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Irrigation Solenoids - Strange Failure

Discussion in 'Electronic Repair' started by Matthew Smith, Feb 17, 2005.

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  1. Hi All

    I have an irrigation system using 24V Hunter solenoid valves (hydraulic
    assist). Solenoids have DC resistance of 24.5 Ohm when healthy.

    The solenoids are controlled by a microcontroller driving BUZ11 mosfets;
    +24V is fed to the solenoids, the mosfets ground a return wire to turn
    them on. SMD equivalent to 1N4007 from mosfet drain to +24V to protect
    from reverse EMF.

    There are two sets of solenoids, both fed by standard irrigation cable.

    I find that two of the solenoids - one set, both in the same valve box -
    have suddenly gone high resistance. The four solenoids on the other
    cable are still fine. There is no problem with the controller - still
    delivering 24V down both lines quite happily.

    The 24V power supply is actually derived from a large dot matrix printer
    and holds a steady 23.79V.

    I am totally mystified as to why this pair of solenoids would both
    suddenly go high resistance at the same time, leaving everything else
    working fine.

    Ideas anyone?

    Cheers

    Matthew Smith
    South Australia
    http://www.mss.cx
     
  2. Guest

    The theory used to compute MTBF's assumes that all failures are spread
    out over time by some distribution function. (Often the distribution
    function is flat). And that failures are uncorrelated.

    But now you see a correlated failure. One really vague possibility is
    that the failure of one solenoid caused the other. More specific
    examples of this could include one overheating next to another, causing
    it to get too hot too.

    Another possibility is that the same environmental condition common to
    both solenoids caused both to fail. For example, vibration. Another
    example might be a large voltage on the housing (is it metallic?)
    holding both solenoids. Or a large voltage lighning-induced on the
    wiring (which is after all nearly identical for both solenoids.)
    Although you'd expect lightning to take out the drivers too.

    Finally, some devices that come in pairs are well-known for failing in
    pairs. Car headlights, for example. They were manufactured
    identically, installed identically, operated identically, so you
    shouldn't be surprised when they fail at nearly the same time. This is
    sort-of modern manufacturing coming back to bite you... (Of course part
    of this goes back to the possibility of an common extreme environmental
    condition causing the fault - vibration, overvoltage, etc.)

    Tim.
     
  3. Tim wrote:
    The two solenoids are in a fairly large space under a plastic manhole
    and are separated by about 6" of air. The only real proximity is the
    adjoining conductors in the cable.
    That's what I've been puzzling over. The solenoids are plastic potted
    (as things can get wet in this type of environment). I've been
    wondering if the 30' or so of cable might have had current induced into
    it - but can't figure out from where.

    The controller is protected against reverse transients of 1kV, although
    100V would happily fry the mosfets. So we could have had a large
    voltage of a polarity that would be dissipated by the diodes - but I
    think we're getting into the realms of rather unlikely.

    A vehicle did pass close by to the manhole, although I wouldn't have
    thought that there would have been that much vibration as to upset the
    electrics - more likely to bust one of the PVC pipes that the valves are
    solvent welded onto.
    And this could be it; a statistically possible example of s***[1] happens.

    Thanks for your comments Tim.

    Cheers

    Matthew Smith

    [1] Not sure on the Newsgroup policy of the use of Anglo Saxon words ;-)
     
  4. Most of these valves are rated at 24VAC. If you are using DC, that
    could explain the coils burning out. The inductance of the coils limits
    the AC current thru the coil to something less than what Ohm's law might
    tell you. If you must run them from DC, you should determine the lowest
    voltage that will make the solenoid function correctly and limit it to
    that.
     
  5. NSM

    NSM Guest

    I'd be looking at the connections, mate. Could be corrosion.

    N
     
  6. I wrote:
    </blockquote>

    NSM replied:
    </blockquote>

    Possible but, in this case, I think unlikely. The solenoids are potted
    in solid lumps of plastic - they are, after all, designed for a wet
    environment.

    I thought that corrosion/damp in the joints between the flyleads from
    the solenoid to the feed cable so I cut off several inches of the
    flyleads and put my meter onto clean, dry, wire.

    Still read several MOhms rather than 24.5Ohms.

    Thanks for your thoughts, anyway.

    Cheers

    Matthew Smith
     
  7. NSM

    NSM Guest

    Crappy manufacture in that case, assuming you are feeding the right sort and
    quantity of volts to them.

