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Relay voltage spikes

Discussion in 'General Electronics Discussion' started by JFL110, Aug 2, 2012.

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  1. JFL110

    JFL110

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    Aug 2, 2012
    Hi I'm using some 12V relays to control some solenoid valves which are fed by a 240V to 12V mains transformer, the problem is that when the relays are engaged or disengaged it causes a voltage spike which can be heard through speakers attached to the same mains socket and is actually causing my USB breakout board to crash when its connected to a computer on the same socket, what can I do to suppress these spikes?

    thanks for your help
    James
     
  2. GreenGiant

    GreenGiant

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    Feb 9, 2012
    Try connecting up some capacitors from the relay connection to ground, this should help filter some of the noise, you are going to need caps with voltage ratings higher than the voltage that runs through the relays

    This is simply chatter on the connections of the relays, you could also try replacing them with solid state/reed relays which *shouldn't* have chatter since there are no moving parts
     
  3. mechtronics

    mechtronics

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    Aug 7, 2011
    yes i agree with green giant about connecting capacitors to deal with the noise.
     
  4. JFL110

    JFL110

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    Aug 2, 2012
    ok thanks how would i calculated what capacitance to use?
     
  5. john monks

    john monks

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    Mar 9, 2012
    I'm assuming that you are using a 12 volt transformer to drive a 12 volt AC relay. But where is the switch in relationship to all this? If they are long wires they could be radiating energy. If the wires are short a .1uF capacitor across the output of the transformer along with a voltage suppressor across the relay coil may help. Too many unknowns.
     
  6. JFL110

    JFL110

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    Aug 2, 2012
    Hi thanks for the help I've found that a capacitor across the transformer and a diode across the solenoid has solved the problem cheers
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    We like to keep our threads nice and tidy, so perhaps you could clarify that this is, in fact, a DC relay coil. You made no mention of a DC power supply, just the transformer.
     
  8. JFL110

    JFL110

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    Aug 2, 2012
    Hi yes it was a 240V mains AC to 12V DC transformer powering a group of solenoids with a DC relay coil operated by a 5V micro controller circuit, the problem wasn't with the relays it was with the solenoid valves generating voltage spikes but fixing zener diodes directly across the solenoids and capacitor across the transformer supply has solved the problem.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    You didn't need Zeners across the Solenoid coils. A common reversed biased Diode is what's normally used. The Zeners are working because the BEMF sees a .7V Diode junction, which is a Zener reversed characteristic. What is the Zener voltage rated at? If it's 12V they may eventually pop.

    A forward biased 12V Zener starts to conduct long before it reaches 12V. Unless these Zeners are rated a good margin above 12V they will be needlessly conducting when the Solenoids are energized.

    If I were you I'd replace them with common 1N4002's
     
  10. Electrobrains

    Electrobrains

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    Jan 2, 2012
    Hi James

    Better than "raw capacitors" as sparkle absorbers, you should use RC components (snubbers)! For instance 0.1uF with 47 ohm in series.

    Without the resistor, you will create a resonance circuit that will oscillate high voltage for a considerably long time.

    Worse than that is usually that you will quickly wear out whatever contacts you are switching those capacitors with. There will be very high inrush current through the switch-capacitor branch (sometimes hundreds of amperes, just limited by the source impedance and cable resistance/inductance).
    In worst case that poor little contact will weld together... RIP :(
    I have often seen this problem, especially when relay contacts are used, switching heavy loads of "modern equipment" with built-in X-capacitors.
     

    Attached Files:

  11. cvicente

    cvicente

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    Sep 6, 2013
    I'm sorry if I'm ressurrecting this post, but I have a very similar some problem.

    In my particular case I'm using a 5V relay to control wall AC socket to power up whatever AC appliance I could think of. But for the sake of testing I'm using 45W fan. For that purpose an AC/AC transformer (240VAC to 6VAC and later rectified and regulated to 5VDC) and wall socket are fed from the same mains socket.

    Like JFL I also hear it on my speakers which are attached to the same mains socket.

    I've carefully read all the answers, but one thing I couldn't quite understand is the Electrobrains spikes snubber circuit, because it seams to be directly attached to the relay coil. Shouldn't I be worried about filtering the spikes going to the mains soket instead?

    (All this considering that in an extreme situation I might plug-in a 240VAC at 16A appliance going through the relay)

    P.S. I'm actually a Software engineer giving hardware a shot, so I'm kinda green in what electronics is concerned.
     
  12. john monks

    john monks

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    Mar 9, 2012
    cvicente, I am not sure what you mean by "it" If you mean you hear rectifier noise on your speakers you might try placing a non polarized capacitor across the output of your 6VAC transformer. Maybe 1 microfarad will do. If you year a click when you change the state of your relay then you may try placing a 240 ohm resistor across the coil of the relay. Often times to get rid of noise you just have to try things until the noise disappears.
     
  13. cvicente

    cvicente

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    Sep 6, 2013
    Thank you answering John.

    I guess what I mean by "it" it's the click noise (more like a "BUMP") on the speakers everytime I engage or disengage the relay.

    By the way, if the voltage spike is due the trasient generated by the 230VAC going through the relay, how a resistor across the relay coil would help to supress it? I'm genuinely asking...
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Does it "click" if there is no load on the relay?
     
  15. cvicente

    cvicente

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    Sep 6, 2013
    Hi Harald.

    I don't know if I quite understood your question. It clicks everytime I engage or disengage the relay.

    What I mean is... the AC appliance is always "on", waiting for the relay to ti give it power to actually work. The load (230VAC) I think its always present on the relay, I just close (I have the 230VAC phase wire connected to the relay) the circuit on the relay whenever I engage it (giving it 5VDC through the coil).

    ok, I guess what you're trying to suggest is for me to engage and disengage the relay without the AC appliance. Right?
     
    Last edited: Sep 11, 2013
  16. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What I mean is: remove the AC load from the relay. Does activating the relay without load generate the "click"?
    If yes: the problem is with the low voltage driver side of the relay (but i doubt it).
    If no: the problem is with the AC load side of the relay (my guess).

    Another thing you can try: put a switch in series to the relay. Engage the relay. turn the switch on and off. does it "click" Then the problem sure is the AC load side. The power supply of your speakres is probably not well filtered which generates the "click" when a disturbance is on the mains voltage (e.g. voltage spike generated by activating the fan).
    You can use another outlet or add a mains filter between the speakers and the outlet.
     
    Last edited: Sep 11, 2013
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Many of us have accumulated as many protected/filtered power strips over the years as we have wall warts, so this is a classic example of field expedience. ;)

    Chris
     
  18. cvicente

    cvicente

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    Sep 6, 2013
    Thank you Harald.

    I'll try to cover all those steps, and get back to post de results.
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    cvicente, do you have a reverse-connected diode across the relay coil? If it's a DC relay coil, you should have a diode across it, to clamp the high-voltage spike caused by back EMF from the coil when the relay is de-energised, unless you need the relay contacts to open very quickly, in which case you need a more complicated spike suppression circuit.
     
  20. cvicente

    cvicente

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    Sep 6, 2013
    @Harald

    So, I've proceeded with the following scenarios:
    1 - Activating relay with no load. Result: no click
    2 - Using a switch in series with an activated relay. Result: click heard.
    3 - Powering on the 45W AC appliance (which is actually a fan) while the relay is already activated. Result: no click

    For some reason I was hoping that by powering up the fan with an already activated relay, would give me a "click" on the speakers... but it didn't.

    @Kris
    No I don't! Do you mean something like the attached picture?
    [​IMG]

    Carlos
     
    Last edited: Sep 12, 2013
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