Understanding single transistor oscillator

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Discussion Overview

The discussion revolves around the operation of a single transistor oscillator circuit, particularly its oscillation mechanism and the nature of the output waveform. Participants explore theoretical aspects, practical implications, and potential applications of the circuit.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the oscillation mechanism involves current flow through the feedback (FB) winding, which turns on the transistor, leading to opposing voltage induction in the primary winding.
  • Others suggest that saturation of the primary winding could stop the oscillation, indicating a complex relationship between the transistor's operation and the circuit's design.
  • It is noted that the average base current of the transistor is limited by a resistor, potentially affecting the oscillation behavior.
  • Some participants argue that the output waveform may not be pulsed and could have a DC component, while others believe it produces pulses of secondary current flowing in one direction.
  • Concerns are raised about the potential for mercury migration in tubes if the output is DC pulses, although some assert that the transformer design would prevent DC on the tube side.
  • One participant highlights the importance of the primary to feedback coil ratio in determining the waveform characteristics, suggesting that a smaller secondary winding might be necessary for higher voltages.
  • There is a mention of the transistor's temperature dependence affecting its operation, implying that saturation may be necessary for consistent oscillation.

Areas of Agreement / Disagreement

Participants express differing views on the exact mechanism of oscillation and the nature of the output waveform. There is no consensus on whether the output is AC or DC, and the discussion remains unresolved regarding the implications of these characteristics on circuit performance.

Contextual Notes

Participants note limitations in the discussion, such as the lack of specific parameters like the primary to feedback coil ratio and the absence of detailed modeling or experimental data to support claims about the waveform characteristics.

Who May Find This Useful

This discussion may be of interest to those exploring oscillator circuit design, particularly in the context of single transistor applications, as well as individuals interested in the practical implications of circuit behavior in relation to component interactions.

Guineafowl
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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?
 
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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.
 
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Guineafowl said:
I’m not sure of the exact basis of oscillation
I think it is a blocking Oscillator. See this description; https://en.wikipedia.org/wiki/Blocking_Oscillator
Guineafowl said:
Also, is the resultant waveform HFAC, or pulsed DC?
It will produce pulses of secondary current that flow in only one direction through the tube.
 
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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?
 
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.
 
Guineafowl said:
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.
 
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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