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Need Help Making High Voltage Automatic Capacitor Discharge Circuit

Discussion in 'High Voltage' started by ZGrizzlyWord, Nov 19, 2011.

  1. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Hello, I am trying to make a circuit running on 230VAC to discharge ~600µf capacitor to make a magnetic pulser (using a 2.5 mH inductor). I was thinking of using a xenon tube circuit, similar to this design and putting the inductor in series with the xenon tube, but I don't want the heat or wasted energy of the xenon tube. Besides, I want all the energy going to the inductor, not the xenon tube (see page for downloadable schematics http://www.vellemanusa.com/products/view/?id=350496).

    I was thinking of making a circuit with a 555 timer, but with 230VAC, I don't know how it would work. So, is there a way to use simple electronic components to make a simple charging, discharging circuit for the capacitor? I am a Mechanical Engineer, so I don't know how all the electrical components work, but if you make a schematic with part numbers, I can understand it.

    Thank you!
     
  2. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Use a SCR for the switching circuit. It can be triggered off of an optical coupled system as well.
     
  3. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Sorry, but how is an SCR controlled? I saw one in my Google search, but it appeared to only work on 5 volts. If you could give me a schematic, it would be helpful.
     
  4. dave

    Dave New Member

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  5. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I agree with tcmtech, the magnetizing circuits I have worked with over the years used SCRs. Energy stored in a bank of capacitors and bam!

    Ron
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Like this. The firing pulses applied to the SCR gate would be just a few Volts, a few mA, and a mS or so in duration. The interval between firing pulses would have to allow time for the cap to recharge (which would depend on the value of R1). DON'T play with mains voltage unless you fully understand the risks and take the necessary safety precautions. Mains voltage can KILL.

    Edit: Circuit revised. R1 now 100k to assist turn-off of SCR.
     

    Attached Files:

    Last edited: Nov 20, 2011
  7. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Do what Alec mentions and my bad, as I was typing you posted and I never saw the questioning of triggering the SCR. Running slow today and not quick on the uptake. :)

    Ron
     
  8. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Thank you for the schematic, it makes sense. One question, and I know this sounds probably very elemtary, but how do I create the firing pulses automatically? I was going to do something with a potentiometer and a capacitor to make a variable timing circuit, so how would I intigrate that into the firing pulses? When the timing capacitor charges, the node goes to 0V and the SCR fires?
     
  9. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    To make a very simple self firing SCR system all you need to do is connect the gate lead through a number of small neon bulbs to the positive side of the power source.

    Each mini neon bulb has roughly a 60 - 75 volt break over voltage so by stacking 4 - 5 of them in series you could get a high firing voltage with an automatic reset as well. How fast it goes is only limited by how fast you recharge the capacitor back to its firing voltage again.
     
  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Depending on the frequency you want to fire the SCR at your original thoughts of using a 555 configured as an astable multivibrator might be a good consideration. Possibly through an opto coupler or directly.

    Also, along the thinking of tcmtech you may get by using an old fluorescent lamp starter which contain a cap and neon lamp.

    Ron
     
    Last edited: Nov 19, 2011
  11. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    You need a low voltage DC supply to run the 555 timer then feed the output of the timer to the SCR.
     
  12. ZGrizzlyWord

    ZGrizzlyWord New Member

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  13. alec_t

    alec_t Well-Known Member Most Helpful Member

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    If you need guidance about the 555, use the 'Theory' tab at the top of this page.
    The SCR will need to have a voltage rating > 350V and be able to withstand the discharge current pulse of ~ 30 Amps. With the SCR switched on, the current through R1 must be less than the holding current of the SCR, otherwise the SCR won't switch off when the cap has discharged. Be aware that the cap when charged holds a LETHAL charge. I would strongly advise against building the circuit until your electrical knowledge is greater and you fully understand the hazards involved.
     
    Last edited: Nov 20, 2011
  14. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Check the revised circuit in post #5. The time constant (R2+RV1)*C2 needs to be much greater than the time constant R1*C1, otherwise the C1 voltage won't rise very far before C2 charges to ~ 3V and triggers the SCR. To speed up the recovery time of your trigger pulse generator, a reverse-biased 1N4007 diode could be connected between the top of L1 and the SCR gate. Note the above safety caveats.
     
  15. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Thank you for your help Alec. I didn't know what you meant by isolated mains, so I Googled it and I'm glad I did. I went ahead and ordered an isolation transformer as well. For R1, you mentioned that (R2+RV1)*C2 should be >> R1*C1, but you increased R1 from 1k to 100k in your diagram, which seems to contradict your statement. I went ahead and ordered both sizes anyway since they were about $1 each (I got 10w versions), but if you could clarify before they arrive, that would be great. Thank you for your help and I'll be sure to post a picture of this thing flinging bolts into the air.
     
  16. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Yes I did. Increasing R1 was necessary (so don't use 1k) to help the SCR to turn off, but that still leaves 35mA charge current flowing when the cap has discharged. The SCR will need to be of a type with a holding current > 35mA. As I thought you were just experimenting with the triggering and didn't necessarily need the cap to charge to the full ~350V I didn't bother changing the R2, RV1 and C1 values. I left that for you ;-).
    Do be careful when building/using that circuit. Connect a 1 megohm or so high voltage resistor across the 600uF cap before use so that the cap can discharge to prevent shock to some unsuspecting person (possibly you!). Remember, 600uF holds a LETHAL CHARGE at > 100V.
     
    Last edited: Nov 22, 2011
  17. ZGrizzlyWord

    ZGrizzlyWord New Member

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    Alec, I ordered the ZLLS1000, but when I got it here it looks way to small for my voltage rating. Here is the one I ordered:
    http://www.mouser.com/Search/Produc...0TAvirtualkey52230000virtualkey522-ZLLS1000TA

    I went ahead and looked around and found one rated for 600v, but the sucker costs $10.
    http://www.mouser.com/ProductDetail...GAEpiMZZMuIUjt4yeP9c8%2bDq2wh87e4a5Iu5d7cPek=

    Perhaps I can get by with a 300v rated one and just use a small resistor to bump down the voltage.
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMuIUjt4yeP9c9g3GGFcuv2X3kI/ul1Ze94=

    Please let me know what you think I should do. I never heard of these types of diodes before. Thanks.
     
  18. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Well now that we know you'll be using this thing for flinging bolts (rather than for example studying decaying oscillations) we can dispense with the Scottky diode altogether (which, incidentally, is shown with the incorrect polarity in my schematic....apologies. BTW a Schottky diode is intended to break down at a certain voltage, and you wouldn't want one with a higher breakdown voltage than 40V here).
    You might find the triggering is improved by adding a diac (or neons, as suggested in post #8) in series with the gate of the SCR.
     
  19. ZGrizzlyWord

    ZGrizzlyWord New Member

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    What do you think I should pick as my trigger voltage?

    Edit: Nevermind
     
    Last edited: Dec 2, 2011
  20. ZGrizzlyWord

    ZGrizzlyWord New Member

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  21. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Those look ok. Again, TAKE CARE when the circuit is powered up.
     

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