Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss

Bridge or Center Tap?

44 views
Skip to first unread message

Paul D. Spiegel

unread,
Sep 21, 2012, 1:36:11 AM9/21/12
to
For my next project I have acquired a transformer with two identical 450v
secondary windings. I could put the windings in parallel and use a bridge
rectifier. Or, I could put the windings in series and ground the coupling
for a 450-0-450 configuration to use with a pair of diodes.

Which would you do? What are the pro's and con's of the two
configuration?

For this project I am using SS rectifiers. Were I using a tube rectifier
I would go with the center tap, of course.

- Paul

flipper

unread,
Sep 21, 2012, 2:37:34 AM9/21/12
to
A bridge is a little more efficient, say 20%, on transformer loading
than full wave center tap so if the VA is tight use a bridge.
Otherwise you can 'save the cost' of two extra diodes, which doesn't
count for squat in solid state but did with tube rectifiers.

GRe

unread,
Sep 21, 2012, 3:00:33 AM9/21/12
to

"Paul D. Spiegel" <p...@ktb.net> wrote in message
news:k3gucb$cii$1...@ktb2.ktb.net...
> For my next project I have acquired a transformer with two identical 450v
> secondary windings. I could put the windings in parallel and use a bridge
> rectifier. Or, I could put the windings in series and ground the coupling
> for a 450-0-450 configuration to use with a pair of diodes.
>
> Which would you do? What are the pro's and con's of the two
> configuration?

Pro bridge/windings parallel:
- Better B+ regulation because HV-winding resistance is halved.
- Less heat development in HV-winding.
- Diode reverse voltage stress about 635V versus 1270V for the center tap
config.

But... are you sure both HV-winding voltages are exactly identical?
Otherwise one winding is supplying current to the other one by
voltage-difference/2*winding-resistance.

Con bridge/windings parallel:
- 4 diodes needed in stead of 2 for a center tap config.

Rgds,
Gio

Phil Allison

unread,
Sep 22, 2012, 12:55:28 AM9/22/12
to

"Paul D. Spiegel

> For my next project I have acquired a transformer with two identical 450v
> secondary windings.


** You ought to check that by connecting the windings in series and reverse
phase.

Then measure the voltage across the ends - it needs to be no more than 0.1
volts.

If all is OK, parallel connection is possible and preferable.



.... Phil


John L Stewart

unread,
Sep 23, 2012, 10:33:59 AM9/23/12
to
Hey Paul, I think I may have corresponded with you a few years ago with
regard to my take on Crowhurst's Twin Coupled Amp. Was that you?

Anyway, here is my take on the FWB as opposed to the FWCT systems. An
important consideration for PS designers is something called 'Utility
Factor' of the various windings on the PT. Others have already noted the
FWB to be better if that connexion is possible. I've attched a numerical
example here for the secondary winding UF for the FWB with cap input
filter. Since there are no other windings the primary & secondary UF's
are equal.

Other examples will follow but in following posts since file size is
limited here in AudioBanter. But in summary the best UF for single phase
work is the FWB with choke input to fillter. The lost power during the
conversion from AC to DC is in the form of the even order harmonics
(2nd, 4th, 6th...Etc) generated by the rectifying process.

You could check for the possiblity of circulating currents in the
parallel connexion of your PT secondaries as follows. Connect one end of
the secondaries in series opposition. Then with an AC VM check for any
voltage differance.

My guess is the voltages are near equal but with somewhat different
resistance, all OK to parallel permanently. Sounds like you have a PT
built from the core up to be series or paralleled, commonly available.
For example, Hammond lists many such devices.

Cheers to all, John


+-------------------------------------------------------------------+
|Filename: 100v Bridge BW.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=300|
+-------------------------------------------------------------------+



--
John L Stewart

Charles

unread,
Sep 26, 2012, 7:58:40 PM9/26/12
to


"Paul D. Spiegel" wrote in message news:k3gucb$cii$1...@ktb2.ktb.net...

For my next project I have acquired a transformer with two identical 450v
secondary windings. I could put the windings in parallel and use a bridge
rectifier. Or, I could put the windings in series and ground the coupling
for a 450-0-450 configuration to use with a pair of diodes.


If you choose parallel connected secondary windings, a power meter test is
one way to be sure. Fire up the transformer and measure the input power
(primary) with the secondary windings not connected. Repeat the test with
the secondary windings connected in parallel. If the windings are
identical, the primary power will not change.

