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circuit design help...

Discussion in 'General Electronics Discussion' started by 777timer, Sep 4, 2012.

  1. 777timer

    777timer

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    Sep 4, 2012
    I am doing this project where I am running a 350kHz sin wave through a MET-23 transformer (backwards....I am using it to boost voltage up to around 200V). The issue i am running in to is that when ever i connect wires or a connector to the output of the transformer, the voltage just goes down ( I reach my peak to peak voltage goal, but once I attach wires that ar enot even attached to anything, the voltage drops down). Is this an impedance issue? Maybe the line (it's a 1 foot self twisted cable pair) is creating a big cap? The transformer is being fed with an LT1010 opamp. It's max output is 350mA, could it be a power issue?
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi,
    1) what voltage and current are being supplied to the transformer ?
    edit -- ok current in is 350mA maximum which infers its probably less than that

    2) what current do you require at 200V out of the transformer ?

    3) the transformer capable of supplying (operating at the) required current ?

    voltage drop under load most commonly refers to a lack of current capability

    Dave
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
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    Nov 17, 2011
    How much is the voltage drop?
    At 350 kHz the wires' capacitance to ground is not negligible. Some current will flow through this capacitance, thus loading the transformer and, as Dave said, the voltage will go down.
    If you have an ammeter capable of measuring at 350 kHz (which could be a problem) you could attach one probe to the transformer, leaving the other probe lie on your workbench. See if measurable current flows.
    But you will see this effect anyway as soon as you apply a load to the transformer: the datasheet gives an impedance of 1.6kOhm for the primary (in your backwards application this is the secondary). This means a voltage drop of 1.6V/mA from the impedance alone. Plus the transformer isn't designed to transfer power, so effetcs like saturation of the core will further reduce the output voltage under load.
     
  4. 777timer

    777timer

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    Sep 4, 2012
    I am getting something like a 50V drop. I am just puzzled because the drop just happens when I add the line with nothing at the end, so it is an open. I am thinking that the line is acting like a cap.

    The load will require little power, below 1mA.

    Is there anything i can do to counteract this?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
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    Nov 17, 2011
    Use a larger transformer. The transformer you are using is not designed for this operation.
     
  6. 777timer

    777timer

    4
    0
    Sep 4, 2012
    Well, I tried other transformers, and this was the only one that would work.All the others distort the signal or do not output enough voltage....

    Anything I get is running the transformer out of spec...so that is the biggest hurdle.Do you know any company that makes these that do not expect the secondary voltage to exceed 300V? It seems like everything wants to go to more than 1kV.

    As you can tell from the specs, I am trying to get in excess of 200V from a low voltage (7V) source. It's really pushing it, but I do not see any other way of doing it. Any ideas?
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
    1,846
    Nov 17, 2011
    Sorry, I don't know. But if the transformer is rated for 1kV, it can be used for 300V, too - unless it is too big for your project.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I think your suspicion is right. The problem is caused by stray capacitance, because of the combination of relatively high frequency and high voltage. At 350 kHz even small amounts of capacitance represent a significant load reactance, and for a fixed load reactance, the wasted power goes up as the square of the voltage. Your little op-amp running at low voltage will have no hope of doing what you want, and the transformer characteristics are extremely important.

    There is a similar issue with backlight driver inverters in LCD TVs. These run at less than 50 kHz, but they generate over 500V RMS. They use special transformers that are designed for extremely low stray capacitance, and the PCB layout for the HV sections are very sparse. The transformers are driven at 24V by moderately grunty MOSFETs.

    The combination of high voltage and high frequency raises a lot of problems. Maybe you should look into valve (tube) technology!
     
  9. foTONICS

    foTONICS

    332
    9
    Sep 30, 2011
    i agree, you're probably just going beyond the VA of the transformer
     
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