Transfromer for 9V bipolar supply questions.

[BillyJ]

Hello forum,

I have a F36-65 transformer. Single Split Pack.
The kind used in scott swartz 4 output supply for 18V.
I want to use this transformer to make a bipolar supply for a mutronIII.
I have been reading this article:
http://geofex.com/Article_Folders/Power-supplies/powersup.htm

This transformer wired in parrallel only puts out 18V.
If I wire it in series I can get 36V. Plenty for my needs.
Perhaps way too much?

My big question is if I tie Pins 6 and 7 of the transformer to put it in series output where will be my CT?
Will it be the junction of those two pins?

I have a feeling becasue I am asking about deadly voltages I will not get any real answers just a bunch of "when you understand it it will be clear."
So how about some links to how these work that will give me enough to get this project done safely but not so much info that I will be swimming and give up trying.


Help!

[Mark Hammer]

The transformer puts out AC any which way you want to wire it up.  The thing about AC is it keeps swinging back and forth between positive and negative (hence the moniker "Alternating Current").  When you rectify the output of the transformer with a diode bridge, essentially what the diodes are doing is separating the negative swing from the positive.  If you look closely at the arrangement of diodes in any rectifier bridge circuit (4 diodes in a diamond shape)  you'll see that two diodes are connected to each side of the transformer.  The diodes that conduct in one direction direct their "feed" from each output line of the transformer to one place, and the diodes that take the other directional "feed" go to another place.  At the output of the bridge, you have one "pile" of positive current swing and another "pile" of negative current swing (and at this point the veins on the foreheads of EE's everywhere are bulging and throbbing in frustration with the metaphor).  

If the transformer is centre-tapped you have two "piles" and a neutral point.  But what if there IS no centre-tap?  Where do you get the neutral point from?  Easy.  Take your two "piles" from only one of the transformer lines and treat the other line as if it were your neutral point.  An illustration of this in action is here: http://www.paia.com/tubehsch.gif

This is a schematic of the PAiA "Tubehead" tube preamp.  You will note that the circuit uses a 12vAC wallwart but it NEEDS +/-15v in paces.  Although rectified, a 12VAC transformer it is able to deliver more than +15VDC, how the heck do you get a bipolar supply out of it?

As you can see in the schematic, one of the lines from the transformer is treated as ground, and the +/- part of the supply is derived from the other line using *two*, rather than 4, diodes.  Remember that in the center-tap scenario, each half of the supply takes some current from this side of the transformer and from that side, such that the + output has "taken" current from both sides of the transfromer, as has the - output.  In the non-centretapped scenario, you do that too, except one of the sides is missing, hence only 2 diodes.

I hope that's clear, if a little "nonaligned" with theory.

Your 36vct transformer, if used in the "classic" manner, would yield a rectified output of a bit over 25vdc per side, or just under 51vdc if you ignored the centre-tap and treated it like a single-ended transformer.  Were you to use the two taps in parallel for more current but less voltage, you'd have a single 25vdc rectified output.  If you derived a Vref from that, you'd have something that functioned like a +/-12.5vdc supply.  If you do the trick used in the Tubehead supply, you're back to having a +/-25v supply, which is more than what you want to use for a Mutron.  

The smart thing to do here is to pick up a pair of 3-pin regulators (neg and pos) which should be able to handle that voltage and regulate it down.  *Which* voltage starts to get tricky and into judgment-call territory.  Typically, you want what goes into the regulator to be at least 2 volts higher than what you plan to have coming out of it, but not dramatically more.  They can handle up to 30vdc input, but large gaps between input and output show up as heat.  You haven't said what the current rating of the transformer is, but lets assume it is a modest one (<200ma).  Although you can get 100ma capability regulators (same size as a transistor) and may not draw anywhere even remotely near that much for a Mutron, it is still probably smart to get some 1A rated regulators to handle the excess heat IF you plan to use 9v regulators (LM7809/LM7909).  If you go with 15v regulators, you may not need to worry about the heat quite so much since the in/out gap is smaller.

Okay, my fingers are tired.  Somebody else's turn.

[Paul Perry (Frostwave)]

That untapped transformer with two diodes that MarkH talked about, is how I do it in the Frostwave effects (most of which run at +-15 volts, which was standard for the original analog synths that a lot of my stuff is based on).
A source of 16v AC plugpacks, is burglar alarms, and very old modems.
Remember that a 16v tranny, when rectified, gives a peak voltage of  almost 20v. So be careful not to overload filter caps! Most regulators need 2 or 3 volts headroom, but in an emergency you might be able to get "low drop out' regs that get by on a lot less. They cost more & I havn't seen them for high voltages, though.

[BillyJ]

Thank you gentelmen!!
You both have been a great help not only to me but many many others.
I am still groking this but I have learned more and will get it.
Thank you Mark for the awesome writing. I have a whole folder on your stuff.
I cannot thank you both enough for the assitance!!!
THANK YOU!!!