6v to 75v PSU diag

[ed731pdh]

This circuit diag popped up on a french magazine a while back, I've had the opportunity to re-draw the diagram for clarity.

The circuit was done on EasyPc, I've just upgraded my donkeys old DOS package as it would not operate under the latest Win environment.

Moderators, might be an idea to have an area put aside just for assorted circuit diags, build up a directory for others to reference.

[Skywave]

Hmm: that I do not like the look of. The HT bridge rectifier will produce at least 300v. across its reservoir capacitor. Between that cap. and the Zener diodes we have R5: 470 ohms. Calculation gives the power dissipated in R5 in the region of 100 watts! And a current through it of about 450 mA. On no-load (or a low load current) that 450 mA will flow through the Zener diodes. So each one will dissipate about 13 ~ 15 watts.
Unless my arithmetic is wrong, alarm bells are now starting to ring! I can just about accept that it will be necessary to mount the Zeners on a very substantial heat-sink, but what about R5? The problem is clearly the 'secondary' of the mains transformer: it needs to be a 12v - 120v item.
Could there have been a mistake in the drawing?

Al.

[pwdrive]

The transformer says 12 to 240 v but the multivib and driver runs off 6v maybe it's not 240v on the secondary?

Lawrence.

[Mark Hennessy]

Not sure how you're getting there, Al. Indeed, I'm not entirely sure how it's going to work at all, as the primary is not being driven at all well. I assume it's a 50Hz transformer...

OK, when Q3 saturates, there is 6V DC across the primary. But what happens when it switches off? Presumably C3 is meant to resonate with the inductance of the primary? If not, I can't see where the flux goes. Perhaps it's being used as a flyback converter? In which case, you'd only need half-wave rectification on the secondary. On the other hand, if there was another power transistor, driven from Q1, and a centre-tapped primary, then I might be happy.

Without building one and experimenting, I'd have to advise caution with this circuit. Certainly it has a low component count, but there are better and more efficient ways of doing it.

[ed731pdh]

Possibly, it was french, and I hasten to add that I have not had the chance to knock it up and test it. Mearly clean up the poorly printed original. Then again in reality the efficiency might be so bad etc.

[Alistair D]

I don't particularly like Zeners in parallel. Ensuring equal current distribution is difficult. Better to have devices of half the voltage in series. The power dissipated in them will still be the same.

Al

[Skywave]

Not sure how you're getting there, Al. Indeed, I'm not entirely sure how it's going to work at all, as the primary is not being driven at all well. I assume it's a 50Hz transformer...

Quite: I overlooked the simple fact that the sec'y. of the transformer is not producing a sine-wave. Thank goodness for that; I knew I had missed something somewhere: my calculated watts for R5 was just not believable!
So thanks for the correction. And like you, I don't care much for the design either. For example: Zeners in parallel: how can you be sure that there will be an equal distribution of current between them?

Al.