Auto-Resonant PWM circuit won't run in LTspice w/o Load between circuits.

Discussion in 'Off-Topic' started by lost_bro, May 7, 2010.

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  1. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    9
    2
    Good Evening All.

    I am trying to combine two separate circuits. The 1st. is a self-resonating TANK/LC circuit derived from the days of radio about 80 years ago. The second is a Current limiting PWM circuit which I have been using now for about 2 years in my vehicles (brute force hydrolysis). I have All three of the circuits set up and running in LTspice. Both the first and second circuit run fine and as expected in LTspice.
    When I combine (or try to combine them, I am probably missing some obvious solution to this), I get the wanted waveform and function ONLY when I connect a simulated load of 29 amp.s or greater between the two circuits.
    I'm not sure what effect this simulated load has on making the two circuits function as wanted. Apparently something is not balanced in the circuit.
    The idea to combine the two circuits is to end up w/ a circuit which automatically finds resonance, takes that waveform and gates it via the current limiting PWM. This would allow some level automatic regulation of the in-resonance waveform.
    This is indeed what happens w/ the LTspice simulation.
    Here is the link for the files: http://tech.groups.yahoo.com/group/LTspice/files/ Temp/
    I have already uploaded to the LTspice group at Yahoo.
    The file is : PMW modified_self_resonating.zip
    Just go down the page and double click on the file.
    I would appreciate any and all comments regarding this circuit. I am by far only a novice, and could benefit from the experience of a forum such as this one.
    Thanks in advance
    Lost_bro
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The link does not work.
    You can upload the attachments here in the AAC.

    Bertus
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You have to be a member of the Yahoo! LTSpice group in order to have access to the files area.

    To save some time/trouble, the .zip file is attached to this post.

    LM339.sub goes in \Program Files\LTC\SwitcherCad\lib\sub
    potentiometer_gm.lib goes in \Program Files\LTC\SwitcherCad\lib\sub
    potentiometer_standard.asy goes in \Program Files\LTC\SwitcherCad\lib\sym
    There was no .sym file supplied for the LM339.
    The .ASC files go in \Program Files\LTC\SwitcherCad

    There is a .jpg image in the .zip that does not get extracted to the SwitcherCad directory.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I don't suppose you noticed, but you have your load on the source terminal of the MOSFET, and as a result, there is no way for you to maintain Vgs=10v once the MOSFET turns on. If you hold down the ALT key while clicking on M1 during the simulation, you'll see that the MOSFET is dissipating about 150 Watts of power.
     
  5. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    9
    2
    Good afternoon SgtWookie

    Yes, your absolutely right. I went ahead and modified the diagram, placing the the connection to the TANK cir. directly to the emitter on M1.
    Please note that the Simulated load is necessary to get the PWM to simulate in LTspice.
    Both circuits as drawn do what I want them to do. Check the waveform of the votage of TANK circuit, and the waveform at the emitter of M1. Looks Good.
    Then: disconnect the Simulated Load from the emitter at M1 and connect it to Point X , don't forget to erase the ground at Point X also.
    Now, the timing Cap in the PWM (C1) is no longer regulating the frequency of the LM339. The entire PWM side of the circuit is operating at the resonant frequency of the tank circuit.
    How to uncouple them so the PWM will actively gate (I know it is NOT technically gating because I am manipulating the Collector and not the Gate) the tank circuit but not be affectd by the resonant frequency of the tank circuit?
    Is it possible to make the circuit do what I want it to?
    The JPEG in the file is what the gated waveform looks like (the wavefrom I am trying to produce). I produced it manipulating the last version of the Pwm, but of course the circuit was not functional in real life.

    Thanks for the suggestions
    Lost_bro
     
    Last edited: May 8, 2010
  6. SgtWookie

    Expert

    Jul 17, 2007
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    You have used the current source as a constant current.
    Right-click the current source, click the Advanced button, and check the "This is an active load" box on the right, under "Parasitic properties".

    Otherwise, it will continue to output current even while the MOSFET is off.
    Did you know that the 4148 diodes are only rated for 100mA? They would get rather crispy rather quickly with 30A flowing through them.

    M1 does not have an emitter. M1 is a MOSFET. It has a gate terminal, a source terminal, and a drain terminal.

    I don't know why you're trying to do this. The "electrolyzer cell" will have some resonant frequency due to the capacitance of the plates and the inductance of the wiring, but in order to break down the water to it's elements, you need to pass current through it. About the best you can possibly hope for is 70% efficiency on a very good day with a tailwind while going downhill.

    Of course not.

    It's based on wild-eyed claims by people who somehow think they can overcome the laws of conservation of energy. What they're actually doing is reaching their hand into your wallet while you're not looking.
     
  7. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    9
    2
    Good evening SgtWookie

    Thanks for the fast response.

     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I eliminated your 20A current source model because I don't feel that it's realistic to assume that your cell will have a constant current across it.

    What I do feel is realistic, is a relatively constant voltage drop across it, depending upon the current flow through it. This is better modeled using a semiconductor diode, or a series of diodes.

    I've replaced your current source with four diodes rated for 25A; they're in the default LTSpice library.

    Changing to a more realistic load model made it necessary to decrease the values of R1 and C1. Since there is no inductance in the load, the current is an "all or nothing" affair. Attempting to control current through a diode load w/o inductance using simple PWM will be rather fruitless, as you will see. Adding an inductor in series with the load will smooth things out.

    I did some cleaning up on your schematic. All of the spice parameters were combined into a single list. I cleaned up the potentiometer symbol file, since it was off-center. If you start using this new .asy file, you will see that the alignment is different from your original. It has the potentiometer_standard.sub as a model included in the .asy file so you no longer need to have that .include in spice directives.

    You seem to like to sprinkle lots of diodes around, for what reason I don't know.
     
  9. lost_bro

    Thread Starter New Member

    Mar 17, 2009
    9
    2
    Good evening SgtWookie

    Thanks for the fast reply.

    I will review your suggestions and get back to post ASAP.

    Have a great evening

    Lost_bro
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    How lovely of you to patronize us -
    We've had run-ins with similar free spirits in the past. There was never a good outcome. Something about pointing out that science doesn't respond well to wishing.

    You are free to peruse the threads linked to in the sticky - http://forum.allaboutcircuits.com/showthread.php?t=28067

    But I'm afraid the topic has come to a close.
     
    Last edited: May 9, 2010
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