Single or dual bridge and why?

[ßart West-VL.]

Hi everyone,

I'm building a psu from a toroïd: +12V/0V/-12V AC 160VA. (2x 6.66A)
Nothing high end for now, because everything that comes behind it, is simple at the moment.
However the supply should be very good so that I can use it in further projects. Besides, if the psu doesn't work good, everything next will never be good, right?!

Something that I can't seem to find out is, why should I use two bridge rectifiers if one can do it equally well and even have about 1.2V more output and have less components. (one vs. two bridges)
Or shouldn't I ...?
What is the better choice and please include 'why' it is so.

Thank you!
Bart

[AndrewT]

Hi,
a 12-0-12 is a centre tapped transformer and suits a single rectifier.

To use a dual rectifier you need twin secondaries 0-12 & 0-12.

[ßart West-VL.]

Thanks for your answer AndrewT, but...

This toroïd has dual secondaries. (four wires)
So, should I connect the end of the first with the beginning of the second to have a center and use one bridge?
Or use two bridges at each secondary?

Bart

edit: I see now how my initial post is confusing, sorry.

[AndrewT]

Hi,
in a mono (single channel) amplifier, I have found no difference in noise and/or hum with the single rectifier off the centre tapped compared to the dual rectifier off the dual secondary.

However, when building a two (or more) channel amplifier, some builders find that it is easier to get a quieter output by adopting the dual rectifier topology.

[mikelm]

Originally posted by ßart West-VL.
why should I use two bridge rectifiers

This is my understanding of the situation:

If your circuit draws no current then one bridge is ok.

But if you are drawing current from the transformer then you have a volt drop across the transformer windings.

With an AC music signal - lets say 500hz - you draw current alternately from each transformer winding, another alternation also happens when the 50hz AC power switches polarity.

These two circumstances that cause alternate current draw between the two transformer secondary windings means that the volt drop in the transformer is transferred alternately and somewhat randomly ( depending on the mixture of AC 50 or 60 Hz and whatever frequencies the signal is creating ) between the coils. Because the two halves of the transformer secondary are joined together AND connected to earth this transference of volt drop pulls the earth around and this is effectively injecting a spurious signal into the earth line.

If you have 2 bridges, the two halves of the transformer are independent from each other and the star earth point is established AFTER the bridges. So now, the two coils can move independently of each other according to what ever the current demand & corresponding voltage drop is and, because the earth is not connected here, this switching transient is not generated

That is the complicated answer & I hope I have explained it clearly enough for you to see it.

The simple answer is that it sounds better ..

[mikelm]

thinking further about it - it is slightly different from what I explained above .... but someone else can get into that if they want to !!!

more simply put - the middle of a transformer secondary is not an ideal place for a star earth point !

[megajocke]

The reason isn't that compicated and doesn't need any assymetry or randomness or equally strange concepts. The reason is: High current pulses flow between the caps and transformer.

Which is why you should never take the transformer center tap to a "star" ground. It should go directly to the caps. One wire connects the point where caps and center tap meet to the star ground. If the center tap is connected to the star ground you'll get problems as the high current charging pulses will create a voltage drop over anything they flow through. Like the star ground bolt if you are using one. This will easily cause hum in amps with more than one channel.

[mikelm]

it is a little more complicated that you say

It is important that the earth is not connected to the secondaries at all - not just the star earth point.

[megajocke]

Do you mean safety earth? If it is connected to signal ground you can have problems when interconnecting different pieces of equipment. Then again, if you don't you will have a safety hazard if you use a transformer without reinforced insulation or a grounded shield. With balanced interconnections this problem does not exist.

[mikelm]

you miss my point

the mains safety earth can be connected but that is not the what we are discussing.

with a dual bridge the point is that the main "power earth" is not connected to the secondaries......

it is connected after the diodes.

the two secondaries are then left to float separately.