# Understanding what happens in transformer with a center-tapped primary

In the above schematic of a timing light made in the 60's by Heathkit, there is what amounts to an oscillator, a step-up transformer, a capacitor, and a xenon lamp. The xenon lamp fires when the high voltage from the car's distribution coil passes a threshold enough to establish rapid conductance, at which time the xenon effectively shorts the capacitor, creating a bright flash.

In this design, I can see where positive current comes in from the transistor's collector on the 'top' of the primary, and moves half-way down the primary to the grounded center tap. However, there is also the additional bottom half of the primary that ends up going through a capacitor and to the base of the transistor, no doubt to get it to oscillate.

In the case of a center tapped secondary, the secondary would produce two waves - one at the top of the secondary coil and one at the bottom, out of phase with each other:

My question: Is what's happening here that current is being passed through the top of the primary, and the bottom of the primary is simultaneously conducting, in opposite phase? Or, is a totally different phenomenon happening: is positive current coming down through C1 and C2, and entering the bottom part of the primary?

If the former, this throws my brain off a little because it makes intuitive sense that in the secondary-tapped transformer, all the input current is across the primary, and it is coupled entirely to the secondary magnetically, whereas in this case input current is placed across the top half of the primary, and must be magnetically coupled to both the secondary and the bottom part of the primary coil. (Correct?)

Another question: In either case, what are the specific purposes of C1, C2 and R3?

Here is a waveform capture. Channel 1 is the bottom of the primary (point D on schematic), and channel 2 is the collector of the transistor (input to the top of the primary).

Thanks

My question: Is what's happening here that current is being passed through the top of the primary, and the bottom of the primary is simultaneously conducting, in opposite phase?

The primary can be broken into two windings that share a common node: -

simulate this circuit – Schematic created using CircuitLab

That common node is grounded and therefore the only relevant question that needs to be asked is not about current but about voltage: -

Is the voltage at node $$\\color{green}{\boxed{C}}\$$ in phase or out of phase with the voltage at node $$\\color{red}{\boxed{D}}\$$.

Because a BJT in a common emitter configuration produces a collector voltage that is the inverse of the base voltage, the transformer has to do the same to ensure that there is an overall 360° feedback i.e. the feedback is positive and causes oscillation.

Another question: In either case, what are the specific purposes of C1, C2 and R3?

C2 blocks low frequencies thus ensuring that the oscillation frequency is not below a certain value. C1 shunts higher frequencies to ensure that the oscillation doesn't occur at too-high a frequency. Somewhere between "too-low" and "too-high" is the goldilocks point where the circuit is most likely to oscillate i.e. where the sum of the voltage at node C and the voltage at node D is precisely 180° compared to the collector voltage. That's the frequency of oscillation.

R3 ensures that there is just enough negative feedback to keep gain sufficient for oscillations to be sustained without too much distortion.

• Thanks for your detailed answer Andy. You are truly an expert. I think I'm still stuck on 2 issues: You mentioned this was common emitter. Forgive me for asking, but isn't this common base (since the base leg is common to the input voltage source and output xfrmr)? Also, the voltage at point D is the result of the diode drop across the transistor's emitter/base. Are you saying that current flow isn't particularly relevant in analyzing this and to only see voltages? I ask because I see the base current as being so limited in value, but perhaps I'm not thinking about the problem correctly. Aug 30, 2021 at 17:55
• No, it's CE because the output is the collector and the feedback is to the base. Emitter is stuck on +15 volts. A common base would have feedback to the emitter (go figure!!). The DC voltage on the base is as a result of the internal diode of E and B but the AC signal at D is as a result of the AC signal at point C. Aug 30, 2021 at 18:18