Make cool gadgets from a deceased fluorescent lamp

Step 4 End result.

This is my version of the circuit.

Comments

[hd196]

This site has spms step down regulators schematics with only 2 transistors:
http://www.romanblack.com/smps/smps.htmhttp://www.romanblack.com/smps/smps.htm

very interesting

[acmefixer]

I built his very interesting circuit. But I found one problem with it, The circuit depends on itself for oscillation. If you put a few hundred milliamps load on it while it is running, it will work. But if you then turn it off, and turn it back on, it may not oscillate. Instead of getting 5V out, you then may get 12V out, and that is VERY bad.

If you use his circuit, you should include overvoltage and overcurrent protection.

[hanelyp]

Looking over the diagram, if the output voltage is above the zener voltage minus the base-emitter drop of Q2, Q2 is shut off, denying base current for PNP Q1, turning off the feed from the input voltage.

[acmefixer]

I didn't investigate to find out the cause. One thing I remember is Q1 got hot so I decided to use a heftier transistor, might have been a BD135.  It may be that the emitter to base breakdown voltage of Q2 is being exceeded, and it starts to conduct.  Perhaps you could build one and see for yourself.

[hanelyp]

Replacing a BC327 PNP transistor with a BD135 NPN transistor would be expected to not work.

[acmefixer]

My error, I should've said BD136.

[unknownpocketnerd]

Yes, they are great. Just 2 transistors and it is self oscillating.