single transistor osciillator with feedback coil

[TheMartian]

hey
i want to make power supply from this schematic:

I tried transistors:
- BDY25
- 2N2222
- 2SC867
And I tried transformers:
- old from TV like on picture (schematic)
- self-wound small transformer from ATX power supply
I power it from 13.5V and have 1000uF cap at circuit.

In every confiiguration (except TV transformer +2N2222, where 2N2222 burn instantly) I can hear metalic pitch sound about every second. It's like circuit tries to oscillate but fails. There is no vvoltage at output. What's wrong?

 

[betwixt]

It isn't the cleverest of circuits but it should work. The most likely reason it would fail is if one of the primary windings is reversed. Try swapping the wires between the collector and '+'. Don't swap the base wires as well or you will be back where you started from.

Brian.

 

[barry]

Dont you at least need a resistor or capacitor somewhere in the circuit?

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Dont you at least need a resistor or capacitor somewhere in the circuit?

 

[betwixt]

I think its relying on the low impedance from the bias resistors to build up enough voltage on the feedback winding but it is far from efficient. A capacitor across the bias source and a clamp diode across the transistor B-E would help. It relies entirely on the transformer characteristics to set the frequency so the number of turns will make a big difference to the output. However, I would guess with the windings the right way around it would probably work to some degree.

I had to make a super-cheap 75V inverter for a special lighting job last week and did lots of experiments. With self oscillating designs like this one I couldn't get more than around 60% efficiency but using a simple 555 timer and MOSFET driving a tuned transformer I eventually managed 98% !

Brian.