Understanding single transistor oscillator

  • Thread starter Guineafowl
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  • #1
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I see this circuit has many applications for creating high voltage from a battery source in a very simple and compact manner. However, I’m not sure of the exact basis of oscillation - is it:

1. Current flows through FB, turns on transistor.
2. Primary induces opposing voltage in FB due to opposite winding directions.
3. Since FB is off, transistor is off and cycle starts again.

Or is the mechanism to do with saturation of the primary?

Also, is the resultant waveform HFAC, or pulsed DC?
 

Answers and Replies

  • #2
.Scott
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You basically have it. In fact, if the primary saturates, it is possible that the oscillation would stop.
I would also note that:
1) Ultimately, the only current available to the base of the transistor comes though the 470 ohm resistor. So the average base current cannot exceed 10mA. It's probably a lot less than that.
2) voltage on the two caps are roughly 90 degrees out of phase with each other.

The data sheet for the transistor is here:
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00062867.pdf

The waveform will not be pulsed. The current through the transformer will have a DC component. Other than that, it could be close to a sine wave. Hard to tell without modelling the load, transistor, etc. in a full circuit .. or just building it.
 
  • #4
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Thanks both. The tube is incidental - I just wanted a pic of the primary side. If the output is AC, then great, but if DC pulses, wouldn’t the tube suffer mercury migration?
 
  • #5
NascentOxygen
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It looks like a battery-powered device, so it's reasonable to expect it has been designed with efficiency a high consideration, and this means switching transistor operation. It's probably designed to alternate between saturation and cutoff.

It is amazing to see how much can be achieved with a little ingenuity.
 
  • #6
.Scott
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Thanks both. The tube is incidental - I just wanted a pic of the primary side. If the output is AC, then great, but if DC pulses, wouldn’t the tube suffer mercury migration?
Because of the transformer, there would be no DC on the tube side.
 
  • #7
.Scott
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One of the critical parameters for the waveform would be the primary to feedback coil ratio. And that is not given.
The tube will need higher voltages, which means a pulse with rapidly rising and falling edges would likely require a smaller secondary winding.
On the other hand, a more rounded pulse would avoid saturations and likely deliver energy to the tube more efficiently.

I think @NascentOxygen is right. Because that transistor operates quite differently at different temperatures, they almost have to saturate it to make sure it will oscillate over the full range. And that would mean pulses - although, I believe, somewhat rounded ones.
 

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