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> i have a big trafo

OK, _how_ "big"? As big as a bus? 477 Volts? 57 pounds over ideal weight for its height?

> on: August 08, 2012, 01:53:28 pm
> on: August 08, 2012, 08:55:41 pm


And how much of a rush? This week? 7 hours? Yesterday?
 
user 37518 said:
Is a very simple circuit, however  eventhough it makes sense to me,  and ive done it "on paper" several times, for some reason im unsure on how "audio worthy" or clean will that PSU be, as opposed to using a bridge rectifier.

So show us.

It may make sense to you, but from your (sparse) description I cannot picture any scheme that might come close to doing what you want. Post a schematic (hand-drawn is better than nothing), so at least we know what you're talking about.

JD 'a schematic says more than 1000 hand-wavings' B.
 
With 48V (rms) you will have a peak of about 68V, so for a low current 48V (phantom) will work fine, but you don't need such high voltage, with a 36/40V trafo will be enogh, but beacouse it's low current you wont have troubles.

In the other hand, if you use a bridge rectifier or 2 diodes halfe wave rectifier you'll have de same DC voltage, but using half wave or the full wave, so you still have the 68V less 2V from the diodes so 66V, the LM317 wont be happy. if you have no other choice than use that trafo you can put a lot of diodes (about 40 instead of only 2) so each diode will disipate low power or a reversed 40V zenner in series with a 1n4007 but it will work for low currents... IMO it doesnt seem a good idea.
 
user 37518 said:
The size of the trafo is in the title ;)
Seems only the transformers secondary voltage rating is in the title. A transformer (EI-core, toroidal, autotransformer, ..) has some more data.
Is 'the trafo' singular? A little insane, but if you have two, maybe feed the primary windings of both transformers -so these exist, whatever VA these might be rated and whatever the load current might be- in series to get your secondary windings voltage down.
 
If this PSU is meant to provide dc for the active electronics and 48V phantom power from the same secondary then there is another potential problem. For best noise performance,the 0V of the phantom power should be connected to the pins 1 at the mic input sockets and nowhere else then the signal 0V should be connected to the chassis where the mains enters. If you have a common transformer then the phantom 0V is already connected to the signal 0V within the PSU itself so you cannot do this.

Cheers

Ian
 
I'm with JD and PRR.  Haven't a clue what you are talking about.  How about a pic?  Maybe we're stupid  :eek:
 
Dual, have you built this?  The 24V supply is a conventional full wave supply.  But the '48V supply' will have less voltage as it receives the same rectified AC but via an extra diode & capacitor. ie the 48V supply dun wuk.  [EDIT] Rubbish

You may be better off arranging the normal voltage doubler chain to get your 48V  [EDIT] It IS a voltage doubler

Try taking the other end of the bridge rectifier, not to earth, but to the 24V supply capacitor.
 
Looks just like Keith's 9K psu with fullwave C_ockcroft-Walton / Villard multiplier for the +48V part and two diodes left out for the left out negative rail.
 
Sorry, my post was wrong, i didn't think you have a CT point, this trafo is called 24+24, 24-0-24 or 48CT, that's why my confussion. It should work as shown, but still have the problem of grounding, you should split grounds of 48V at the output of the bridge maybe.

Cheers

JS
 
[EDIT]My apologies.  Your circuit is indeed a voltage doubler, giving over 70Vp on your '48V supply'.  I'll remove my doubting posts.

As with all voltage doubler/multipliers, regulation is poor so you'll do well to simulate your max. load current too.  Don't forget transformer resistance and supply capacitors.

taking the other end of the bridge rectifier, not to earth, but to the 24V supply capacitor
is taking the other end of the bridge rectifier to the '24V supply' output.  There will be a capacitor there that you haven't shown.  This will give even higher output voltage but you don't need this.

And YES.  This is a good way to get 24V & 48V from a single transformer.
 
>>> i have a big trafo and i need to lower for my 24V psu.
>> OK, _how_ "big"?
> The size of the trafo is in the title


Um.

If you want to _solve_ a problem, you list the needs and resources. If you work alone, maybe you can keep the list in your head. If soliciting help from others, you want to lay-down most key details so everybody pulls the same way.

I still don't know what "big" means. Even the "48V" in the title is insufficient.

JD and Ricardo were willing to help but, like me, didn't have enough information for a good suggestion.

> 48-52V with center tap

The CT is vital information and should have gone on-stage sooner.

Ignoring the still-unknown current available from the transformer and required on the two outlets, there is a _simple_clean_ way to get what you want.
 

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> 48V for phantom, no more than 10-15mA, and the 24V PSU would deliver no more than 300mA

That's an issue, though minor.

"My way", the 68V is somewhat more efficient than the 34V. The 68V uses all the winding on both cycles, the 34V uses half the winding on each cycle. That's not a big difference (much less than 2:1 as you might first think). Your transformer should be about 50% bigger (in VA) than if it were designed solely for a 34V/24V load.  If the load was one Phantom and a complete BBC-style 24V broadcast console and rack, then it might be bucks cheaper to right-size the 24V (these days with single 24V AC winding plus FWB) and make Phantom with smaller iron or multiplier.

300mA is not necessarily "small", but it is down in the range where transformers don't get cheaper when they get smaller, so you are probably OK.
 
Oh Great Guru PRR, I grovel at your feet, in awe at the simplicity and efficiency of your solution.  :eek:  Also kicking myself as I've designed exactly that in da dim en distant past for somewhat different voltages.
 
> I've designed exactly that in da dim en distant past

If the problem were stated clearly and more completely, a dozen of us would have popped the same stock answer.

> is the transformer going to overheat

Why I was asking what you mean by "big".

 
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