transformer to step up 1V to 2000V at 100kHz

Discussion in 'The Projects Forum' started by moot, Nov 11, 2013.

  1. moot

    Thread Starter Member

    Sep 20, 2009
    46
    1
    I'm wondering whether I can take a 1 Vpp sine wave (frequency = 100 kHz), and step it up to a 2000 Vpp sine wave using a transformer. The load would basically be a capacitor (literally two thin electrodes spaced 5 mm apart, with a nonconducting dielectric crystal in between).

    The application is this: I have a "reference" sine wave from a commercial lock-in amplifier. It is 1 Vpp at 100 kHz. I would like to use it to switch a Pockels cell, to modulate the amplitude of a laser beam. The Pockels cell needs 2 kV. It acts as a high impedance load.

    Can this be done? What kind of transformer would I be looking for? (keywords?)

    I spent some time searching for transformers. I looked at power, audio, and pulse transformers. They all seemed to be tailored to other applications, such as stepping up/down line voltages or high power applications.

    Note: If my specs are an issue, I could possibly do with stepping 10Vpp up to 2kVpp at 10kHz instead.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    What about using your capacitive load with a series inductor so that it is at resonance? This will then be a series tuned circuit. If the Q is high enough you will get 2000V without a transformer.

    If you can't get the Q high enough then a combination of a loosely-coupled air-cored transformer and a tuned secondary with your load as the capacitor might do it.

    A few hundred turns on a toilet roll tube might be good place to start!

    A more sophisticated design might use a ferrite (probably a pair of U shaped cores) with a large air gap.

    Edit: I have attached a drawing showing some possibilities. If the load is a very low value of capacitance, you might want to add some extra in parallel so that the series inductance is a sensible value and stray or changing capacitance of the load is swamped out.
     
    Last edited: Nov 11, 2013
  3. moot

    Thread Starter Member

    Sep 20, 2009
    46
    1
    JDT - Good idea. I have a few capacitors rated to 10kV; I have to check, but I think they are around 0.1-1nF. I also have several toroidal ferrite cores. I'll see if I can play around with them today and respond with my findings. Thanks!
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    How about a two-step process? Take the 1V signal, pass it through a "power amp" (NFET?) to boost the signal to say, 50Vpp, and then use that to excite the primary of an aircore transformer with a more reasonable turns ratio.
     
  5. moot

    Thread Starter Member

    Sep 20, 2009
    46
    1
    For the preamp ("power amp"), could you imagine an N-type MOSFET doing this job? I have many 100V (and higher) N-channel MOSFETs lying around. I'm used to using them for switching, but here I imagine one would need to be operated in the "linear region" to get a sine wave output. I just haven't done that before.

    Here's an example of a linear JFET amplifier, but a MOSFET could be used, too (source):
    [​IMG]
    This could then drive a step-up transformer and/or high-Q resonant LC circuit.

    Thinking more about this, I think the series inductor is important. Without it, the primary's voltage would rise in response to the amplification from the high-Q resonator on the secondary. That would expose the function generator to a high voltage. The series inductor (assuming it's rated to HV) would protect the function generator.

    My initial check suggests that the Pockels cell has a capacitance of around 10pF (measured with an Agilent 34405A Digital Multimeter and 8inch leads). I'm going to see if I can improve that measurement.

    (Also - there's a chance that I won't see this through. I'm considering another path if the time investment for this project gets too great. We'll see.)
     
  6. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    And the resonant series inductor has another benefit: If the load changes - goes short-circuit, or if you touch it, the circuit goes out of resonance and the voltage collapses. So short-circuit proof and safer!
     
  7. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    That depends on how quickly the circuit rings dowm from its 2kV resonant voltage to the input voltage or below, you might still get a nasty RF burn if you touch the wrong part.
     
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