How to switch a latching solenoid?

Discussion in 'The Projects Forum' started by Ged, Nov 14, 2007.

  1. Ged

    Thread Starter New Member

    Nov 6, 2007
    3
    0
    Hi,

    This is my first post here and I hope I explain this correctlty.
    I have very little electronics knowledge but am seeking to make a solenoid valve switch on and then off (for water) after about 10 mins. It will be battery operated to minimize current drain and will be switched on by a sensor.
    I had hoped to have the sensor switch one 555 for 25ms and a second running for 10 minutes and then using the down mark/edge to turn it off.

    Any clues as to what I should be looking at?

    Thanks in advance.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    A 555 won't source enough current for most relays, but you can control your solenoid with a FET. The ten minute 555 output would be tied through a current-limiting resistor to the gate of an N-channel FET. Your solenoid would connect between V+ and the FET drain.

    Good information can be found here:
    http://www.allaboutcircuits.com/vol_3/chpt_5/1.html
     
  3. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    I don't understand why you need two timers. Why is one timer not enough? What is the purpose of the first timer?

    Also, ten minutes with a 555 is really pushing the limit. In my experience you need a really high R (like 10 Mohm) and if the capacitor has or develops any leakage current then the timer will never finish.
     
  4. Ged

    Thread Starter New Member

    Nov 6, 2007
    3
    0
    Thanks for responding.

    What I am trying to achieve (through my limited knowledge) is a battery powered device utilizing a latching solenoid water valve to irrigate for a given time. The time "on" will be started by a sensor tripping. I had intended using an inverting op amp but this would give a constant supply to the valve when it didn't need it.
     
  5. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Without knowing more about the sensor it is impossible to say anything definite but a falling edge can trigger the main timer directly.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    My experience with magnetically "latching" solenoids is that they need to be driven in both directions. To accomplish this I have used an h-bridge with good success.

    hgmjr
     
  7. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    I have no idea why you say this because a normal gardening solenoid watering valve is usually driven with 24 V and is open while voltage is applied and closes when voltage is removed, just like the valves of a washing machine. These are all AC but, in any case, if they were DC they would actuate independently of the polarity.

    A solenoid fluid valve is actually actuated by the pressure of the fluid and the solenoid only actuates a tiny pilot valve. The solenoid works against a spring which keeps the pilot valve shut.

    For a really good explanation and schematic see Wikipedia: Solenoid Valve. (Of course it's good. I did it!) ;)
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Not all solenoids have return springs...
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You are correct thingmaker3. This is what I had in mind by "magnetically latching".

    The advantage of a magnetically latching relay is that it retains its state even with power removed from the drive circuitry.

    I interpreted the term "latching" in this thread's title to refer to a magnetically "latching" relay, hence my comment about the need to pulse the relay energizing coil in either the close or open direction. An h-bridge performs the bi-directional pulsing function very well.

    hgmjr
     
  10. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Oops. I missed that part. Sorry.

    Still, using a latching valve appears like an unnecessary complication unless there is a very specific need or you happen to have such a valve. Home appliances and gardeniong systems use regular valves. The OP mentions using batteries and the need to conserve power but I am not sure if this justifies the complication. You can buy gardening valves (24Vac with step-down, isolating transformer) and appliance valves (at mains voltage) very cheaply anywhere and, in any case, the circuit only has to control a tiny relay and the mains does the rest. Not only that but you can supply the circuit from the mains too using an adapter and save batteries and trouble.

    Now, If you absolutely HAVE to use a latching valve, then what you need are two pulses of different polarity ten minutes apart which can be done in many different ways but it certainly complicates the circuit.

    Note that using a latching valve is intrinsically unsafer because if anything happens to the circuit after the opening pulse (like a low battery condition) then the valve will stay open indefinitely. This is one important reason whi most appliances use standadard valves.
     
  11. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    I have had experiance with 9 volt latching water solenoids as used in remote irrigation/frost systems, due to their low power consumption as most are ultimately powered via solar.
    They require a 9 volt positive pulse of around 1/2 second to latch on, and a negative pulse to switch off. You can either use an H bridge or relays to achieve this. My last design used 4 SPDT relays to control 2 of these solenoids switched from a PICAXE and a temperature controller...(this unit was used for frost protection on a vineyard). I normally feed them with 12 volts due to the cable run distance and to improve reliable operation.
    They are generally not as reliable as the standard 24VAC units used in mains powered controller systems, so this must be factored into the equation
     
  12. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    No prob:)!

    Magnetically latching relays are less common than their more common energize/de-energize cousins.

    The additional complexity required of the driving circuitry is offset by the power efficiency due to the fact that they are energized only briefly when the closed/opened state needs to be changed. Gadget's experience is typical in this regard. This makes them well suited to an application such as the one under discussion in this thread where power is at a premium.

    hgmjr
     
  13. Ged

    Thread Starter New Member

    Nov 6, 2007
    3
    0
    Thanks for all the replies.
    I would like to say that my use for using a latching relay was to save power whilst using batteries. On the otherhand, I had thought of the benefits of a normally closed, low voltage solenoid, fed from a 12V source remotely. The reason to not use this type was to make it good to use in a remote location.
     
  14. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    It depends on the duty cycle. If the valve will be energized 10 mins out of every 15 mins and power is at a premium then the latching valve might make sense. But if the valve will be energized for 10 mins out of 24 hrs then it makes more sense to use a regular valve. Regular solenoid valves should not consume much power (I'm guessing in the order of 3 W for a small one) and, like all solenoids, holding current is less than energizing current so that you can send an initial activation pulse and then keep a lower maintenance current for the rest of the 10 mins. This is what many starter motor solenoids in cars do. In other words, you can have a regular solenoid valve which will not consume more than 1 or 2 watts when energized. If it only needs to be activated 10 mins/day then it can be done with batteries without consideration for power.
     
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