Transformer Help

[crazeydazey]

Hi all..

not been around for a while had lots to do, new job, babies etc..

But now I'm back and hopefully gonna get some of my projects finished.

First thing I thought I'd best finish my PSU... so I have bought a 240v 2A secondary 48VA 0-12-0-12 VAC transformer... great I should be able to get some power now and start making some noise

It's come today and the sticker on it has baffled me somewhat, so before I blow myself up I just want to check with you guys (if thats ok)

it has a brown and blue wire, which I think is fair to say is the primary and on the sticker it says brn 0V - blu 240V (I know it doesn't really matter, but isn't that the other way round???)

then the other side has Red, Yellow, Black and Grey wires and the sticker says - Red 0V - Yel 12v - Blk 0V - Gry 12V, so I guess that means this...

[mrkva]

So first of all - watch out when dealing with AC mains, its not fun to touch the wrong wires!
Suggested setup should be giving you +/-/12 and ground, BUT for PSU giving you 12V you probably need to provide higher voltage! I've succesfuly built one with transformer giving 15V.
You can definitely test it out with this setup and see for yourself if the regulators give you proper voltages.

[Uncle Krunkus]

I would agree,
A 12-0-12 transformer will really equate to +/-9VDC once you get past the rectifier and any kind of regulation.
For a solid +/-15V system, you really need an 18-0-18 transformer, at least.
Don't be discouraged by buying the one you did though.
A good +/-9V supply on your bench will prove it's worth over and over and over.

BTW,.... "shouldn't that be the other way around?"
Well, yes,... in a way.
The Neutral should be across a fuse to ground at you main power box. The Active,... is just that... active (that is 240VAC (around here)), but if the electrician swapped the leads at the input, it wouldn't make any difference. Remember, we're in the land of Potential.
And,............... Keep in mind that,..... inspite of the fact that vitually everyone speaks of the +ve as the "resource", all current flow is actually from the -ve to the positive (as far as electron flow goes)

Welcome back to a very rewarding hobby!

[crazeydazey]

ah bugger... I am a little bit gutted, but like you say it will still come in handy, maybe for my soundlab as thats +-9V.

the reason I'm gutted is the board I have is for +-15v and I was told I could use a 24VCT transformer.. have I misunderstood.. I thought 24VCT meant 12-0-12-0 and the centres joint together??

is my diagram correct though.. do I join yellow and black, I wouldn't like to join the wrong two (say red and yellow.. I guess thats not a very healthy idea???)

Is there any way I can test with a meter before wiring up.. say a resistance test on red and yellow and black and grey and then red and grey and black and yellow (I assume as they are too separate windings I shouldn't get anything from say red and black or red and grey for example??)

[jordroid]

The easiest way to determine the phasing of a transformer is with a dual channel scope, but there is a trick at the bottom of the (very informative) article here http://www.freewebs.com/valvewizard2/Transformers.html

Sounds like you are on the right track.

regards,

jordan

[Skrog Productions]

Hi There

just to re-cap , can i ask what the total load per regulator will be , type of regulator 78 / 79 or adjustable ones , and whether your using 1 bridge rectifier or 2 bridge design , thanks .

Regulators usually need a sweet spot 3volt drop over them (example - 15 in 12 out) but when i was first designing my supply , in error i thought i could load up the whole modular on a pair of regulators , i did create some fantastic heat from the 5 inch X 5 inch heatsinks (there is still a bare patch where hair used to grow on my arm ) .

My original transformer was a 18/0 18/0 , after the rectifying stage the voltage was 20v , 20v in 12v out was a 8v drop over the 12v regs and that dissapated as heat , searing heat with 40 odd modules running , then they shut down in disgust .

Your transformer may or may not satisfy the regulators by the time it goes through the bridge but a 15/0 15/0 will be better in the long run.

hope i didnt mumble too much there

good luck & fire any questions to us here.

Dave

[Skrog Productions]

here's the thread for the blue wardrobe supply http://electro-music.com/forum/topic-48193.html&highlight=

[Osal]

[crazeydazey]

Hi skrog..

I don't think I can answer most of your questions I'm afraid, I bought the board ready built, just needed to add the transformer and was told the one I got should do..

I can see that the regs are 7815 and 7915 and I can see a bridge rectifier on the AC in and a couple of 1n4002 on the DC out (guess that doesn't make it a 2 bridge design though eh - sorry, I'm still pretty new to synth DIY)

seen your PSU, it looks pretty dam cool and all your modular in the back (I'm jealous - I've just about finished WSG, Soundlab and a PSU, I guess we all have to start somewhere)

I have done a bit of poking around with the meter and everything looked ok (thanks jordroid for the link that was quite useful), so went ahead and connected it up and plugged it in and it didn't go bang, so that was nice , took some readings.

I get +15.1v and -14.8v. is this ok, do they both need to be the same?? do they both need to be bang on 15v??

now I do get 15v is it still not really gonna be good enough, am I better getting say a 18-0-18-8 (allowing then for the 3v drop) and using this one for my soundlab (if they ever start selling 7809 and 7909s in england again, I remember last time I looked I couldn't get em anywhere, except ebay from china)


btw..cheers everyone for your help

Daz

[Skrog Productions]

Hi Daz

"I get +15.1v and -14.8v. is this ok, do they both need to be the same?? do they both need to be bang on 15v?? "

That's not bad for 78 /79 regs .
Yes , an adjustable reg design would give fine tune 15V but unless your performing with ultra sensitive vco's and need critical tuning stabillity , that will be sound as a pound for diy fun synths.


Farnel do 9v +/- regs , or they did do .

Dave.

[Skrog Productions]

Sorry , just to add , i mis-read your transfomer voltage.
Monitor the voltage after the regulators when you connect the circuits up, the reg's may act funny under load with your 24vct xfrmr.

Dave.

[Skrog Productions]

If you get an 80VA 15/0 15/0 torroidal transformer that would future proof for quite a while & help reduce heat problems with regulators when you add more modules , it can become mind spinning planning a supply for a big system but your well on your way so far .

Keep us updated .

Dave.

[crazeydazey]

cheers mate, I might get me one of those torroidal transformers, then I know I'm good for a good few modules