The output is a reasonable-ish sine wave. The waveform on the drains is rather like a class C output stage with some ringing part way up and down the waveform. Fets are on small bits of ally for heatsinks but they are not much more than tiepid. I realised what the problem was after I closed the PC or at least I think so.
The problem is that at resonance the inductive reactance is zero so that the signal voltage is across the low R of the coil,hence the high I and power dissipated in the
coil. The coil with the higher no of turns (30 as against the present 12) did not get hot at all, just warm. The only thing I can think of is to make another coil with just a few more turns so that the resistance increases a bit. If I was to use thicker wire the problem would be worse, I seem to remember that some single turn coils made from tubing have to be welded or bolted together because the solder would melt.
I am rather green as regards Fets and their biasing requirements etc. The cct is extremely simple. one drain is connected to the other gate with a cap. the gates have a 220r and a 10V zener to 0V, the drains are connected to the ends of the Tx coil, the CT of which goes to the supply. The tuning cap is 3 x 0.33uFd in parallel across the coil. I'll draw the waveforms on the gates and drains tomorrow.
Mike
The problem is that at resonance the inductive reactance is zero so that the signal voltage is across the low R of the coil,hence the high I and power dissipated in the
coil. The coil with the higher no of turns (30 as against the present 12) did not get hot at all, just warm. The only thing I can think of is to make another coil with just a few more turns so that the resistance increases a bit. If I was to use thicker wire the problem would be worse, I seem to remember that some single turn coils made from tubing have to be welded or bolted together because the solder would melt.
I am rather green as regards Fets and their biasing requirements etc. The cct is extremely simple. one drain is connected to the other gate with a cap. the gates have a 220r and a 10V zener to 0V, the drains are connected to the ends of the Tx coil, the CT of which goes to the supply. The tuning cap is 3 x 0.33uFd in parallel across the coil. I'll draw the waveforms on the gates and drains tomorrow.
Mike
