My brain got stuck at Power Supply question

[KarenColumbo]

- I'm not comfortable with 240 volts
- I'd like to have several 9V DC outputs that are galvanically separated (plus one or two 18V outputs but here's just for 9V)

My chain of thought was:

1. 24V AC Wallwart
2. "Step down transformators" 24V -> 9V AC for every rail
3. Rectifying
4. extensive filtering
5. LM7809 (plus filtering)
6. Stompbox

I googled away - my problem, though, is missing vocabulary (no formal traning at all).

Does this make sense?

[tonyharker]

If you want them galvanically separated then you need a separate wall wart/ transformer for each 9v /18v supply.  Why do you need that?

[EBK]

You can buy transformers with multiple secondaries to build this.

Or, you can search for a commercially available power supply with multiple "isolated" outputs.  If you look at the commercially available ones, note the prices.  You should see the prices clustered in two groups.  Buy from the higher priced group.  Voodoo Labs makes good ones.  There are several other brands that are good as well.

Cheaper power supplies may use the word "isolated" but may have outputs that share a common ground connection.  Those will be the cheap ones.  Don't buy one of those.  With a truly isolated supply, you can stack two 9V outputs to create an 18V output.  With a cheap one that only claims to have isolated outputs, you'd end up shorting one of them if you attempted this.

[GGBB]

If you want them galvanically separated then you need a separate wall wart/ transformer for each 9v /18v supply.

Step 2 takes care of that:

2. "Step down transformators" 24V -> 9V AC for every rail

[tonyharker]

Do you know of any 24v AC - 9v AC  step down transformers?  I don't unless they are specially wound to order - expensive. How many are we talking about?

[KarenColumbo]

Hm. What about some 1:1 )isolation?) transformers and stepping the voltage down afterwards?

[GGBB]

Do you know of any 24v AC - 9v AC  step down transformers?  I don't unless they are specially wound to order - expensive. How many are we talking about?

They don't need to be 24V -> 9V. An inexpensive isolation transformer would suffice as long as it had the required power handling. The point is that galvanic isolation doesn't require a separate transformer connected to mains for each supply as you stated. It can be achieved as proposed which is also multiple transformers connected to an AC source except that the AC source is 24V not mains. It's not that different from what the commercial supplies with true isolation do using a single transformer with multiple secondaries. Here, mains is stepped down to 24VAC by the AC supply, then that 24VAC is fed into multiple transformers for isolation (and optionally further step down) for each 9/18V supply. The step-down from 24VAC can be done either by a step-down isolation transformer (before rectification) or by the regulator with a 1:1 isolation transformer(after rectification). The proposed design is sound. In theory at least. I have no idea whether or not it is cost effective.

A couple of years ago I planned to build my own 9V supply as well. I spent some time designing an 8x9V true isolated supply using four 120V to 2x14V transformers. Then I priced everything out and it would have cost only a bit less than a PP2+. I ended up buying a couple of used PP2+ units and saved money plus have nicer more compact units (and probably better made). The used PP2+ units cost me a little less than what the parts for my supply would have cost.

[anotherjim]

Any 1:1 isolation transformer is likely to be bulky/expensive - the market doesn't want small power isolation traffys, but generally to serve a rack of equipment.

The industrial "safe" supply is 110VAC and the isolation transformers are widely available, but again heavy - I've never seen less than 750VA examples.

You could use a 12-0-12 (24v overall) secondary of a normal step down traffy like it was an auto transformer - or inductive voltage divider. The full secondary is connected across the 24VAC supply, there will be 12VAC between any one end and the centre tap. The unused primary will have dangerous high voltage (>200VAC) as it is "backdriven" from the 24VAC on its secondary, so should be well insulated. If you used the centre tap as supply 0V, you can use it like any other centre tap 2 diode rectifier scheme.

[GGBB]

I've never seen less than 750VA examples.

Much smaller ones are readily available. And while also much less expensive, still kind of bulky and IMO not inexpensive enough. I still say it's not worth it to build your own when commercial units are reasonable.

https://www.mouser.com/ProductDetail/Triad-Magnetics/N-48X

[Rixen]

Use some Recom (or other brand) isolated DC-DC converters, followed by filtering and (if required) linear regulators. Power it from a wall-wart

For the ultimate in isolation, a box with an array of IR LED's and some photovoltaic cells. 

[KarenColumbo]

Thanks, gents, for your invaluable input. I'd like to clarify that it never was about saving money - if it was I'd have abandoned this addiction years ago But if it's not worth the time and collateral damage (my "box of shame" is pretty cramped already) I'll stick with what you proposed and buy something ready-made.
Edit. But then again I thought ti myself "what the heck" and ordered 6 18-to-9-V isolated dc-dc-converters (thank you very much for the hint - never knew those existed!). So 18V AC it will be at the input. Those converters are 11 Euros each, so it's not cheap, but I think it's within certain limits of decency.
Now to some reading about proper power noise filtering

[antonis]

Now to some reading about proper power noise filtering

Especially for DC-DC converters...

[KarenColumbo]

Well, I know I know nothing But my quite expensive 9V iSpot Wallwart is noisy as hell compared to my 200 Euro bench supply. So I guess a few uF and nF here and there won't hurt. Or is this stupid?

[Phoenix]

I thought ti myself "what the heck" and ordered 6 18-to-9-V isolated dc-dc-converters (thank you very much for the hint - never knew those existed!). So 18V AC it will be at the input.

By 18-9V isolated, I assume that 18V is the max input voltage? Better be sure not to use a supply derived from 18VAC for those, that'll give you ~24VDC peak under normal conditions, around 26.5VDC if the mains is high. Hope I'm wrong.

[KarenColumbo]

Very little time so it's been a while. But I can't let this just go
Just found those and bought a couple:



Directly from the manufacturer in China. They do 240V AC to 12V DC in a very small package. Not sure yet if they will make do but they are "dead cheap", so if I burn some it's not the end of the world.

[ElectricDruid]

Those look like little switched-mode power supply modules. They're very widely used and mass produced these days, which is why they're so cheap. That's the good news.

The bad news is that a lot of them are pretty poorly designed, they tend to use under-rated capacitors for the levels of the voltage spikes they deal with, and the lifespan is often about one week longer than the one year warranty. Oh, and they may not be that quiet either - they tend to throw a lot of high frequency switching spikes back onto the mains voltage - where it will hit all the other power supplies you've got tied to that same mains input. At that point "isolated" supplies don't help, because the one thing even isolated supplies share is the mains input.

I'd be cautious, and that's partly because I don't like switched mode PS. But me and my fellow power supply dinosaurs might die out, dragged under the rising tide of cheap, lightweight adaptors by our huge chunks of soft iron and wound copper!

There's a good discussion of the technology here:

http://sound.whsites.net/articles/smps-primer.html

[KarenColumbo]

I know you're right, of course. I got the stuff for a "real" PS here - but that's gonna be H.U.G.E., which I want to try to avoid. To no avail, I'm afraid Just look at those transformers

[Groovenut]

What about something like this as a base?

https://www.tedweber.com/wpdlxfmr-1
https://www.tedweber.com/wpdlxfmr-2

[KarenColumbo]

Yeah, they're ideal. But 118,- USD postage is too much. Just put a request at a local manufacturer in Vienna, Austria Let's see how they react to my price fantasies (40-50 Euros) ...

[vigilante397]

https://www.tedweber.com/wpdlxfmr-2

I know this is the wrong side of the world for Andreas, but I was looking for something exactly like this for a project I'm working on. Thanks!