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Circuit design help

Discussion in 'Electronic Design' started by Mechdesign2k4, Mar 22, 2007.

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  1. Having not much knowledge in the field of electronics, I neeed
    assistance in designing a circuit. Basically I want to control a
    solenoid air valve. I would like to be able to push a button and have
    the valve pulse on and off and be able to independently vary the on
    and off times. I was thinking on basing this circuit around a 555
    timer. QAny help would be greatly appreciated.
     
  2. D from BC

    D from BC Guest

    If I understand the prob correctly..
    When the button is pushed an oscillator with independent on/off
    control is turned on?

    I'm guessing at the function in words..
    A Ton timer triggers on a rising edge.
    Ton elapses.
    Ton's falling edge then trips the Toff timer.
    Toff elapses.
    Toff's rising edge is fedback to the Ton timer.

    Yup...sounds very 555ish to me.
    D from BC
     
  3. right now the solenoids are 120volt, NC. I'm guessing a relay would
    between the "circuit" and the solenoid. Within a 3-5 second cycle, I
    would like to have the solenoid fire about 6 times. In the energized
    state, the solenoid open, it would remain that way for only 20-30
    milliseconds. Again I would like to be able to adjust these times
    manually, potentiometer maybe? I have a 5-15v power supply, 13.4 amp,
    as well as a 24v 2.5 amp supply that I could use for this.
     
  4. Rich Grise

    Rich Grise Guest

    Even though this sounds suspiciously like homework, I'll try to help
    withoug doing the whole thing for you. A 555 should do the trick,
    although you'll most likely need to add a transistor to control the
    solenoid. You could use a pot to adjust the "on" time, but I'd think the
    "off" time would be determined by "until you press the button again". You
    might want to differentiate the button-push to the trigger - I'm not sure
    what happens to the output if the trigger stays active through the timing
    interval, but I think it just stays high; ergo, you want a very narrow
    trigger pulse.

    Good Luck!
    Rich
     
  5. Hawker

    Hawker Guest

    How big a solenoid's?
    You will probably need a second 555 (or small MPU) to act as a PWM
    modulator. for all but very small solenoids you need to give a high duty
    cycle pules to close it then back off the duty cycle by a large amount
    to hold it close or you will burn up the coil.
    It may just be easier to pick a small 8 bit processor to handle all the
    timing from the push button and PWM. But you are correct a 555 should be
    able to do it for you. I would need more info to give you a drawing.
    Look for "1 shot" drawings in National Semi's web site of LM555 for some
    ideas for your timer part of it.

    Hawker
     
  6. Noway2

    Noway2 Guest

    The first thing you need to do, is determine what the product
    requirements are. Spell them out, write a specification, add drawings,
    etc.

    An example of what you might include: you say independently vary the on
    and off times, but how do you want them set? buttons, dials, or other?
    Do you want a digital display, 7 segment LEDs, or none?. These types
    of decisions will at least impact, if not determine many aspects of the
    system design.
     
  7. LOL thanks Rich,
    I can assure you this isnt homework. We do have an inhouse electronics
    engineer, but lets just say if you want it done in a hurry, you might
    get something in a couple of months from him...
     
  8. MooseFET

    MooseFET Guest

    The 20-30mS time worries me. Are these things run from 60Hz AC or
    120V DC?

    Doing a time that small on an AC powered thing adds a layer of
    trouble.
     
  9. John Fields

    John Fields Guest

    ---
    From what you've written, I think you have a solenoid that uses
    120VDC to go from what you call the "normally closed" state to the
    energized state, which is what you call "open".

    Also, you have a "cycle" which can last from 3 to 5 seconds, and
    during that time you'd like the solenoid to go from the normally
    closed (de-energized) state to the energized "open" state, to stay
    open for 20 to 30 milliseconds and then revert to the closed state
    six times during that 3 to 5 second period.

    Also, you want to be able to adjust the cycle time, the number of
    times the solenoid fires during that cycle and the "open" times when
    the solenoid is fired.

    Am I right?

    Oh... and how much current does the solenoid need to operate and how
    long does it take to go from closed to open and from open to closed?
     
  10. Guest

    Yep. He's going to ask the same sorts of questions as John Fields has
    asked, and you haven't answered, so he'd have to go and find out the
    answers himself, which does use up more of his time. For some odd
    reason, people who have a more detailed idea of what they need done
    get faster service from in-house engineers.
     
  11. Guest

    Don't think: Instead BUY a couple of DIN rail timers. They will work directly
    with the solenoid too. Once you have done the thinking, the power supply, the
    relay driver, the PCB and the EMC stuff then the 555 is very expensive indeed!
     
  12. Noway2

    Noway2 Guest

    Your comment about "all but very small solenoids" caught my attention.
    How small is very small?

    I have an application I a 12Vdc 20W solenoid I am planning on driving
    with a ~24V PWM. I was planning on adjusting the PWM duty cycle to
    provide an RMS equivalent of 12Vdc with a frequency of about 100Hz.

    I hacked together a test circuit and verified that it worked and let it
    run for a while. I monitored the RMS voltage and current and the
    readings look fine. The coil did not appear to get hot or suffer any
    side effects either.

    It sounds like you have some experience with doing this sort of thing.
    Based on your experience, does this approach sound reasonable or would
    you recommend an initially large duty cycle and then back it off as you
    say? The hold current is far less than the pull in current, so backing
    off on the duty cycle makes sense.
     
  13. Hawker

    Hawker Guest

    You will have to measure the temperature of the coil.
    Your approach should work as you have it, but as I think you know, you
    will need a much higher pull in current than holding current. For this
    reason you can shift your PWM after about 200mS to the holding current
    and save power. If heat is fine (because the coil was built over sized
    enough) then the only advantage is power saving. If you don't care then....

    What often happens, to save cost is manufactures make the coil smaller
    than can sustain the pull in current indefinitely. In this case if you
    don't back of the current after pull in the coil will eventually burn
    up. Not everyone builds them that way. The very large stuff has to be
    built that way, but you are in the size that could go both ways. 20W
    seems like alot of juice to have burning up if you only need 2W to hold
    it though.

    BTW for whatever reason I don't know the PWM frequency is usually in the
    1-5Khz range. Much higher the effects of the inductor take over.

    I have designed a few PWM controllers for very large 3-phase
    contractors. For them I think we had something like 90% duty cycle pull
    in with 10% duty cycle holding or some such.

    HAwker
     
  14. Noway2

    Noway2 Guest

    Thank you for the reply.

    You have given me some things to think about and experiment with. You
    are right, 20W does seem like an awful lot of power just to hold in a
    solenoid. In this case, it is on a little valve on a 1/2" plumbing
    line. I have honestly been wondering why such a large coil was used in
    this application. The problem is that I don't have any specifications
    telling me what the requirements are and there is no history from the
    person who chose the original to explain his rational. I do have
    confidence in that person's abilities based on what I know of them, but
    there is no way to ask them anymore.
     
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