Why 24vac won't actuate a solenoid

Discussion in 'Irrigation' started by 1idejim, Jan 26, 2013.

  1. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,098

    Quite a few times i see a post where the OP states that he has 24 volts but the solenoid won't work.

    I always ask the same few questions, what is the resistance of the wire path and solenoid and what short to earth is.

    I took a call from a friend in Texas this week, 24 vac, but he also had 180 ohms resistance from the controller.

    Most troubleshooters will begin fooling with the splices because that's what we're taught.

    Before i look at anything i will take a short to earth measurement to test the wire insulation. If the insulation is good i look at the splices, if the insulation is bad i grab the A-frame.

    Most solenoids require 300mA - 500mA inrush to lift the plunger. That's 7 - 12vA.

    Using Ohms law you can see why the valve won't open.

    24vac/180ohms=0.133333mA.

    There are 24vA in one Amp. 1A/24vac = 0.041667mA or 1vA.

    0.133333/0.041667 = 3vA, less than half the current required to magnatize the coil and lift the plunger.

    Electrical troubleshooting can be made a lot easier by doing a little more testing and a lot less guessing.

    Hope this helps someone.
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  2. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,298

    Too many numbers .... brain is boiling .... can't reme
     
  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,943

    A current reading is the most helpful measurement. Aside from a leakage meter, what's a good clamp-on ammeter with a low range?
     
  4. Kiril

    Kiril LawnSite Fanatic
    Posts: 18,298

    Too expensive, that what it is.
     
  5. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,943

    C'mon, techies, there has to be some sort of improvement in the state of the art

    [​IMG]
     
  6. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,098

    I had one of those analog amperage testers Shoes, i will see if i didn't toss it. Sperry made some tough instruments.

    Armada, greenlee, excell, fluke, all make good stuff.

    Accurate measuring instruments are hard to find at Harbor Freight though.
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  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 46,943

    You have to wonder if all the technical ends of a multimeter haven't been condensed into a single chip by now, leaving only the quality of enclosure and switches as factors in determining the worth of these instruments.
     
  8. 1idejim

    1idejim LawnSite Fanatic
    Posts: 9,098

    There are a number of inexpensive and yet reliable clamps out there that measure .000 Shoes.

    Point of the thread is that for the tech that actually uses a multimeter, values can be measured or calculated close enough to make critical path decisions.

    Line voltage / transformer output, voltage drop as well as resistance and current can be measured or calculated to an acceptible degree of accuracy. Close enough to make decisions rather than guesses.
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  9. cjohn2000

    cjohn2000 LawnSite Senior Member
    Posts: 553

    Doesn't help with someones upside down profile pic. :dizzy:
     
  10. irrig8r

    irrig8r LawnSite Platinum Member
    Posts: 4,535

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    Last edited: Jan 28, 2013

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