Why AC/DC **REALLY** rocks

[Mark Hammer]

People often post queries about needing/wanting to get a higher supply voltage than 9v.  The standard recommendation is to use some sort of voltage multiplier like a 7660 or one of the MAX chips, or something using a 555.  All of these solutions use a clock to produce an AC voltage that can then be summed in productive ways to yield a higher output voltage.

But the problem is that these chips are generally limited in terms of how much current they can supply, and that will often be part of the responses, and a source of disappointment to the OP, who is probably looking for something to provide 50ma or more.

So, yesterday, I'm looking for something to browse through during the commercials, while I'm watching TV with my wife, and grab a little Sams paperback called "99 Electronic Projects" By Herb Friedman, from 1973 to keep me company.  I figure there's gotta be something simple-but-interesting in there.  And sure enough, I was right.

If a person wants to multiply a supply voltage by means of using an AC source, you need look no farther than your wall.  Well, not your wall current exactly, but a stepped-down version of your wall supply.  As Friedman shows below, take the output of whatever AC wallwart you might have, and use the standard series of diodes and electros to produce a multiplied version of that, but without the traditional current limitations of the 8-pin multiplier chips.

So someone who wants a 24v DC supply, but only has a 9VAC wallwart on hand, can simply tack on a multiplier circuit, maybe a zener or a coupla series diodes to drop things down to exactly 24v, and voila! 

The output is, of course, unlikely to be as ripple free as the much higher-frequency clock of the multiplier chips, but you can fix that with the appropriate regulation or simple cap-smoothing on the DC output side.  And I imagine someone is goin to respond to this thread with some better suggestions about the caps, and some specific diode part numbers.

Just one of those things that makes you smack your forehead and mutter "Well, DUH!!".  Thanks, Herb.  (Snappy bowtie, by the way.  )

[Morocotopo]

Great find! could be useful for those circuits that require a wall wart, I´m thinking tube preamp, for example.

[Gus]

Mark

I posted some time ago about a tube microphone article that has a clever power supply that uses a 24VAC transformer to supply about 95VCD and the heater voltage.

http://www.diyfactory.com/projects/royerproject/royermod_2.pdf

[caress]

finally a good reason to get at all of those crummy thrift store AC adapters!   

[Slade]

This is very cool, thanks Mark.

[frequencycentral]

Gosh, I thought this was general knowledge or I would have said.    Used a lot in various tube amps. And is also the basis underlying MAX type voltage multipliers.

[CynicalMan]

Gosh, I thought this was general knowledge or I would have said.  

Same here.  I just came back from a garage sale with a 15VAC 1.1A wall wart for this exact reason!

[Mark Hammer]

A big part of what this forum does, actually, is taking the tacit knowledge stuff that so many of us think is just....basic and obvious....and making it explicit.

[Hides-His-Eyes]

The issue of course is that ripple is proportional to current draw...

Still a good move in many cases. You can picture this as the same as most charge pump cases except that here the AC is from the wall, not the IC.

[smallbearelec]

Hi Mark--

Friedman was an old-time, long-time EE and broadcast engiineer, also a prolific contributor (sometimes under other names) to the DIY magazines of the period. He did hundreds of projects and articles for the Davis group and for Fawcett, as well as books like the one you picked up.

I didn't know him well, but I had the pleasure of meeting him briefly in college days. I had been doing a few articles to make tuition money, and one of the editors introduced me to him. He was nice enough to have me over to his basement one afternoon, and he helped me get a project working. He died many years ago, but his contributions live on.

[Mark Hammer]

Small world, and a sweet story.

Gotta love all those old school guys, both the hard-core EE types and the classic stereo guys: Sidney Harman, Julian Hirsch, Len Feldman.

We have a guy in town here, by the name of Hy Bloom, who is about to turn 90 ( http://www.ottawacountrymusichof.org/inductees/1987/hy_bloom.htm ).  He still has his shop open about a 6-block walk from my place of work.    I've bought a couple of parts from him.  Delightfully crusty old guy who I suspect comes to work every day because he can smoke cigars there without anyone bothering him.  He's probably forgotten more about tube amps than everybody else in the region put together knows.

[SISKO]

Isnt current limited by the diodes current capabilitty?

[R.G.]

Isnt current limited by the diodes current capabilitty?

Probably. However, silicon diodes have enormous peak current capacity. In practice, it's not an issue.

[Renegadrian]

so this is another method to multiply AC - we also have c0ckroft walton - a couple of multipliers from AC are in the VOLTAGE MULTIPLIERS topic

And Yeah AC/DC (the band) really rocks!!!

It's a long way to the top if you want to multiply voltage!!! HIGH VOLTAGE!!! 

[davent]

Good article here on the creative use of AC wallwarts.
http://www.audioxpress.com/magsdirx/ax/addenda/media/stamler2874.pdf

A couple things to keep in mind with the voltage multipliers, your available current goes down by a similar factor that your voltage goes up, (not likely to be an issue when power a pedal or two.)

A safety factor that wasn't obvious to me was, the body of the capacitor whose negative lead is connected to the transformer  secondary is not at ground but can have quite a substantial voltage on it depending the transformer you're using so beware if you're poking around inside.

Take care,
dave

[Mark Hammer]

Thanks for that link.  That article's a keeper.  Makes me glad I started the thread, just for that.