Massive Power Supply?

[bluetubes]

Hi all,

I have a rat shack 12.6 - 0 - 12.6 2A transformer that has been kicking around my basement forever.  I don't want it to go to waste so I figured I would try to make something out it.  The only thing that strikes me is a Pedalboard power supply.......the only problem is, I have no idea which schematic to use or what options to include.  I have mostly Boss pedals with one Digitech PDS1700 Chorus/flanger. 

I have researched it here on the board and have a found a few comments stating that a 25.2V transformer might be a bit too big for a pedal power supply.

Any comments or suggestions???

Thanks,
BT

[JimRayden]

Check generalguitargadgets.com for regulated power supplies. If you use half of the secondaries, you'll get 12.6VAC. That will be 18V when rectified and the 7809 regulator will make that into beautiful 9V.


------------
Jimbo

[bassmeister]

In order to not lose any current capabilities, I'd suggest you connect the two secondary 12.6 VAC wires together. Let these two go to one AC input of the rectfifier bridge, and connect the center tap to the other AC input. This is effectively the same thing as having two secondary windings in parallell, which doubles the available current compared to if you only connect the center tap and one 12.6 VAC output.

[R.G.]

In order to not lose any current capabilities, I'd suggest you connect the two secondary 12.6 VAC wires together. Let these two go to one AC input of the rectfifier bridge, and connect the center tap to the other AC input. This is effectively the same thing as having two secondary windings in parallell, which doubles the available current compared to if you only connect the center tap and one 12.6 VAC output.

ACK! ACK! ACK! ACK! ACK! ACK!

Smoke alert!! Smoke alert!!

Do NOT connect the ends of a 12-0-12Vac transformer together. That shorts both windings and sends up smoke signals at great expense.

If you have two 12Vac windings separately , not centertapped, you can parallel the windings **IF** the manufacturer recommends it. The centertap of a winding is a hardwired connection of the start of one of the windings to the finish of the other winding. Connecting the outside free ends connects the other two start/end and is effectively a short; worse actually, because the other winding is trying to pull this winding to the reverse of its voltage. If the maker recommends paralleling, they have taken the care needed to guarantee that the two voltages of the two windings match well enough to be paralleled.

[Connoisseur of Distortion]

i was just thinking that hooking together two windings 180 degrees out of phase could make a bit of a problem...

[bluetubes]

Could I make a dual supply out it?  say, 9v per side with a common center???  Would there be any benefit to that?

Cheers,
BT

[R.G.]

Sure. Read "Power Supplies Basics" at GEO, http://www.geofex.com

[bluetubes]

Holy smokes!!!  That place is a mountain of information.  Thanks R.G.!

[bassmeister]

Whew! I guess nobody will believe what I say after this... Anyway, when I was thinking this appearently bad idea up, I couldn't see why it shouldn't work. What I didn't think of was the "dot" theory that applies to transformers. As I have shown, I'm not the man to explain it (the dot on a winding marks out the phase as a function of direction of magnetic flux or something like that, perhaps someone else wants to shed some light on that), but I do realise why my suggestion won't work.

I hope I'm right this time, but parallelling two secondary windings with the dotted sides opposite to eachother would cause the same effect as connecting the two secondary 12.6 VAC wires together, right? The right way to go is to connect the dotted sides together, but don't take my word on it... 

But why do centertapped transformers exist anyways? Two separate windings allow the user to decide for himself if he wants duble the voltage/double the current/bipolar with centertap. Are separate windings more difficult to match when producing transformers, therefore making it more expensive?

I hope you didn't get the time to do as I suggested, bluetubes...

[R.G.]

Don't be too embarassed. You were only missing one small - albeit critical - bit of info.

the dot on a winding marks out the phase as a function of direction of magnetic flux or something like that, perhaps someone else wants to shed some light on that

The dot convention is kind of a coordination mark. Transformer windings only make AC voltages, not DC voltages, so polarity is not the right word. What it means is that when one dotted end of a winding goes positive with respect to its non-dotted end, ALL dotted ends go positive with respect to the non-dotted ends. They're all marching in step, and the dot indicates which foot forward.

I hope I'm right this time, but parallelling two secondary windings with the dotted sides opposite to eachother would cause the same effect as connecting the two secondary 12.6 VAC wires together, right? The right way to go is to connect the dotted sides together, but don't take my word on it...

You're right. A center tap is made by connecting one dotted end to a non-dotted end. Paralleling is done by connecting two dotted ends together and separately, the non-dotted ends together. One is a series connection, one is parallel. But like diodes, the direction the windings are oriented matters.

But why do centertapped transformers exist anyways? Two separate windings allow the user to decide for himself if he wants duble the voltage/double the current/bipolar with centertap. Are separate windings more difficult to match when producing transformers, therefore making it more expensive?

If what you want is flexibility, your view is correct. If what you want is the least amount of labor to wire things up the same way every time, you'd like to eliminate even one more wire. And the people who buy thousands of transformers a week want least labor, so they buy them with centertaps.