Simple bridge rectifier question.

[paulb]

Originally posted by Geoff
The first circuit does not fully utilise the transformer secondary and so potential VA capability is being lost.

Well, the transformer's VA capability is likely based more on heating effects due to losses than anything else; a full-wave CT arrangement only draws current on alternating half-cycles but you can pull more current on each pulse because the heating is based on average, not instantaneous current. However, there is effectively more winding resistance involved so you would indeed lose some VA capability.
Of the three I like Geoff's proposal the best if you can afford the 4 bridge rectifiers.

[Geoff]

Paul

The utilisation factor for a transformer/rectifier circuit (defined as the ratio of the dc output power to the transformer rating required by the primary and/or secondary) is 0.572 for a full-wave centre-tapped circuit and 0.812 for a bridge rectifier circuit.

This means that either a smaller transformer can be used with a bridge rectifier or, for a given VA rating, more dc output power is available.

The foregoing figures apply to rectifier circuits providing a single polarity output (as was being done in blmn's circuit). When a centre-tapped arrangement is used to provide dual supply rails, the utilisation factor increases to that of the bridge rectifier.

Geoff

[blmn]

Subwo1,

If the transformers were perfect you would be right, but filtering needs are defined by load, transformer regulation and frequency. Since the transformers and the load are defined (JoeBob's case), for the same transformer, for a given VA rating, as Geoff said, you have more power available and, if I don't miss something here, better regulation (less core losses, less wire losses etc).

regards

[JoeBob]

I can see how Geoffs diagram works, and 2 more 35A bridges doesn't cost more (it's just 50A and above are ALOT more expensive for some strange reason). And if ti's the most efficient way I think I'll go that way. The PSU box is almost done so I'll wire it up tomorrow.

And Geoff, you're correct, two windings, not center tapped...

[AudioFreak]

Yeah, I agree with Geoff. His design probable contains the least number of potholes so i'd got with that.

[paulb]

Geoff,
Do you have figures on efficiency as well? Utilization (oops I mean utilisation) you've defined as power out / VA rating required, but I think this might be different because of the pulsating nature of the current draw. What would the actual losses in the transformer be for the two cases?
My thinking is that by utilizing less power from the transformer it keeps its internal heating down. I normally like to run a transformer well under its VA rating.
Anyway, it's a moot point: JoeBob has the cash for the extra rectifiers; we all agree it's the best approach. Go for it, JoeBob!

Paul

[Geoff]

The title of this post was 'Simple Bridge Rectifier Question'. Well, it wasn't so simple after all, was it? :-)

Geoff

[Geoff]

Paul

Your last post came in whilst I was composing my previous comment. I agree with you about oversizing transformers, or putting it your way 'running them well under their VA rating'. I normally recommend a VA rating of 5 times the the amplifier rms power (in watts) for Class-A amps. Class-AB can be somewhat less.

I was quoting the Motorola (now ONSemi) 'Power Supply Design Manual', an essential read, though many other sources make the same point. The figures are calculated from the relationship between the rms and average current and so take into account the pulsating nature of the current draw.

You ask if I have figures for efficiency as well. How are you defining this if it isn't 'power in to power out' i.e. the utilisation factor?

Geoff

[paulb]

Hi Geoff,
Yes, I was enjoying the irony of this thread's title as well.
I asked because power is not the same as VA. I'm curious as to what happens with the "extra" VA capability. I know that with a power factor other than unity, amperage is wasted in "imaginary power" that doesn't actually appear at the load; I'm wondering whether the extra VA capability required for a fullwave CT (for the same load) actually results in extra heat in the transformer - i.e. losses.
Just rambling, I haven't actually thought this through any further. I really do enjoy these discussions, there are a lot of very knowledgable people here and I always come away from these learning something new. Thanks for the reference on the ONsemi manual, I may pick up a copy.

Paul

[subwo1]

blmn,

Actually it does make sense that there would be a difference. I could see loading the same winding for a whole cycle instead of a half cycle could help utilize core flux and keep the windings more compact.