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Impedance Matching Transformer

  1. Nov 6, 2008 #1
    Hi All,

    I am building a circuit that i need to get a better understanding of before jumping in head first, it says it requires an Impedance Matching Transformer.

    I don't get this with the Attached Circuit, is it the whole Primary circuit has to match the whole Secondry Circuit Impedance including the work coil??

    Any advise would be appreciated.


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  3. Nov 6, 2008 #2


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    The circuits is showing the 20:1 matching transformer.

    Just curious. What are you making?
  4. Nov 6, 2008 #3
    It's for a small Induction Heater, I may be way off but the thing i Don't get is surely the impedance a 40 turn primary is going to have a different Impedance than a 2 turn secondary.

    So is it the Impedance of the whole Primary Circuit Vs the Impedance of the whole Secondary circuit.

    Or is it just the Transformer that has to have the same Primary & Secondary Impedance which is what i don't get.

  5. Nov 6, 2008 #4
    are you trying to model the impedance in one lump on the primary side? it's been a very long time, and my brain is a little fuzzy, but you can "reflect" the impedance back through the transformer for purposes of analysis if that's what you're looking for.
  6. Nov 6, 2008 #5

    Just trying to understand things better instead of just making making it without understanding things correctly.
    It's the Impedance matching transformer bit i can't get a handle on & what has to match what.

    This is the site the Schematic comes from:

  7. Nov 6, 2008 #6


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    Here's the deal: Your induction heater requires a certain number of watts to do the job. The 'work coil' is just a few turns and as required by most induction heaters will need to be driven with a high current low voltage source. You do not have that. You have a high voltage low current source, or at least relative to what the work coil needs. To remedy this a transformer is used. Generally, any transformer is able to transform impedance. Think about it for a minute. It may help to visualize impedance as a ratio of voltage to current.
  8. Nov 7, 2008 #7


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    Also, you should put a large value bleed resistor around each of the input center-tap-forming caps (the two caps between the rails and the bottom of the input winding), to better define the center tap voltage. Relying on the leakage current of the caps alone will not give you a reliable 320V/2.
  9. Nov 7, 2008 #8
    Thanks to everybody that has helped with this, i now have a better understanding of the workings of this particular setup.
    I will note the use of the bleed resistors as suggested.

    Thanks Again
  10. Nov 8, 2008 #9


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    Because the output loop is resonant there should be some energy storage in the output loop (like riding a swing).
    It will be important to keep the output loops resistance as low as possible.
    You will probably need to adjust the drive frequency, depending on what object is in the heater coil, before applying full drive power.

    Also, I'm not sure that a class B drive is the best choice for this application.
    There should be some similarity to transmitter output stage design here.
  11. Nov 8, 2008 #10
    Could you explain more about the drive please, you mention class B drive???

  12. Nov 8, 2008 #11


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    There are four different classes of output circuits.
    A, B, C and D
    Generally in order of increased efficiency and increased distortion.
    The particular circuit example you posted is class B.

    I suspect class C might be the best choice for your application.
    Using class D (square wave switching) will destroy any nearby radio, tv or cellphone reception without very careful attention to detail.
    Note that this will be a problem with class B or C as well, just more constrained to specific frequencies.
    RF heaters will act as a transmitter unless you pay attention to proper RF shielding.
  13. Nov 9, 2008 #12


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    I would say model the output transistor stage after a switch mode power supply. That IS what the circuit in question is. With proper power supply decoupling and shielding a switcher can be quite noise free. It is not uncommon for switchers to run above 125 Khz.
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