Power Supply Units - The Spyder and Transformer ideas

[hungry_pete]

OK - I know PSUs have been much discussed, along with the well established Spyder design (http://geofex.com/article_folders/oldspyder/oldspyder.htm)

-but-

In the UK I'm struggling to source a transformer with multiple secondaries. The next best is to either home-brew a transformer or use multiple transformers - one for each channel (or 1 for 2 channels with twin secondaries). Typically this means a lot of real estate in a pedal board mounted box, and a -lot- of weight. This seems a little sub-optimal to me, but I'm keen for really stable voltages and isolated grounds between power channels. I've seen ideas using 18V DC from old laptop PSUs, but ultimately this either leads to lack of ground isolation or some introduction of switch modes and DC to DC converters (which can cause other problems down the line).

So - I've sketched up the following concept (see diagram). Why not use a cheap-ass step down transformer plug to create, say 12V AC which forms the main power to the power unit. AC is easy to isolate into a series of channels with a bank of 1:1 isolation transformers. My amateur untrained instinct is that this bank of lower voltage, low power 1:1 transformers should be a lot smaller and lighter than 240V primaries to 12V secondaries. I'd have guessed the trade-off would be quite obvious by about the 5th or 6th channel. And I'd still have all the variations available for the outputs (low output, -9V, even different voltage levels with a few adjustments) as has been show in previous versions.

Now the problem. I would have thought that 1:1 transformers designed for around 12V and ~200 mA or so would be common as part of production for DC-DC isolation systems using switched mode solutions. But I can't seem to find the right spec! 1:1 seems to be reserved for low power applications, or the ones of the right power rating still be specified to be step down.

How to help:
- Has anyone done anything like this for weight/space saving in the box?
- Is it a silly idea? Would it result in more economic use of space or would it just not work
- Has anyone come across a transformer like this? Is there anything I could use instead?

[iainpunk]

welcome to the forum

most 1:1 transformers on the market are for either audio isolation or are designed for way way way higher frequency's, above audible that is, so they need way less metal for the same power rating.
there are some cheap switch mode dc-dc galvanic separated units for sale for quite cheap.

i remember a guy on 4chan's /DIY/ who build his modular synth with 42 5v-2A phone charger boards in a big ass box, putting out 5v each, and jumpers can stack them for higher voltages, since the total separation between the board outputs, ill dig around to see if i can find the posts again.

cheers, Iain

[mozz]

Weber sells a transformer with multiple secondaries.

[davent]

When i built mine i used 5 small pcb mount transformers with dual secondaries, paralleled one for a higher current capacity so 9 outputs altogether. Transformers bought at Digikey, if i recall were maybe $3 - $4 cdn each.

dave

[Phoenix]

My amateur untrained instinct is that this bank of lower voltage, low power 1:1 transformers should be a lot smaller and lighter than 240V primaries to 12V secondaries.

That's where your assumption falls apart. For a given frequency and power (VA), a transformer core needs to be the same size, regardless of ratio. A bank of 12V:12V 50Hz 2.4VA transformers would be exactly the same size as a bank of 240V:12V 50Hz 2.4VA transformers.

[PRR]

Transformer bulk does not go by voltage until you get to the kilovolts where you need more insulation than wire.

As said, normally VA/pound is near constant from thumbnail to size of a baseball. Efficiency and $/VA normally improves significantly with size. (Partly scale effect, also market demand.)

Today, a low-DC rail and DC-DC modules are practical.

Is there a good SEARCHABLE general vendor in the UK? RS/Allied is impossible.
https://www.alliedelec.com/product/mean-well/sbt01l-09/70388282/
MEAN WELL SBT01L-09, DC-DC Converter,9V,0.111A,4.5-5.5V In,Enclosed,PCB Surface Mount,SBT01 Series

[hungry_pete]

My amateur untrained instinct is that this bank of lower voltage, low power 1:1 transformers should be a lot smaller and lighter than 240V primaries to 12V secondaries.

That's where your assumption falls apart. For a given frequency and power (VA), a transformer core needs to be the same size, regardless of ratio. A bank of 12V:12V 50Hz 2.4VA transformers would be exactly the same size as a bank of 240V:12V 50Hz 2.4VA transformers.

