Thanks AudioFreak that diagram's really helpfull. I'll go that route, I'll feel alot safer then just using one 35A bridge.
As for why I didn't just go buy a 50A, because the only ones at my local shop was 35A and lower, and the 50A is like 5 times the price of the 35A if they order it. So I find this solution alot better.
Thanks again for all the help guys.
As for why I didn't just go buy a 50A, because the only ones at my local shop was 35A and lower, and the 50A is like 5 times the price of the 35A if they order it. So I find this solution alot better.
Thanks again for all the help guys.
i think audiofreaks' hook-up is better then blmn's 'cos you cannot garantee that the two transformers are exactly the same voltage and hence have slightly uneven rails. Even tho' this doesn't matter much is just better design.
BUT make sure the tansformers are the right way round with respect with each other. Check the AC voltage between taps with DMM, if 0 volts then they are in-phase, if the expected voltage then they are 90 degree out out phase. Just a thought
BUT make sure the tansformers are the right way round with respect with each other. Check the AC voltage between taps with DMM, if 0 volts then they are in-phase, if the expected voltage then they are 90 degree out out phase. Just a thought
Um, audiofreak's hookup wires two transformer secondaries directly together - the consequences of uneven secondary voltage would be much more severe in this case.Helix said:
I think JoeBob is using two identical transformers; if not, I wouldn't use audiofreak's arrangement. Even if they are, I highly recommend fusing each transformer separately.
I totally agree with the comments about checking carefully for the right phasing of the transformers.
no, it's more lightly that the secondaries are more closely matched with each other then two diffrent transformers, in which case, if one is slightly lower from the other, the output voltage is the mid point.
HOWEVER how about using 4 bridges
that way there is less current in each bridge, if thats the problem
Time to put in my two-penn'orth. Both the previous proposals have problems. The first circuit does not fully utilise the transformer secondary and so potential VA capability is being lost. The second circuit requires closely matched secondary windings (< 0.5V difference) to avoid a high current circulating in the two parallel windings causing a heat build up and reduction in usable VA.
May I suggest that, for the cost of two additional 35A bridge rectifiers, the following circuit be used which will avoid these problems. This circuit has the added advantage the diode current will be halved. I am assuming that I have read this thread correctly and that you have dual-secondary, not centre-tapped, transformers
Geoff
May I suggest that, for the cost of two additional 35A bridge rectifiers, the following circuit be used which will avoid these problems. This circuit has the added advantage the diode current will be halved. I am assuming that I have read this thread correctly and that you have dual-secondary, not centre-tapped, transformers
Geoff
Attachments
Since Paulb's and Audiofreak's diagrams show full wave rectification, the ripple is 120hz, and the filtering effects will be virtually identical.
Geoff's way is a good way, providing full wave rectification also. I would connect one bridge output from the top tranformer to the bottom capacitor, and one from the bottom to the top.
Geoff's way is a good way, providing full wave rectification also. I would connect one bridge output from the top tranformer to the bottom capacitor, and one from the bottom to the top.
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.Geoff said:
Of the three I like Geoff's proposal the best if you can afford the 4 bridge rectifiers.
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
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
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
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
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...
And Geoff, you're correct, two windings, not center tapped...
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
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
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
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
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
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
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