Single Coil Two Transistor Boost Circuits

[Farmhand]

Hi all, I found that this circuit works ok for boosting from a "single cell type voltage" to over 5 volts,
I can charge 3.7 volt cells with it.

This circuit is a variation of the "Stingo" circuit, I simply used a capacitor in series with the upper pnp transistors base and
used a variable resistor across the capacitor to adjust the frequency/power.

Feel free to share any single coil "feedback" type oscillators here.
..

[Vortex1]

This is the elementary schematic for the circuit I posted some time ago. I have not included values as they will change over a wide range depending on the application.

There are many enhancements to this circuit that are valuable. I will post them in time.

Note that the "stingo" circuit is different in that the pnp is connected C-E in reverse in most all of the google images I have seen. I don't know how that can work, but I haven't tested it yet. Is this an error by Sucahyo?

The schematics I posted are shown as led pulsers but can be simply converted to boost converters in the usual manner.

Note: For operation, a current limiting resistor should be in series with R1 or R3 so as not to blow the npn B-E junction at an extreme pot setting.

[Farmhand]

Yeah Vortex I think you're right, it looks like a mistake in the Sucayho drawing.

Here is the wave forms produced at the NPN collector in yellow and the NPN base in blue.

It's using 31.6 mA from 1.25 volts and working into two tiny 20 mAh 3.7 volt batteries for the shots, charges them well too.
..

[Vortex1]

It is fairly straightforward to calculate the operating characteristics of the circuit, considering the current gain in each transistor, then the maximum npn collector current can be known and this will be the point where the  inductor ramp current switches state.

I have been trying a number of things to avoid current wasteful "hard turn on" of the pair.

I have used a variable resistor in the collector base connection which seems to work.

Another method is to turn the pnp into a limiting current source. With drive limited to 1 mA at the base of the npn, and considering a gain of 100, then 100 mA will be the switchpoint for inductor current.

The simplest is to just limit the base drive current to the pnp by a suitable resistor in series with the pot, but then the resistor in series with the cap becomes the peak current adjustment limit.

[TinselKoala]

I know this one doesn't meet the criteria, but when I look at Sucahyo's circuit I just have to respond somehow.