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Proper way to drive a 24VAC solenoid? — Parallax Forums

Proper way to drive a 24VAC solenoid?

CounterRotatingPropsCounterRotatingProps Posts: 1,132
edited 2009-05-28 04:55 in General Discussion
Hi All,

Picture·a 24V AC sprinkler-valve control solenoid and you'll know what I've got here on the bench.

It has about 400mA surge at pull-in and about 120 mA run.

It may toggle on/off·upwards of 50 times per day and needs to run like this for, preferably, years. (They can do so·with 24VAC relay.)

I'd like to control them with an SX or B.Stamp.

Was thinking of PINOUT to a driver/buffer transistor·then gating a properly rated triac.

Is that overkill ?

Is there a simpler way ?

What would you recommend to avoid inductive coil kick-back ?

thanks so much for your time!
Howard

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No matter how much you push the envelope, it'll still be stationery.

Comments

  • mctriviamctrivia Posts: 3,772
    edited 2009-05-25 18:26
    Use an opto isolator and triac would be my suggestion cheap and guarantees no feed back to chip

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  • PrettybirdPrettybird Posts: 269
    edited 2009-05-25 18:57
    Get a solid state relay. That is what theu use in the industry. It is a cube with an opto input and isloatted output. 24V is an industry standard for noise and long wire runs. Just remember the clamping diode across the coil.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-05-25 19:07
    Prettybird said...
    Just remember the clamping diode across the coil.
    Um, it's AC. Don't use a clamping diode unless you want to see some smoke! Most solid state relays include a snubber circuit, anyway.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2009-05-25 19:11
    They often have an LED built-in which comes on when they are actuated - useful for testing.

    Leon

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  • kwinnkwinn Posts: 8,697
    edited 2009-05-25 21:30
    Zero crossing optically isolated scr (moc3030-3033) driving a 2-5A scr.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-05-25 22:00
    Thanks all for the prompt replies.

    kwinn's suggestion seems the best option because it has all the parameters you all have suggested - and the moc3030 is $6 - certainly cheaper than an SSR.·· But kwinn, you probably mean driving a triac?

    That said, I was thinking about SSR's for another application... but...

    It's been literally decades since I"ve used an SSR. Back then we put a cap across either the load or trigger --- but I can't recall which (I smell smoke [noparse]:)[/noparse] --- but it was for snubbing I think. (I can image that the manufacturers realized the oversight and put these inside.)

    Phil, do the built in snubbers make this unneed?

    thanks again·!
    - H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    No matter how much you push the envelope, it'll still be stationery.

    Post Edited (CounterRotatingProps) : 5/25/2009 10:05:35 PM GMT
  • kwinnkwinn Posts: 8,697
    edited 2009-05-25 22:11
    Yes, I meant triac. Sorry about that, glad you caught the mistake. BTW I don't think the snubbers are necessary when you are turning on at or near the zero crossing.
  • Alan BradfordAlan Bradford Posts: 172
    edited 2009-05-26 11:27
    I use Grayhill SSR Modules in all of my industrial control projects.
    The 70M-OAC5 is $9.50 from Digikey. Part Number GH7022-ND
    A few more $ but you get a 140 VAC 3 Amp Triac with the snubber built in.
    The MOC3030 is rated at 1 amp and has no snubber.

    You can connect it directly to a Stamp pin.

    You need to look at the cost of a part failure in your dsecision.
    Is it worth saving $3 when the solenoid stays on due to a shorted triac.
    Will this flood a lawn, drain a well, fill a basement, run up a $500 water bill, etc.
    The data sheet on the MOC3030 says it is rated at 100 MA with 1.2 Amps peak.
    If it takes 400 ma to pull in the solenoid, what is the holding current?
    This is not enough to reliably run your solenoid. This is a maximum or higher load.
    You also need to derate the chip as it gets warmer.
    A 3 Amp SSR is less likely to fail than a .1 Amp Dip chip Triac.
    You will be running it ar about 15% of its maximum rating.
    It seems worth the extra $3 to me.
    I have attached a copy of my StamPLC that uses 16 SSIO modules.
    We have installed around 100 of thesedriving 120 VAC Gas/Water Solenoids,(the same ratings as your solenoid)·and have not had a failure yet.
    Hope this helps you out.




