very curious, I've tried to make this "circuit": it works but is very weak, compared to the circuit seen on the video (I have no intention to make a jammer, also considering the fact that no transistor - for amplification purpose - is used): is barely able to make some little interferences only if is nearly placed to the television or near the TV's antenna. I used a 12 V relay, a 22 pF capacitor, and a simple copper antenna.
    I just wish to understand which features (eg the kind of relay and the capacitor value) are able to causes this particular phenomenon, maybe a bigger relay?

    Thank you very much and excuse me for my bad english.

    [1]

     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    It's the spark created between the contact that causes the interference. It like having a motor without any suppresion capacitors.
    Adam
     
  3. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    Hi Adam, thank you for your reply!
    Well, and which value can determine the "power" of the spark? The voltage? The dimension of the relay? In the above videos, the relay has the same dimension as mine, but his action is more powerful (again: is just curiosity, I don't want to cause discomfort to other people).
     
  4. Arouse1973

    Arouse1973 Adam

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    Oh thats a shame, dont you have any friends you dont like. :) its only the voltage that causes the spark. Current will depend on the charge stored in the circuit. Because charge and time is current.
    Adam
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    To get a nice spark you need high voltage say 16KV and say 100pF of capacitance and disharge this to a very low resistance like ground. This and the energy in the capacitor gives a nice spark.
    Adam
     
  6. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    Ok: so the guy of the video, maybe is using a 24 V relay, which act better due the higher voltage, right?
    Maybe, since I don't have any friends which I dont like, I could consider to make some joke to my brothers :-D Ehehe ;-)
     
  7. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    Yeah, or I should also build a Tesla coil :D
     
    Arouse1973 likes this.
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    No I was joking, you have to carefull with high voltages. Basically it the voltage that jumps the gap and energy that make the crack sound you hear.
    Adam
     
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  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The energy stored in the relay coil (which is an inductor) essentially increases the voltage across the coil as the current is quickly interrupted to try to keep the current flowing. This causes a spark across the relay contacts until such time as the energy in the coil is depleted.

    So yeah, this circuit actually does generate high voltages.
     
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  10. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    Thank you for the explanations, Adam and Steve!
    So, apart the fact that I don't want to build a tesla coil or something similar (since I perfectly know that I don't have the skills to achieve these voltages and I don't want to risk) the circuit with the relay (as shown in the videos) is able to work from some distance only if the voltage is higher than 9 Volts? In facts, there is a comment in that video which talk about a 24 Volt relay. I just want to understand the "equation" which can define the range of action of that relay which can produce sparks, and I think, in facts, that the the power of this "sparks relay" is related to the voltage (9V against 24 V).

    There is some book for novices (also in english) about these topics?

    Many thanks.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The voltage produced as the relay contacts open depends on numerous factors. It's based on the amount of energy stored in the delay coil, the speed at which the contacts open, the contact characteristics and many other things. Since this is not a desirable behaviour you're not going to be able to choose a relay based on any published specifications. Try a few and see which one is best (or worst).

    Also note that if you find one that works really well you might get a visit from people who frown on your activities.
     
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  12. MisterBeppe

    MisterBeppe

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    Feb 27, 2015
    As I've said (and please, trust me :) ) I don't have any intention to bother anyone: I'd just like to understand (and considering that I am a novice) which could be the practice difference between the circuit seen on the video (it seems a similar relay as to the one which I use) and the relay which I have, since the circuit on the video is able to cause interference from some meters of distance and the mine is able to cause interference only from a distance of few centimeters (5 CM, about 1,9 inch).
    I know that circuits which cause radio interference are not allowed, but first of all is an ethical matter, avoid to bother someone else :)
     
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