    N
     
  8. Franc Zabkar

    Franc Zabkar Guest

    The combined heat dissipation is about 47W. Could ventilation be an
    issue, especially in an enclosed box on a hot day?


    - Franc Zabkar
     
  9. How do you come up with that? 24V across 24.5 Ohms is only ~1A,
    therefore <25W. I still say the problem is using DC where AC is called
    for, resulting in cooking the coil. Hunter valves use AC.
     
  10. Oops, sorry about that, I just realized you mean for two coils. :) I
    haven't had my coffee yet.
     
  11. NSM

    NSM Guest

    I'm assuming the OP hasn't done anything that foolish.

    N
     
  12. James Sweet

    James Sweet Guest

    Didn't he say he was using DC?

    DC would work fine so long as the voltage was reduced below specification.
     
  13. The landscapers who installed the valves advised that these AC units
    were suitable for 24V DC (no reduction) and were in regular use on water
    trucks, running off the 24V truch supply. They claim that they had
    never had a failure before - but then every vendor says that...

    This was why I went with these valves - they can be switched easily with
    a MOSFET.

    Of four that were installed a year ago, one failed fairly recently,
    shorted out. The pair that failed had only been in service for a couple
    of months.

    As for getting warm, they can. The original system has cycles lasting
    up to 30 minutes and yes, the solenoids get pretty warm, but not too hot
    to touch. The ones that have just failed have a programmed runtime of
    only 10 minutes each.

    Cheers

    M
     
  14. NSM

    NSM Guest

    What do they know about electrics? FWIW, you could check that the amps are
    OK, see what they draw on AC and don't exceed that on DC.
    Another way, run them up on a DC supply until they operate then add 10% to
    that.

    N
     
  15. I replied to this post earlier, but haven't seen it appear on the list,
    so assume that it got lost in the aether...

    NSM:
    I measured the inductance of the solenoid (120mH) and calculated that
    the total impedance at 50Hz (R=24 Ohms, Xl=37 Ohms -> Z=61 Ohms) would
    make for a current of 400mA - less than half the DC current (1A). To
    achieve 400mA on DC, I would be putting in only 9.6V.

    I'll do tests with a 24V transformer on a Variac - I'm guessing on 12V
    DC operation at the moment.

    I'll modify my power supply to run at the appropriate voltage and never
    again will trust the word of a landscaper on electrical matters, no
    matter how convincing they sound ;-)

    Many thanks to NSM for these suggestions.

    Cheers

    M

    PS: N - did you try to mail me? My server bounced a "nowrite" e-mail
    address earlier today. (It rejects anything that hasn't a real return
    address.)
     
  16. David

    David Guest

    The total impedance is not the sum, but the square root of the sum of the squares.

    David
     
  17. NSM

    NSM Guest

    Like I say, amps closes the valve. If 400 mA AC will do it 400 mA DC will
    too.
    Not me.

    Kia Ora.

    N
     
  18. Ross Herbert

    Ross Herbert Guest

    Anthony Fremont is correct. 24Vac solenoids will definitely fail when
    operated from 24Vdc. Some manufacturers do make a 24Vdc solenoid but
    the coil is completely a different spec from the 24Vac coil.

    Find your particular valve on
    http://www.hunterindustries.com/Products/Valves/default.htm and read
    the technical data. Then if you need to find out more use the
    Support/Ask the Expert link to ask whether a standard 24Vac solenoid
    should be connected to 24Vdc.

    PS. It might just be possible to get away using 24Vdc (100Hz or 120Hz
    unfiltered), ie. straight out of a bridge rectifier. Carry out some
    testing and measurements.
     
  19. Thank you for correcting me on that - it's amazing what you forget over
    18 years when you don't use stuff... (Now re-reading my text book.)
     

  20. After much searching in the "treasure house" of my shed, I have finally
    found a transformer in the 9-10V range. Hooked it up to a spare
    solenoid via bridge rectifier and filter cap an bingo - 398mV! Couldn't
    have got closer if I'd tried. And yes, the solenoid operates fine.

    I now need to incorporate this new power supply into my controller, but
    that's a minor issue.

    Many thanks for all assistance on this matter. Now to tell the
    landscapers that they've been dishing out bad advice...

    Cheers

    M
     
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