OT: transformerless step-down DC power supply

It makes sense to me if the input current is AC, even though the symbol implies DC.

OK thanks Tom. I was wiggling the current both ways but could only make it flow through L1 and L4. I'm missing something stupid... which is typical.

George H.

What values are you using for those inductors?

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Thanks, Jim. You've always been very generous in your assistance with various things here over the course of the last 20+ years. Bitrex (who is KF'd here and only visible in TP's follow-ups) completely misunderstood the question - or rather inferred things into it that were not relevant. For the avoidance of doubt, this is not a PSU that I'm creating for anyone else. It's for *me* only and for one specific purpose only, where all the safety issues some folk are obsessing over will simply not arise. Thanks again.

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I did state in the original post that the design did not consist solely of diodes and I suspect trying to come up with a step-down power supply built from diodes *alone* would prove a completely fruitless endeavour!

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What Cursitor Doom wrote was "consisting of little more than a network of plain silicon diodes", which doesn't give much insight into what he had in mind.

"Step-down" implies either a step-down transformer or a stack of capacitors that get charged up in series and discharged in parallel (which is lot trickier to make work than winding a step-down transformer).

The most likely explanation of his post is that he has completely lost his marbles.

--
Bill Sloman, Sydney

Well, there's some hope that this was what was intended:

which is voltage-dropping as long as you make sure there's a good load on the end terminals (a zener would be ideal, but parallel resistor and capacitor would also work). For minimal parts, use a Zener for D1, omit D2, and let it ripple.

It's not dissimilar to some of the LED driver schemes, suitable for replacing fluorescent tubes in an oldstyle ballasted lamp.

No, that's nothing like it. Thanks anyway. BTW, what is that facility you referenced? Does it enable users to produce schematics that others can view? I'm rather fed up with trying to do that kind of thing using ASCII characters.

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It's DigiKey's SchemeIt tool; the thing is very nice for making simple pretty-pix, and (naturally) supports BOM from the DigiKey catalog... because it's cloud-based, you can read/write from any computer, anywhere, without worrying about software (other than a browser) being able to do the edit or display. Just remember your account info to log into your own little private saved files (or make public ones, with the 'export' feature).

Useful at work for simple test jigs that just have to be documented, but not easy for complex schematics.

More info here:

Worth knowing; thanks, dude. ;-)

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Right, because the voltage flies back to keep the inductor current running in the same direction.

How does this function as a current multiplier though? I still don't get it.

Clifford Heath.

Yup, and the waveform (for square wave input) looks chopped up: the first inductor gets a full square wave, the second gets chopped a bit shorter, the third is flybacking off /that/, and the Nth gets a measly 1/N the pulse width.

Just as the currents in a C-W taper off, because the first cap delivers current to the whole string, and the next one to everything but that, and so on...

...For the same reason, tapered inductor values (smaller at the start, larger at the end) would be desirable to improve performance.

The currents in a C-W may not be as intuitive, because you're probably not used to seeing them measured. But that's precisely what you measure (capacitor current inductor voltage) after the transformation. Fun, huh? :)

All the inductors carry current to the output, cranking it up as you go. At no load (shorted output), each one carries the same current*, so the output is the sum of all those currents. Simple, right? ;-)

*Give or take half vs. full currents, because there's an even/odd thing with the stages.

Tim

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Hah! Nice. I hadn't realised that they all parallel to the output. It seems obvious now.

Clifford Heath