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Transformer design

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szzuk

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I need to make a new transformer. I designed and made some transformers a few years back, but I've now forgotten what to do. I still have spare cores, bobbins and wire but I'm scratching my head on what I need to do. The circuit is Amplifier - transformer - piezo. The amp is the OPA548, it can supply around 50V and 5A. The piezo is 1.2uF with a resonant frequency around 25KHz. The amp and piezo work with 50V so there is nothing wrong with them it is a simple matter of me needing 100V across the piezo.

The physicality of making the transformer isn't a problem. The turns ratio needs to be 1:2. But what do I need to do? What sort of inductances do I need?

The impedance of the capacitnce of the piezo at 25KHz is about 4 ohms, I've not tried to measure the real resitivity of the piezo but usually it is around 6 ohms. So I'd say the piezo had an overall impedance of 10 ohms at the resonance it operates at.
 
The transformer primary inductance determines the magnetizing current at the operating frequency. For example, for a magnetizing current of 1mA @ 25kHz and 50V, the inductance should be ≥318mH.

The transformer inductance has nothing to do with the load impedance.
 
Hi Crutschow,

Thanks for the reply. Are there some calcs you could point me towards? I've tried a few transformers I have around but none are doing much, so I would like to know how to match impedance.
 
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I guess I could make one with a primary over 318mH and secondary of 4 times that, it will work. I'm not sure how much current I need, I haven't measured that.
 
I seem to remember I set up resonance between the secondary coil and the piezo at the resonant frequency of the piezo. It seemed to work but I'm not sure why?
 
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