Ah shucks - there goes physics getting in the way of a good idea. You'll really hate my free energy machine plans...

I fancied the higher current at lower voltage might be more effective at increasing the magnetic flux density changing the requirements slightly - but I guess you've still got to get the same amount of energy through a transformer core - regardless of electrical requirements. Its the magnetic circuit requirements I guess I'd overlooked. Some solutions I've seen seem to still have nearly 1kg of mass just in transformers which feels excessive, but might be necessary. Maybe this is one of those inverse things where a lower voltage actually increases the problem (apart from the one where you're playing with mains voltage in a home-made box). I guess I'm hitting the search engines for ~2VA transformers.

When i built mine i used 5 small pcb mount transformers with dual secondaries, paralleled one for a higher current capacity so 9 outputs altogether. Transformers bought at Digikey, if i recall were maybe $3 - $4 cdn each.

dave

Do you have a part number? I'd be keen to see specs so I can optimise this with parts available. This is the route I thought about going down but the units seemed quite large so I thought I'd investigate.

Transformer bulk does not go by voltage until you get to the kilovolts where you need more insulation than wire.

As said, normally VA/pound is near constant from thumbnail to size of a baseball. Efficiency and $/VA normally improves significantly with size. (Partly scale effect, also market demand.)

Today, a low-DC rail and DC-DC modules are practical.

Is there a good SEARCHABLE general vendor in the UK? RS/Allied is impossible.
https://www.alliedelec.com/product/mean-well/sbt01l-09/70388282/
MEAN WELL SBT01L-09, DC-DC Converter,9V,0.111A,4.5-5.5V In,Enclosed,PCB Surface Mount,SBT01 Series

The search engines and stores are a little tough to work through in the UK, hence the questions here. DC - DC might be the way to go for space saving but sometimes the units seem a little mysterious in the specifications as to whether they have an isolated ground.  I'm not sure I've seen this without an isolating transformer following some sort of HF pulse generator.

Thanks for all the replies. My wife is tired of me bouncing ideas about this of her so needed to ask someone else.

[davent]

This is the one i used, cost now is more than i remembered.

https://www.digikey.ca/en/products/detail/tamura/3FD-324/98321
dave

[PRR]

> bouncing ideas about this of her so needed to ask someone else.

You can buy pre-engineered multiple-hole power packs. If you have to ask family, or internet strangers, maybe best to go to the experts? $100-$200 may be cheaper than a fire or a heavy shock?

[hungry_pete]

> bouncing ideas about this of her so needed to ask someone else.

You can buy pre-engineered multiple-hole power packs. If you have to ask family, or internet strangers, maybe best to go to the experts? $100-$200 may be cheaper than a fire or a heavy shock?

You can - but where's the fun in that!? 

A few hints from internet strangers can be useful. I can assure you its backed up by lots of other research going on and experience in other applications. Just not worked with transformers much which leaves a bit of hole in my knowledge. And to minimise risks I'm gathering as much information as I can - you're right in that this stuff needs to be treated with a lot of respect. Ultimately I might choose the risks aren't worth it in my case and I'll buy one, but I really want to explore that first.

(btw - the wife's a senior lecturer in Engineering. She knows some stuff too and can be a useful sounding board in all things technical, she just get tired of guitar talk  )

[FiveseveN]

I'm fortunate enough to have a winding company in my relatively small city, they do any custom toroid. Surely there must be something similar close to you.

[hungry_pete]

I'm fortunate enough to have a winding company in my relatively small city, they do any custom toroid. Surely there must be something similar close to you.

Its a good shout - I'm not so sure many would be keen for a production run of 1 

I think I'm going to pursue a DC-DC converter based design. Otherwise I'm going to end up making something that will weigh a tonne. I'm a bit worried about switching noise from any mains adapter I use and the switching circuits in the DC-DC converters, but that's nothing new with these things!

There's a few designs out there I can adapt (obviously http://www.muzique.com/lab/9v_iso.htm as a starting point). I'll have a trawl to see if there's any good threads here from others to see if there's any lessons learned, to support the hints from a few people on this thread. Seems like it'll be fairly cost effective and small so worth a punt. I'm reluctant to let go of a transformer approach but without a multi secondary option it feels like its a non-optimum solution for me.

Thanks for the comments folks!