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    Alan Bradford

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  • kwinnkwinn Posts: 8,697
    edited 2009-05-26 16:00
    Alan has a good point. I doubt that you can assemble a circuit using a MOC 3030 and triac for the $9.50 the SSR costs.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-05-27 01:31
    Encouraging information!

    Alan and kwinn,

    thanks for answering my next questions - before I asked them. ( Ever heard of Psychic Pizza ? It's guaranteed delivered five minutes before you think of ordering [noparse]:)[/noparse]

    Very cool, last time I used SSR's they were really expensive - but how can you beat <$10 ? The solenoids are 400 mA pull and about 200mA run (at least that's how they're labeled) so the Grayhill device will certainly have plenty of headroom.

    Thanks a lot, Alan, for the pdf - yes, it will decidedly be helpful in many ways - I'll study it carefully.


    Taking a related tangent here a second...

    Kwinn, that zero crossing detection implies that snubbing is unnecessary helps clarify what snubbing _really_ does. (I've always considered snubbing, shunting, esi, esd

    - and lightning-rods! amoung the Grey Arts.)

    It's just basically the reverse of current, the collapse of the coils mag. field, and the resulting kick back that needs to be snubbed out, right?
    So, at zero crossing, the flow goes to zero and *shazam* no kick back - at that instant?

    But, as my 12 year old nephew (and I) wonder, AS it is crosses zero, where does the coil's discharge actually go ?? Into itself? Into Some Alley in New Jersey?

    And really basic here:

    Is the term "Snubbing" properly used only for A.C. (Or A.C.-like)?
    and "Shunting" only to D.C. (Or D.C. like)?

    thanks
    - Howard

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    No matter how much you push the envelope, it'll still be stationery.
  • kwinnkwinn Posts: 8,697
    edited 2009-05-27 03:55
    The snubbers and "kick back diodes" or whatever name given to them are really there to absorb the energy from a rapidly collapsing magnetic field. A sine wave at 60HZ is not rapid, and only produces the counter emf that limits the rate of current change in an inductor. If the magnetic field is produced by a DC current through a coil a diode can be used. If it is an AC current then a diode would present a short in one direction so a snubber is needed. With a triac or scr, once it starts to conduct it remains conducting until the current and voltage drops to a level too low to maintain current flow. At this point there is very little energy in the magnetic field, so no snubber or diode is necessary.
  • CounterRotatingPropsCounterRotatingProps Posts: 1,132
    edited 2009-05-27 19:35
    Thanks for the very clear explainer, kwinn
    I guess the old SSR's·didn't have zero crossing detectors ... I recall if we left off the cap., they'd zonk out after a while.
    Alan Bradford said...

    You need to look at the cost of a part failure in your decision.
    Is it worth saving $3 when the solenoid stays on due to a shorted triac.
    Will this flood a lawn, drain a well, fill a basement, run up a $500 water bill, etc.

    Pretty good guesses!
    In my case, failing part would·result in: "run the well dry and have tenants complaining that their A.C. isn't cooling." jumpin.gif

    Avoiding that is worth almost $300 !·· So thanks again for the bargain, Alan!

    - Howard in Florida
    ~~~~~~~~~~~~

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    No matter how much you push the envelope, it'll still be stationery.
  • PrettybirdPrettybird Posts: 269
    edited 2009-05-28 02:56
    Opps too tired. I ment supressor. 24vac is unusual. Why not go 110vac coil or relay?
  • kwinnkwinn Posts: 8,697
    edited 2009-05-28 04:55
    It is on a sprinkler system, so near water, which would be hazardous with 110V. 24 V is much safer.
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