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Can't have high voltage function generator. What about low voltage Tesla coil?

  1. Dec 22, 2012 #1
    According to many knowledgeable people on the internet, an affordable "high" voltage function generator does not exist. I would have liked to have had electrodes operating at >50v with frequencies up to 15 Mhz but it seems such a function generator is out of my reach.

    So I want to know if a "low" voltage tesla coil (below 150v) would be a working substitute. I would love to know your thoughts.
     
    Last edited: Dec 22, 2012
  2. jcsd
  3. Dec 22, 2012 #2
    I should add I am only interested in the electric fields produced, as far as the similarities between a function generator and a tesla coil goes
     
  4. Dec 22, 2012 #3
    What kind of electric field do you want as a function of time?
     
  5. Dec 24, 2012 #4
    right, i should have specified. Im trying to get a sinusoidal wave. the purpose of this is to do a project involving dielectrophoresis, so a simple sine wave will do. I also thought about making a 100:1 ratio transformer to turn 10 volts into 1000 volts, but I suspect it will be complicated to keep the integrity of the field at high (20mhz) frequencies after transforming the voltage.
     
  6. Dec 25, 2012 #5

    gneill

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    At those frequencies and voltages you're looking more at radio transmitters than function generators... Google a copy of the Frequency Allocation Chart for your country to see what services/agencies/organizations you could potentially interfere with if signals manage to leak.

    Hopefully you only need microwatts of power? Be prepared to implement grounding zealously, and Faraday cages if required!
     
  7. Dec 26, 2012 #6
    I imagine after ramping up the voltage 100x, the current will be ramped down 100x, thus it will be relatively safe. I only need a rapidly alternating electric field strong enough to affect dipoles up to 5 inches away from the electrodes.

    I'd rather have a frequency generator than a radio transmitter simply because i have no need to modulate anything, and a radio transmitter only has a narrow band to work with, and i wont know where to plug in the wires for my two electrodes. My brother is willing to get me a decent function generator (~$300) as a late christmas gift

    But my main concern isnt safety, its whether i can actually ramp up the voltage with a simple iron core transformer. I've heard a lot of talk about people needing amplifiers for their function generators, but I dont think i need to go that route because the current is not important for generating electric fields
     
  8. Dec 26, 2012 #7

    gneill

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    I think that an iron core transformer will be unsuitable for the frequency range you have in mind; parasitic losses in the core will bite you as the frequencies get higher. An air core transformer is probably the way to go, something you might tackle yourself. As the impedance of inductors goes up with frequency, you might be looking at winding several coils to cover different frequency ranges. It's the same issue that crops up in radio tuning section design (analog).

    While it's true that stepping up the voltage on the secondary side of a transformer means proportionately lower current on the primary, you still need to consider the power required to drive the load. This may increase significantly as the frequency goes up if the impedance of the load drops. That is, current requirements may increase with frequency. If the load is mainly capacitive (parallel plate electrodes?), can you estimate the impedance from the geometry? Do samples between the electrodes introduce significant dielectric effects?
     
  9. Dec 27, 2012 #8
    hmmm, hopefully i can just buy some suitable transformers from the internet, but if i have to build my own, i suspect 99.9% of it will be trial and error. are there any resources available to help me cut out some of the guesswork? are commercial wires standardized with universal levels of impedances or will i need to use a voltmeter?

    this is my first time actually "engineering" anything, but im learning that all the theory i learned is grossly misleading
     
  10. Dec 27, 2012 #9

    gneill

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    There are coil-winding calculators and wire tables on the web. The wire tables will give you diameters and resistivity (Ohms per meter) for standard wire gauges. Take a look at some of the "Tesla coil" projects to get an idea of what others do when they wind coils.

    I think, though, that you should get a better handle on the parameters of your project before you start winding coils. Perhaps play with a circuit simulator first (such as LTSpice, which is both powerful and free!).

    Driving a high AC voltage across even a teensy capacitance (your probe load) will entail a relatively large current (at a guess, several milliamps). That'll translate to maybe a few amps on the primary! (I misspoke before when I said that the current would be lower in the primary --- it's power that's conserved, so V*I on the secondary = V*I on the primary). I'm not aware of a function generators that'll put out several amps without using an amplifier of some sort.

    You should start by trying to characterize your "load" as best you can, starting with all the physical dimensions and known properties. That'll give you at least a ballpark estimate for a physical model (electrical equivalent). Then calculate (or simulate) your desired voltage driving this load and check the current.
     
  11. Dec 27, 2012 #10

    berkeman

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    To add to the points that have been made already...

    To make a 15-20MHz sine wave at 150Vpp or so, you could use a signal generator and follow that with a HAM radio power amplifier (you can buy kits for these as well as buy pre-built ones). A 50W power amp for the 15m band (20MHz) would put out about 61Vrms into a 75 Ohm antenna load. That gives you 2*√2*61Vrms = 173Vpp.

    But there are several problems with that approach. First, RF power amplifiers are designed and built to drive the load (a 75 Ohm antenna in this case), and they generally will blow up if you try to drive an open circuit (because the output transistors are not rated for the higher output voltage that you get when you drive an open circuit). You might be able to find amps that are rated to still operate into an open circuit, but I'm not sure your sine wave fidelity will be very good. Is that important to you?

    And as already pointed out, when you are working with RF signals in your lab, you have to be *very* careful that they do not become propagating EM waves outside of your lab, or you will get in serious trouble with the FCC or applicable government body where you live. You also put equipment in your lab at risk, since interfering RF fields can cause nearby electronics (like computers) to fail, sometimes permanently.

    So whatever means you use to generate these HV RF signals, you need to have safety considerations in mind (shock hazard) and RF Interference considerations in mind. Do you or somebody you are working with have the experience to deal with those two issues? Are you wanting to do this research at your school's lab, or is this a home project?

    Do you need the waveform to be sinusoidal? Or can it be triangular or squarish? Tri and square waves will cause even more RF interference issues due to harmonics, but are much easier to generate. You could build a tri wave HV amplifier that is driven by the signal generator fairly easily with a HV power source and some HV transistors, for example.

    Can you say more about the size of your test cavity and its shape? The smaller you can make it, the lower the voltages you can use, and the less power you need to drive with the amplifier...
     
    Last edited: Dec 28, 2012
  12. Dec 29, 2012 #11
    im really grateful for all your help, admittedly i know very little about this. i dont even know what kind of metal my load is made of (atwometal miniature wire fence looking sheets, separated at a fixed distance by some cardboard wrapped in electrical tape), but youre saying i wont get the fields i want without a more power current, and a more powerful current means risk of damage electric equipment in the room... and on top of that using an amp on an open circuit could cause it to blow? how does an open circuit differ from a closed circuit to cause that?
     
  13. Dec 29, 2012 #12

    berkeman

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    Thread closed temporarily.

    Look, you said that you are working on an dielectrophoresis project, which understandably involves HV RF E-fields. But, you have repeatedly demonstrated a very worrisome lack of understanding of HF and RF and even basic electronics. We do not help people with dangerous projects here on the PF. We can help people with the correct background and experience and with professors/Mentor supervision to do more advanced projects.

    So, you need to PM me with much more explicit details of what you are working on, what school you are attending, and what the details are of your current project. It's just too easy for you to hurt yourself or others without the appropriate supervision.
     
    Last edited: Dec 29, 2012
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