Snubberized gainclone psu with Centre Tap transformer

[Sadface]

Hi Guys,


I am looking to upgrade the power supply in my gainclone to Carlos' snubberised version.

I can get a 250VA transformer with a single centre tapped 28-0-28 secondary for half the price of one with dual secondaries.

Anything wrong with using a 28 - 0 - 28 transformer like so?

Also, the idea is to do this on the cheap. Any reason not to use 10000uf filter caps I have lying around?

 

[FauxFrench]

No (yes, it is wrong), you will short-circuit the transformer windings through the diodes supposed to generate the potential close to the mid-point. You need floating windings.
Try to find a current path from the mid-point and you will see the problem.
You can no doubt use your 10000uF instead of the 4700uF.

 

[Sadface]

Hi FauxFrench,


Thanks for that. I think i see the problem.


Would it work if I was to ground the centre tap to the main ground star like so.

 

[rayma]

Would it work if I was to ground the centre tap to the main ground star like so.

No, it'a a fundamentally flawed approach.

With a single, center tapped secondary, you have to use
the FWBCT circuit to get a bipolar power supply.

 

[Michael Bean]

Hi FauxFrench,


Thanks for that. I think i see the problem.


Would it work if I was to ground the centre tap to the main ground star like so.

You can't use a transformer with a center tapped secondary in that circuit, it needs to have two totally separate secondaries.

Mike

 

[Sadface]

Yeah I was just working my way through that. Transformer wiring always does my head in a tiny bit. I think i have it right this time round.



Would it work to wire the rectifiers to the centre tap as though it was dual secondaries?



The ultimate question here is can a centre tapped transformer be wired as dual secondaries?

 

[rayma]

The ultimate question here is can a centre tapped transformer be wired as dual secondaries?

See post #4.

 

[Sadface]

Thanks Rayma.



I was afraid of that.



It looks like I have to spend the extra money for a transformer with dual secondaries.

 

[rayma]

It looks like I have to spend the extra money for a transformer with dual secondaries.

That's the way to go, but it shouldn't cost much more.

 

[Panelhead]

I purchased a board with two diode bridges per channel. The bridges are parallel. Cannot see why circuit in post one will not work.
Using two single output center tapped winding transformers for stereo.

 

[Panelhead]

Guess it is incorrect. For one you are putting 56 volts AC across each bridge.

 

[FauxFrench]

Thanks Rayma.

I was afraid of that.

It looks like I have to spend the extra money for a transformer with dual secondaries.

Or, you use a symmetrical regulator design such that you have a regulator for the positive rail and a regulator for the negative rail. You can use LM317(positive)/LM337(negative) but they have to be buffered with an NPN and a PNP power transistor. Then, you can use your transformer with a center tap.

The need for individual windings arise from your intention to stack positive supplies.
Such less trivial rectification arrangements require us all to think at least twice before being sure it will work well. It is certainly not only you having overlooked a problem with such constructions.