Is it resistors I need? Diodes? If so, what type??

Discussion in 'General Electronics Chat' started by adam763, Sep 8, 2014.

  1. adam763

    Thread Starter New Member

    Feb 27, 2014
    15
    0
    Hi there,

    After scouring the forum for what seems hours and hours, I'm still no closer to finding the exact answer I'm looking for...
    Here's my dilemma:

    I'm running a 12volt supply to activate a solenoid. Within the initial wiring I have a push button switch (obviously to allow me to activate the solenoid).
    In tandem with the solenoid I have a relay. Why? Because as I activate the switch, the solenoid activates and so does the relay. In turn, the relay then completes a switch to a PC keyboard encoder (if you're not sure what one of these are, it doesn't matter, because I simply need to know...>>)
    The keyboard encoder requires/requests NO voltage. It simply needs the connection complete (as what a standard button would do). This is why I thought the relay would be ideal. Unfortunately, I've since found that current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which knocks out my keyboard encoder.

    So guys, what do I need to implement into my design to ensure the relay doesn't send any voltage through it's COM/NC connections? A diode? A resistor? And what type and where would it go to protect my encoder?

    I've tried to find the answer for AGES, so all help will be greatly appreciated.

    Thanks.
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    Add a diode across the relay coil so it opposes the 12 volt supply. When the magnetic field collapses, that power will circle through the diode until the resistance of the coil turns it into heat. Something like this:
     
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  3. adam763

    Thread Starter New Member

    Feb 27, 2014
    15
    0
    Thank you #12, that's very helpful.

    Can I just ask, is there a particular diode I should look for? Your image shows a 5v diode, will this guarantee absolutely no high voltage 'leaks' to my encoder?

    Thanks again.
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    The diode isn't 5 volts, the power supply is. A 1N4001 through 1N4007 diode will work in this position.

    And, no, this isn't a guarantee. The fact that you are using a push button and have not provided a schematic means I am guessing. I am guessing that you will need an electrolytic capacitor on the power line AFTER the push button to keep voltage spikes absorbed.

    Can you post a drawing?
     
  5. adam763

    Thread Starter New Member

    Feb 27, 2014
    15
    0
    Excellent. Thanks again. :)
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    Something like this. If the first drawing doesn't work, try the second one, or use 2 diodes and snuggle them up real close to the coils.
     
  7. adam763

    Thread Starter New Member

    Feb 27, 2014
    15
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  8. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    Or this....

    Theoretically, all of these will work except there is inductance in wires, so physical placement counts. Actually, theory says the relay isolates the Encoder and you're not having this problem. So much for theory!
     
  9. adam763

    Thread Starter New Member

    Feb 27, 2014
    15
    0
    So much for theory indeed!

    Thanks again for your invaluable help. It really is appreciated.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    One down side to the diode is that it delays the drop out time of the relay, for small inductive devices, it is not really a problem, unless it operates a pulse dispensing valve or very large inductive device such as a brake/clutch etc, where the delay is perceptible.
    Max.
     
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