circuit needed, 70kv/20A

Discussion in 'The Projects Forum' started by yikes, Jul 17, 2008.

  1. yikes

    Thread Starter Member

    May 19, 2007
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    Here is what I want: a circuit that will provide 70k volts/11mA and 13.8v/20A. In other words, a car coil and battery amps going to the same connection. These inputs could be flip/flop'ed or timed hi/lo.

    Here is what I need: baby talk,,,seriously. I try to sound intelligent, but I'm faking it.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That's a rather absurd combination, but I guess you have your reasons.

    The only kind of switch that could handle that combination of voltage and current that I can think of offhand are the huge old knife-type switches like were used in the Frankenstein movie. 70kV can arc for quite a distance, particularly once the air between the potentials has been ionized. :eek:
     
  3. yikes

    Thread Starter Member

    May 19, 2007
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    I was thinking a 555 timer circuit to trip the coil, the secondary going to an open air spark plug to prevent the amps from flowing back up the 70k line, maybe a diode to prevent the 70k from going to the battery.
     
  4. theamber

    Active Member

    Jun 13, 2008
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    Where are you getting the 70Kvolts souce from?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    He's getting the high voltage from an automotive coil. Actually, then it would be more in the range of 40kV.

    I'm just wondering what you're planning on using this for?
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Just remember that the 'conductors' carrying the HV in an automotive system are often high resistance for suppression purposes. 13.8 volts would be lost in these cables.

    The catch 22 is that you need this type of insulation to prevent the HV transferring along the outside of the cable. If you have ever been bitten by damaged or substandard automotive ignition cable you will know what I mean.

    I too am interested in your application. Is the switching frequency low enough to use electromechanical (relay) switching? That would be much more reliable.
     
    Last edited: Jul 17, 2008
  7. SgtWookie

    Expert

    Jul 17, 2007
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    StudioT, that's true. Most of them have a carbon-impregnated string of sorts. The old rule of thumb was 1k Ohm per foot, and no more than 20k Ohms per wire.

    Racing ignition wires with 8mm insulation are available; they have copper conductors. Wouldn't carry 20A though, unless a bunch of 'em were paralleled. Not cheap.

    I don't know of the availability of 70kv diodes. I suppose they're out there, but would be pricey. Placing several television HV rectifiers in series seems like a good bet. Insulating them could be dicey. Once an HV arc started, it wouldn't stop until the power was cut off, or something got fried.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
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    Out of curiosity, I just checked eBay... there is one 40KV diode with a starting bid of $23. There is also a pack of 5 each 15KV diodes with a "buy it now" price of $20. This is why so many HV hobbyists choose to stack diodes.

    And now some warnings: Just because there is a rating on some label saying "11mA" does not mean the thing won't source lethal current at lower voltages. Air is not a reliable insulator at those voltages. You can kill yourself or someone else with this thing. Exercise all due precautions.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Something else on top of Thingmaker's rather mild warnings - if you're putting lots of power through solid copper wires without any shielding, you're basically creating a Tesla coil, which will generate very broadband RFI noise, and cause every FCC agent within a few hundred miles to beat down your door, confiscate your equipment and throw you in jail while slapping you with a $10,000 or so fine. :eek: Until they get to you, you'll be interfering with television, cellular, police, fire, aviation, ham, and military frequencies. The FCC folks won't be in a good mood when they show up.

    You can't randomly radiate broadband RFI emissions without having dire consequences.

    Auto ignition wires are resistive to suppress the high current and RFI emissions that would otherwise result. Racers have to shield their wiring. If you look at a Corvette engine, you'll see the ignition wiring is all in a metal enclosure; no steel body to shield the RFI.
     
  10. theamber

    Active Member

    Jun 13, 2008
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    Car coils maximun output is 20Kv peak, that is 14140 volts RMS.
    Also, the ignition coil will only produce high output voltages in transients on the input current. He will need to cycle the DC supply into the Coil, at best he will get short bursts of voltage at very low current.
    An oil furnace coil will be more usefull since they have two high voltage terminals instead of one and take 120V directly from the wall but they will need a lot of AC power, and definitely this type of coils is not like a cars coils and these coils really can kill you.
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    I don't think an oil furnace coil will do any good. His i/p is 12Vdc, not 120Vac.
     
