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Oscillator not oscillating

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archiees

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I built this circuit, which is a high voltage oscillator circuit from a published paper, on a PCB. I have connected a tuning capacitor equal to 500 pF. When probing the one end of the 10 turn coil, i measure DC voltage approx. 100 volts.
Why the tank circuit formed by the seconday coil and the tuning capacitor not osciilating.
 

echo47

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Something is terribly wrong if you measure 100VDC at that point. It's connected to a +215V power supply.
 

    archiees

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archiees

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Sorry,
i was not clear. I meant the collector of the BJT voltage when probed was approx. 100 volts. No ac component.
 

Borber

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10 turn coil should have full DC supply voltage if not oscillating. Check 215V supply without a load and when loaded with oscillator. When oscillator is not running it may draw significant current and overload supply. It could be a reason that you have only 100V. The reason why oscillator is not working can be wrong connection of transformer windings.
 

    archiees

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Thanks echo47 and Borber,
So i redid my coil connections, and great i see the 210 volts on the 10T coil. Someone suggested me to decrease the 200 resistors in the feedback with 6T coil. This would increase the voltage fedback to the emitter and might help.
Is there a way to check my coil connections while it is connected in the circuit.
 

archiees

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Great Guys,
My Oscillator is working great as i increased the feedback by changing the 200 ohms resistors in the 6 T feedback coil to 100 ohms.
Now, i want to put some heat sinks on the transistors. Can anyone suggest how to calculate the power dissipated in the BJTs if my secondary coil output is 1000 V p-p?
Thanks
 

Borber

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You may measure current from 215V supply then divide it by 4 and multiply with supply voltage 215V. This way you will get approximation of power dissipated in each oscillator transistor 2SC5392.
 

    archiees

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archiees

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Borber,
Should i measure the rms current? Also this circuit is on a pcb and the 215 volts comes from a voltage regulator (circuit attached). Still to measure the current i think i can put the ammeter in series with the 215 volts coming from the voltage regulator. Do you have any suggestions?
 

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Place ammeter in series with 215V terminal. Average current measurement will be good enough.
 

    archiees

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Thanks a lot Borber,
I did my dc current measurements at the 215 volt supply. For Output voltage 400 p-p at the secondary of the transformer, i measured the DC current drawn is around 113 mA. So the power dissipation comes out to be 6 watts/ transistor.
I might sometime use 1000 volts p-p output, for that case the Power dissipated/transistor wld be approx. 10 watts.
I have also starting using BUH51 instead of 2SC5392 which have gone obsolete. THe BUH51s have Pd = 50 watts @ 25 degree C.
Do i still need the heat sinks, as the actual power dissipation is below the limit? Thanks again for your help.
 

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You still need heatsink. Data 50W@25°C means that allowed transistor dissipation is 50W when the transistor temperature is kept at 25°C. This is not the case at all. Without heatsink transistor can survive dissipation up to 0.6W I think. Look at the bottom picture on:
**broken link removed**
and you will see even a fan in the vicinity of the board.
Total dissipation of 40W shurelly need forced air cooling.
 

    archiees

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thanks for your comments.
I did my measurements of current as i told you. Now, i want to calculate the dc and the ac current in emiitter, collector and base of BUH51. Is there a way to estimate it analytically, to justify the trace widths on the PCB?
Please help.
 

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If you do not need exact analisys of oscillator you can say average value of collector current is 113/4=28.25mA and peak value is 2*28.25=56.5mA. RMS value of collector current is about 37mA.
 

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