Bridge Rectifier with a center tapped transformer

[benoit]

Can you use a center-tapped transformer with a full-wave bridge as long as you don't ground the center tap? Tape it off or whatever?

[UR12]

Can you use a center-tapped transformer with a full-wave bridge as long as you don't ground the center tap? Tape it off or whatever?

Yes

[Phil_S]

Using the CT to ground will either blow the fuse, let the smoke out, or both. Don't ask me how I know this. Good luck.

[benoit]

Using the CT to ground will either blow the fuse, let the smoke out, or both. Don't ask me how I know this. Good luck.

I tend to learn that way myself

[novosibir]

Can you use a center-tapped transformer with a full-wave bridge as long as you don't ground the center tap? Tape it off or whatever?

You can, but you'd get a horrible high B+ out of the rectifier - about 2 times higher, than with the CT grounded and the non-bridge diodes alignment (don't know the description in English for it) - so be careful and don't make your electrolytics to bombs

Larry

[Phil_S]

I'd agree with Larry. You probably don't want a bridge (4 diodes). Just try a "regular" full wave. Actually, this prompted me to do a quick internet search. There is a way to employ the CT on a full wave if you really want to. See Wikipedia:
http://en.wikipedia.org/wiki/Rectifier
http://en.wikipedia.org/wiki/Diode_bridge

[Andy Le Blanc]

I've tried that one......
you will get twice the volts.....
use a regular style full wave set up....
the pop....pop.... sound of your filter caps
exceding their ratings should be avoided.

[David Root]

I learned the hard way too, blew a package of 3A fuses before I figured it out.

I have built one amp, 50w or so Soldano/Hiwatt-ish chassis with a FW bridge. The PT was a custom built toroidal 400-0 VAC secondary (comparable to a 200-0-200 CT but without the CT) and I used a new-ish monobloc hexfred soft recovery bridge from IXYS which bolts to the chassis. It works fine, gives 540VDC "theoretical", and ~495 VDC B+ @ idle connected to a pair of 6550s, two chokes and five 12A*7s.

I would use the FW bridge more if I could find more PTs with secondaries designed for FW bridge. Custom toroids is one way but life is too short to wait 2-3 months for them, plus they don't fit conventional MI chassis dimensions well either.

The FWCT mode is just so ingrained in the MI industry I've just about given up on toroids, especially when also using a toroidal OT as well. It's a shame because toroids ARE worth it for hi-gain crunch machines, IMHO.

Oh, and the CT on a FW bridge is an old trick too, all you Marshall freaks already know that, right?

[rhinson]

this whole discussion is dependent on the trannie, what the hv sec is, and what purpose/amp you're using it for. if you have a 360vct (180-0-180) then sure you could ignore the ct and just bridge it and get a plate voltage in the 460v - 480v range depending on tube type, complement, and biasing.
if you have an amp that is 600vct (300-0-300) you'd get over 800v on the plates, out of the range of most of the tubes we use and guitar amp circuits we build. i didn't see where the original poster gave the specs of the trannie, so maybe he can or maybe he can't. a lot of old pulls have lower hv secs and , within reason, could be bridged for a higher voltage build. of course some old pulls are just the opposite, so it just depends on the circumstance. rh

[benoit]

I actually was thinking of a 12 volt transformer for a multi-tap pedal power supply.

[rhinson]

I actually was thinking of a 12 volt transformer for a multi-tap pedal power supply.

is the full winding 12v (ie---6-0-6, that is 12vct) or is it 12v-0-12v or 24vct? rh

[benoit]

It's 12Vct.

It's shown wired without the CT connected in both the Small Bear power supply and another one I saw somewhere.

[paulster]

As a side note, the centre-tap can be useful when you want a split rail power supply, e.g. for driving opamps and suchlike without having to capacitor-couple each stage. Not what we'd do that sort of thing here, of course!

If you have a 15-0-15 transformer and bridge rectify it, you'll have about 40Vdc, but if you then treat the centre-tap as your 0V line then the +/- outputs from the rectifier are effectively +/- 20Vdc.

[rhinson]

It's 12Vct.

It's shown wired without the CT connected in both the Small Bear power supply and another one I saw somewhere.

well he has a couple on the site--a standard one and a flatpak one. both are 12vct and yeah you could just ignore the ct and use a bridge. the one does have a 110v pri which means it's really a 13vct when 120v is put on the pri.
13x1.4 = 18.2v as your nl voltage which is fine to: filter (maybe a 2200uf/25v), use through a 12v regulator, and then another smaller filter cap after that (100uf -470uf) and you'll a very nice regulated and clean quiet 12vdc supply for your pedal board (or use a 9v regulator if you wanted just the standard 9vdc thing). rh

[Luthierwnc]

Run a Duncan power supply analysis. It will give you the current draw calculations so you can see if your trannie is up to the job. Skip