  12. theamber

    Active Member

    Jun 13, 2008
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    He wants a 70Kv and 20 amp supply.
    Do you think he can get it from a Car battery and a car coil (I already exposed the limitations)?
    An oil furnace coil is the only one that can give him that power or unless he connect to the electrical company transformer.
    And now do you have a better solution for his problem than the solution I gave him???
     
  13. thingmaker3

    Retired Moderator

    May 16, 2005
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    No, he does not. Please read the posts more carefully before composing your replies.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Let's look at some facts here.
    The dielectric strength of air is roughly 3+E6V/meter, or 76.2 volts per mil (0.001 inch)
    In a typical automotive internal combustion engine, the compression ratio is at least 8.5 to 1. This means 8.5 atmospheres, so that translates to 647.7 volts per mil.

    A standard gap used for spark plugs is 0.080", or 80 mils.

    Thus, the minimum voltage to jump the spark gap in a typical modern engine is 51,816 volts.

    In the old days (up to the early 1970's) automobiles did not have high energy electronic ignition systems, and the spark plugs were gapped at 0.035". Thus, these ignition systems required only around 22,670 volts to fire the plugs. High compression engines were up around 10.5 to 1, these required 28,000 volts to fire the .035" gapped spark plugs.
     
    Last edited: Jul 18, 2008
  15. theamber

    Active Member

    Jun 13, 2008
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    Last edited: Jul 18, 2008
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, that was a spark gap for an Oldsmobile V8 that I had a while back.

    Just looked in a Buick V6 repair manual; specs are .060" for the spark gap for the 3.1L engine.
    So if the compression ratio is still 8.5:1, the minimum voltage to fire freshly-gapped plugs is still 38,862.

    People frequently drive their cars without touching the plugs for 50,000 miles or more nowadays. By the time the plugs have that many miles on them, the gap has significantly widened due to erosion, thus increasing the voltage required to jump the gap. The ignition system had to be designed to cope with that increased demand.
     
  17. theamber

    Active Member

    Jun 13, 2008
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    I did modified a 1968 AMC Javelin SST (10.2:1) for high performance a few years ago and I could not find a spark plug with an insulator (material inside the ribs)that would of withstand more than 60,000 volts peak max.I did like a little hotter plugs, 2 heat ranges up borderline than recomended. Mallory ignitions coils claimed to be better by having higher voltages outputs but where just a maketing scam due to the plug limitations.
     
    Last edited: Jul 18, 2008
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Well, that's interesting. I'd never really bothered to look at that. However, as plugs get old, one of the failure modes is arcing over the insulator externally from inside the plug boot to the "nut", or the metal outside of the plug. More common is arcing on the inside due to plug fouling.
    Were you experiencing fouling?
    I seldom would go two heat ranges over, as detonation could be a real problem.

    The Jaguar XJ-S 5.3L V-12 convertible we had used an 11.5:1 compression ratio. Compression in a static test was up around 240 PSI. But, it used a 0.035" plug gap, so the ignition secondary was around 26kV. Getting it to idle properly with such a small plug gap was quite a chore, as the air/fuel charge was rarified.

    Many years ago when I got a 1976 Olds V8, I replaced the sparkplugs and gapped them to the traditional .035" - and I was startled at how terrible it ran. I was stunned when I discovered the spec was for a 0.060" gap. After I re-gapped the plugs, it ran perfectly fine. Later versions went out to 0.080". It seems that with newer ignitions, they've reduced the plug gap, but they are firing the plugs for much longer than they used to.

    But I'm really getting way off topic of the OPs request.

    Let's just say for now that a typical modern GM coil that is properly excited can produce in the neighborhood of 50kv in the secondary.
     
  19. yikes

    Thread Starter Member

    May 19, 2007
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    Guys I really want to thank you for your efforts.
    I would like 7.5 cycles per second. I would like the "corona discharge" to help me. If I had two car coils, could I add the voltages?

    I think a mosfet on a timer could handle the 20Amp part and a timer could be put on the negative primary to the coil, then it would be a matter of timing those two circuits so that I end up with what I want.

    I think I understand the personal danger of touching any of this, but I do not understand the emf danger or how to protect from it. I have heard the phrase "faraday cage", is this what is needed?

    Being repetitious, I am talking over my head here.
     
  20. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Did you already have a look at Tony's Website?
    http://www.uoguelph.ca/~antoon/
    On the right there are links to high voltage projects.

    Greetings,
    Bertus
     
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