Phil Allison

unread,
Sep 26, 2012, 10:34:01 PM9/26/12
to

"Charles"
> "Paul D. Spiegel"
> For my next project I have acquired a transformer with two identical 450v
> secondary windings. I could put the windings in parallel and use a bridge
> rectifier. Or, I could put the windings in series and ground the coupling
> for a 450-0-450 configuration to use with a pair of diodes.
>
>
> If you choose parallel connected secondary windings, a power meter test is
> one way to be sure. Fire up the transformer and measure the input power
> (primary) with the secondary windings not connected. Repeat the test with
> the secondary windings connected in parallel. If the windings are
> identical, the primary power will not change.


** Be far easier to do the test I suggested - OR, with your test, use a
simple current meter in the primary - OR just wait and see if the tranny
warms any more than usual.



... Phil


paul.s...@att.net

unread,
Sep 27, 2012, 1:32:33 AM9/27/12
to

> Hey Paul, I think I may have corresponded with you a few years ago with
> regard to my take on Crowhurst's Twin Coupled Amp. Was that you?
>
>
> Anyway, here is my take on the FWB as opposed to the FWCT systems. An
> important consideration for PS designers is something called 'Utility
> Factor' of the various windings on the PT. Others have already noted the
> FWB to be better if that connexion is possible. I've attched a numerical
> example here for the secondary winding UF for the FWB with cap input
> filter. Since there are no other windings the primary & secondary UF's
> are equal.
>
> Other examples will follow but in following posts since file size is
> limited here in AudioBanter. But in summary the best UF for single phase
> work is the FWB with choke input to filter. The lost power during the
> conversion from AC to DC is in the form of the even order harmonics
> (2nd, 4th, 6th...Etc) generated by the rectifying process.
>
> You could check for the possiblity of circulating currents in the
> parallel connexion of your PT secondaries as follows. Connect one end of
> the secondaries in series opposition. Then with an AC VM check for any
> voltage differance.
>
>
> My guess is the voltages are near equal but with somewhat different
> resistance, all OK to parallel permanently. Sounds like you have a PT
> built from the core up to be series or paralleled, commonly available.
> For example, Hammond lists many such devices.
>
> Cheers to all, John
>
> +-------------------------------------------------------------------+
>
> |Filename: 100v Bridge BW.jpg |
>
> |Download: http://www.audiobanter.com/attachment.php?attachmentid=300|
>
> +-------------------------------------------------------------------+
>
> --
>
> John L Stewart

Hi, John. Yes, that was me who contacted you about the Twin Coupled amp. That project remains the best sounding stereo amp I've built yet. Thanks for your help.

I love choke input filters and use them as much as possible both for the greater current capability as well as the better voltage stability.

The pentode PP parallel amp I am currently building has two high voltage power supplies, one of each type: One FWB feeds a cap input for about ~600v on the EL34 output plates. A second FWB feeds a choke input for ~400v on the EL34 screens plus the input, gain, splitter, and driver stages.

The transformer is from Antek. This is the first project I have tried with one of their products.

Thanks to all of the RAT responses that convinced me to go with the parallel secondaries and FWB rectifiers.

- Paul

Roger Jones

unread,
Sep 29, 2012, 6:43:02 PM9/29/12
to
On Wednesday, 26 September 2012 22:34:04 UTC-4, Phil Allison wrote:
> "Charles"
>
> > "Paul D. Spiegel"
>
> > For my next project I have acquired a transformer with two identical 450v
>
> > secondary windings.

(snip)
>
> ** Be far easier to do the test I suggested - OR, with your test, use a
>
> simple current meter in the primary - OR just wait and see if the tranny
>
> warms any more than usual.
>
> ... Phil

Do it the way Phil says... voltage or current measurement, voltage is a bit easier. The warming-up test is less precise, hard to remember how warm was warm!
Cheers,
Roger

Phil Allison

unread,
Sep 29, 2012, 8:17:40 PM9/29/12
to

"Roger Jones"
Phil Allison wrote:
>> "Charles"
> > "Paul D. Spiegel"
>>
>> > For my next project I have acquired a transformer with two identical
>> > 450v
>>
>> > secondary windings.
>
> (snip)
>>
>> ** Be far easier to do the test I suggested - OR, with your test, use a
>>
>> simple current meter in the primary - OR just wait and see if the
>> tranny
>>
>> warms any more than usual.
>>
>
> Do it the way Phil says... voltage or current measurement, voltage is a
> bit
> easier.
> The warming-up test is less precise, hard to remember how warm was warm!


** All you need to do is measure the resistance of the secondary winding
when at room temp and then when it has warmed.

The higher the resistance, the warmer the windings are getting.


.... Phil



0 new messages