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'Dry' Contact

Discussion in 'General Electronics Discussion' started by IRETST, Dec 7, 2011.

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  1. IRETST

    IRETST

    4
    0
    Dec 7, 2011
    Hi,
    This is my first post.
    Is a relay the only way to get a voltless pulse?
    I need to generate an open/close pulse of Freq < 20Hz, either through a manual toggle switch or from a pulse counter but the pulse must be a 'dry' contact, to avoid damaging the device measuring the pulse.

    Thanks
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I guess that "dry" means galvanically isolated, and that "voltless pulse" means conduction.
    I believe opto-couplers can be used for this. Either with ordinary transistor output or MOSFET.
    There are even optocouplers with a photovoltaic output that can directly drive a MOSFET gate (for higher currents).
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    9,633
    2,013
    Nov 17, 2011
    I suggest using a photoMOS relay (Panasonic trade mark: http://www.panasonic-electric-works.com/peweu/de/html/photomos.php). Similar parts are also available from other companies.Depending on your operating conditions (voltage, current) this is a fast solution and it is free from contact bounce.

    I would not recommend using a mechanical relay since even relays with mercury wetted contacts will wear out rapidly when operated at ~ 20 Hz.

    Regards,
    Harald
     
  4. duke37

    duke37

    5,297
    739
    Jan 9, 2011
    Welcome to the forum IRETST, I hope we can help.

    A pulse is normally considered as the presence of a voltage for a short time so please say what you mean by a voltless pulse.

    A pulse counter does what it says, it counts pulses not generates them.
    I do not know what you mean by a dry contact. Relays may be the best solution for high voltage or high current pulses but semiconductors will be smaller and cheaper for low voltages and currents.

    The device measuring the pulse can have its input protected, specifications will be needed here.

    Be more specific about what you want, I am sure someone can help.

    Duke
     
  5. IRETST

    IRETST

    4
    0
    Dec 7, 2011
    Thanks for the swift replies!!

    I'll try to explain what it is I'm hoping to achieve, I'll start by explaining my current set-up -

    I have a sensor (runs on 3V3 rail) that counts pulses and reports the number it has counted - these are typically short /open - no voltage across the contacts. I'm generating this pulse manually by opening & closing a standalone switch - no other hardware involved.
    I have also got an old "Sine Square Oscillator" (not a Pulse Counter, sorry!) its an old analog style with dials, I've connected a 12V relay to the output from the Oscillator and the O/P from the relay to my pulse counter. The sensor seems to work up to 20Hz accuratly but then loses track.
    I want to be able to set up a test to log the number of pulses input (via a Digital display counter) and cross check this count against the no of pulses measured.
    I like the idea of a PhotoMOS, I don't particularly like the "clunky" nature of the Relay.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,633
    2,013
    Nov 17, 2011
    Hi,
    a typical mechanical relay will not operate reliably at 20 Hz. So the problem may as well be with the sensor as with the relay. Besides, every mechanical relay exhibits more or less contact bounce which may be counted as pulse by your sensor/counter, depending on its sensitivity.
    So a first good step would be to replace the relay by an optically coupled device. A photomos kind of relay can operate up to some 110 or 1000 Hz, depending on the type. If you want to go faster, a photocoupler/optocoupler is better suited. With a photocoupler, however, you have to observe the correct polarity of the sensor signal (whereas a photomos relay can switch AC or any polarity).
    To check the accuracy of your sensor, you will need a second (reliable) counter and measure the pulses output from your generator and compare the result with your sensor output.


    Regards,
    Harald
     
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