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- Nov 17, 2011
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I hadn't thought of using a capacitor to block the DC,
Did you read my first answer (no. 3 in this thread)?
The value of the capacitor should be chosen such that the impedance (Z=1/(2*pi*f*C)at the operating frequency (40 kHz) is considerably less than the impedance of the transformer. In the application without the H-bridge, an electrolytic C can be used. With the H-bridge the polarity reverses each cycle thus a ceramic or foil type should be used, no electrolytic.
The impedance of the transformer at 40 kHz cannot be measured by an Ohmmeter. The Ohmmeter will give yolu only the DC resistance. But the transformer is an inductor and as such will have a higher impedance at 40 kHz. You can measure the impedance by adding aseries ressitor between 555 an transformer and measuring voltage and current at the transformer. You will have to take into account the phase shift between V and I for correct calculation of the impedance, therefore an oscilloscope would be a very useful tool.