Creating a "Mad Scientist" Theater Prop

Discussion in 'General Electronics Chat' started by Yarnell_Dan, Mar 13, 2015.

  1. Yarnell_Dan

    Thread Starter New Member

    Mar 13, 2015
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    I have an oil burner ignition transformer used to drive a "jacob's ladder" encased in a bell jar for safety. I'd like to dress it up for presentation and have obtained some nice, old-school, analog devices to utilize. One is a 150mA ammeter. One is a 30VDC meter. The final is a precision 10 turn Helipot. I think I'd also like to add a purposefully weathered knife switch to energize the device through a relay. The pot will essentially be nonfunctional - for show only - but if possible and relatively simple I'd consider using it to modify the displayed output of one or both meters. It's a 10K ohm unit (Helipot SA1598). I'd like to construct a simple and reliable circuit (or circuits) whereby the two meters would swing a significant amount of their respective full scale in sync with the arc as it moves up the ladder or at least display some deviation whence the arc strikes and eventually flares out at the top of the ladder and re-strikes at the base. Any ideas or thoughts as to how best to proceed? FWIW for the base of the unit I'm planning on using oak.
     
  2. #12

    Expert

    Nov 30, 2010
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    Connecting a killovolt supply to a milliamp meter is a good way to create smoke. The problem is, it will only work once. :(

    You would be safer to use the power to make a 30 volt DC supply and add an oscillator to make the dials move. Immediately, you are looking at buying more parts. Bottom line: You usually can't just find some parts in an old box and connect them together until something works. How hard are you willing to work at this?
     
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    You could place an appropriate shunt in the neutral line feeding the primary of the OBIT -- thence connect the meter{s} (in series with scaling resistors where necessary) across the shunt (one side through an appropriately 'directed' rectifier)...

    Note: when calculating the value of the shunt, Iprimary must = Iprimary(secondary-shorted [i.e. arcing at minimal distance])--- Also please! Never assume 'neutral' = ground!

    Best regards
    HP
     
    Last edited: Mar 13, 2015
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  4. #12

    Expert

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    Yes, testing the primary for current is a better method than using the killivolt secondary but the Terms of Service for this site seem to be in conflict with attaching to the primary voltage. I am being cautious about this part of the conversation.
     
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Fair enough... And, Yes! I concede there are definite safety liabilities attendant to my suggestion :oops:

    With sincere contrition
    HP
     
  6. #12

    Expert

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  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Be sure to check this post out for a proper mad scientist lab. http://forum.allaboutcircuits.com/threads/now-this-is-a-real-ham-shack.107799/
     
  8. #12

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    Big insulator looking things (which are really Styrofoam) with cheap iron wire wound on them look real impressive. :D Tangled wires and some broken instruments with lots of dials and meter faces look good, too! Be sure to throw in some blinking lights. :)
     
  9. Yarnell_Dan

    Thread Starter New Member

    Mar 13, 2015
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    First, thank you for your responses. I was focussing on the primary but had concerns and hoped someone with more experience could offer up a small miracle. I always sleep on ideas a minimum of one night before plunging in tooth and nail and upon waking this morning decided an independent DC circuit with an oscillator was the safe and sane approach. You have firmly reinforced that decision. Yes, there must be lights. :D :cool: I'd still love to sync the travelling arc somehow with the displays though . . .
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You could add some small LDRs to a low voltage DC circuit hidden on the mess. The LDRs would monitor the spark position on the ladder and, as the spark travels, the LDRs cause the ammeter to move to a next position. You may have to hack the ammeter to get it to respond appropriately to the DC circuitry.
     
  11. Yarnell_Dan

    Thread Starter New Member

    Mar 13, 2015
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    Good idea but I don't think I could work LDRs into my concept without significant alterations. Once I get this operational (do not hold your breath! :D) I'll post some images. Thanks for the input people.
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the jacobs ladder works by heating the air which blows the spark up the "ladder. the clear insulator around the ladder should be open to air on top and bottom to alloow a chimney effect of rising heated air. my son built one a while ago using a clear 2 ltr pop bottle for the chimney.
     
  13. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I don't know about yours, but my Jacobs Ladder draws over 2 amps on the primary..And putting the meter on the secondary will smoke it as #12 said..
     
  14. Yarnell_Dan

    Thread Starter New Member

    Mar 13, 2015
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    @gerty: Yes, my primary current draw is similar. I'm already proceeding with constructing an isolated circuit to drive the "for show" meters and lights.

    @alfacliff: I'm fairly certain an enclosure vented as per your description is not a necessity. I've had my ladder energized within a bell jar for well over 10 minutes with no deleterious effects observed; however, I still intend to allow for some gas exchange in my final product - the bell jar bottom will not be sealed. I even vaguely recall someone, somewhere posting they had substituted assorted available gases (likely nitrogen and/or argon) with our Earthly vanilla atmospheric mix and ran a ladder in a well sealed chamber with resultant altered colors being displayed??? Also, I do not anticipate energizing my final product for lengthy periods of time or unattended.
     
  15. BR-549

    Well-Known Member

    Sep 22, 2013
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    Why don't you just watch a Jacob's ladder on youtube?

    Why have a Jacob's ladder that you can't hear and can't smell?
     
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  16. Yarnell_Dan

    Thread Starter New Member

    Mar 13, 2015
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    Hee Haw fan, eh? ;)

    It does sound much better with the bell jar over it! :cool:
     
  17. gerty

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    Aug 30, 2007
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    elect 050.jpg
     
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  18. gerty

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    That pic ^ came out huge, even covered the text. One of my students took the pic, almost looks like time lapse..
     
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