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HELP needed with relay bounce

Discussion in 'Electronic Design' started by chris.knight, Jul 12, 2005.

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  1. chris.knight

    chris.knight Guest

    Hi All


    I've been trying to construct a pressure regulating device, using a 12v

    latching solenoid and conventional valve, to protect a filter in a home

    irrigation system. I am using a pressure switch which goes contacts
    open at high pressure/contacts close at falling pressure. These two
    states in turn trigger 555 timers which in turn drive small 12v SPDT
    relays.


    The relays are configured so that in the relaxed state the circuit
    connects to the 0v line and when exited by the pulse from the 555,
    switches to supply 12v for each relay in turn. The purpose of this
    switching configuration is so that the 12v latching solenoid, which
    operates the valve, can be switched to close the valve when pressure
    reaches a high threshold, but then by reversing the supply voltage, is
    switched to open the valve at a low threshold.


    This all works perfectly - until the latching solenoid is connected,
    whereupon the action of the solenoid causes the relays to bounce
    erratically.


    I have tried various combinations of (non polarised) capacitors and R/C

    combinations across the terminals of the solenoid, and/or across the
    relay. I have also tried using seperate 12v power supplies to power
    the two sides of the device (timing - relay activation side and power
    supply for solenoid side)but the action of the solenoid still causes
    the relays to bounce erretically.


    I feel I am running out of options to solve this dilema, but am now
    considering the use of switching transistors or solid state relays.
    Only I'm not sure how to switch such devices on and off to give the
    necessary pulse to switch the solenoid on then off etc.


    Any help would be greatly appreciated
     
  2. Guest

    The above test may mean this won't work, but: I have seen a 555 circuit
    that behaved badly when switching a large relay. It was supposed to
    turn on the relay for about 2 seconds when power was first applied, then
    turn off the relay. When tested, it just sat there and cycled endlessly.
    The problem was simply that the current initially drawn by the solenoid
    was dragging down the supply enough to reset the 555; a big electrolytic
    capacitor across the power supply leads near the 555 fixed it.

    Were the two supplies totally separate... no common ground or anything?
    If you're not sure, you might use a 12 V battery for one supply or the
    other to make _sure_ the supplies are isolated.

    Random thought: is the problem possibly with the _relay_ coils, instead
    of the solenoid? Have you tried installing diodes "backwards" across
    the relay coils? In other words, orient the diode so that it's not
    conducting when the relay coil is powered. Something like a 1N4007 is
    usually good. You can get quite impressive spikes from a relay coil;
    I have seen spikes to over 100 V from the coil of a 12 V automotive
    relay. If this is the problem, I can't quite explain why the relays
    would behave with no load on the contacts and act up with a load, but
    it's something simple to try.

    Matt Roberds
     
  3. Guest

    The problem has nothing to do with relay bounce. It's due to pressure
    fluctuations in the system because the solenoid abruptly shuts off the flow and
    the pressure changes rapidly. Of course the pressure sensing relay sees this
    fluxuation and appears to chatter.

    The usual fix for problems like this is a pressure "accumulator". A "T"
    fitting is placed in the system between the solenoid and the sensing relay. At
    the open leg of the T a short connection is made to a bottle of some sort. The
    bottle is the accumulator. The bottle is placed with the connection to the T at
    its lowest point. The bottle is half filled with water with air over that. The
    water pressure compresses the air and a little more water enters the bottle.
    When the pressure in the system tries to change rapidly, a little of the water
    flows into and out od the bottle thus not allowing the pressure to change as
    rapidly as it would without the accumulator. This smooths out the pressure
    changes without affecting the overall pressure.

    Jim
     
  4. Guest

    Substitute "pressure switch" everywhere except the first time I wrote
    "relay".

    Jim
     
  5. Dan Hollands

    Dan Hollands Guest

    If the problem is electrical rather than a pressure surge as described in
    another post, the problem is almost certainly the inductive voltage kick
    from the solenoid when voltage is removed. In the old days a snubber (R&C)
    inseries was used but finding the correct values was always a problem.
    Nowadays a device call a bidirectional transient voltage suppressor or
    bidirectional Tranzorb does a much better job - see
    http://www.vishay.com/docs/88378/sa.pdf for an example.

    These devices are widely used to protect electronic wiring from high voltage
    noise spikes

    Dan

    --

    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
  6. JeffM

    JeffM Guest

  7. chris.knight

    chris.knight Guest

    Firstly, I used two batteries to give totally seperate voltage
    supplies. The diode concept is probably worth exploring a bit further,
    particularly the Tranzorb device. I'll try and locate some
    Secondly, the problem is electrical, not due to pressure fluctuations,
    as at this stage I am yet to fit the valve in the irrigation line. The
    comments about pressure fluctuations are well received though. I'll
    watch out for problems along those lines when I finally (optimisticaly)
    get it sorted
     
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