quick note of caution ... the diode-uprating idea assumes a push-pull PSU topology. Make sure this is what you have before you begin to modify it, because many low-power PC SMPS (<250W) these days are actually flyback designs. These use a single primary power MOSFET, lack a separate driver transformer, and have an extra, weaker winding for -12V.
Dalla modestissima esperienza che ho, la tensione -12V in un classico alimentatore per PC è ricavata, nella maggior parte dei casi, da un mini-trasformatore secondario, partendo dalle tensioni principali a +12 e/o a +5V disponibili all'uscita del trasformatore principale. La corrente che ricaverai da un'uscita a -12V sarà quindi comunque bassa (0.5-1A) e determinata dal dimensionamento del trasformatore secondario che fornisce le tensioni negative (-12V e -5V), oltre che alla componentistica di raddrizzamento - filtraggio - stabilizzazione della tensione, che segue.
L'impresa di ricavare le correnti che richiedi a -12V da un classico PSU per PC è ardua, e il gioco non vale la candela.
Puoi però partire da uno schema valido, e recuperare dal PSU un sacco di componenti che sarebbero introvabili...
Buon divertimento
Fabio
L'impresa di ricavare le correnti che richiedi a -12V da un classico PSU per PC è ardua, e il gioco non vale la candela.
Puoi però partire da uno schema valido, e recuperare dal PSU un sacco di componenti che sarebbero introvabili...
Buon divertimento
Fabio
Nicoch-
Most AT & ATX power supplies above 200W are half-bridge, so up-rating the two -12V diodes to the same rating as the +12V diode pair shouldn't be too much of a problem electrically. Just finding a place to mount the new diode pair may be a bit of a challenge.
BTW, what diode pair are U using for the -12V line? I recommend the ONSemi MUR1620CTR if you want to do both ends of the xfmr on the same diode body, much like almost every car amp that has a DC-DC SMPS. If you're doing separate diodes, then most Schottkys up to 200PIV should work OK, though you will have to find homes for two and not one components.
referring th the thread you linked to, Eva is right in that the majority of main power xfmrs actually have tapped, rather than separate windings for the 12V & 5V windings, so full 5V current of 20A should be available at the 12V windings.
Just my two cents.
Steve
Most AT & ATX power supplies above 200W are half-bridge, so up-rating the two -12V diodes to the same rating as the +12V diode pair shouldn't be too much of a problem electrically. Just finding a place to mount the new diode pair may be a bit of a challenge.
BTW, what diode pair are U using for the -12V line? I recommend the ONSemi MUR1620CTR if you want to do both ends of the xfmr on the same diode body, much like almost every car amp that has a DC-DC SMPS. If you're doing separate diodes, then most Schottkys up to 200PIV should work OK, though you will have to find homes for two and not one components.
referring th the thread you linked to, Eva is right in that the majority of main power xfmrs actually have tapped, rather than separate windings for the 12V & 5V windings, so full 5V current of 20A should be available at the 12V windings.
Just my two cents.
Steve
I've modified an older AT supply for +/- 12V rails.
As others have mentioned,be sure that the -12V rail is rectified from the main 12V winding,Sometimes they use a seperate,smaller winding for the -V's and it can't support the current,so you can't use that winding..
But there's still hope!
Basically just tack another diode (pointing in the opposite direction) to the +12V winding,add some caps/coils for some filtering,and you're set...
Usually the feedback (for regulation) is taken from the +5V rail.
I moved it to the +12V rail,and fiddled the resistor values a bit -put a small trimpot on there to adjust the voltage just right.Then you can also adjust it a bit if you need to..(say you need +/-10V or +/-15V,just adjust the trimpot! -but be warned,some PSU's can't be "pulled" from thier nominal values much,it might not go up to 15V,etc. and if it has OVP,it may trip.)
Another note,Since the rest of the rails aren't loaded,you might be able to load the 12V rail beyond it's ratings a bit..
As others have mentioned,be sure that the -12V rail is rectified from the main 12V winding,Sometimes they use a seperate,smaller winding for the -V's and it can't support the current,so you can't use that winding..
But there's still hope!
Basically just tack another diode (pointing in the opposite direction) to the +12V winding,add some caps/coils for some filtering,and you're set...
Usually the feedback (for regulation) is taken from the +5V rail.
I moved it to the +12V rail,and fiddled the resistor values a bit -put a small trimpot on there to adjust the voltage just right.Then you can also adjust it a bit if you need to..(say you need +/-10V or +/-15V,just adjust the trimpot! -but be warned,some PSU's can't be "pulled" from thier nominal values much,it might not go up to 15V,etc. and if it has OVP,it may trip.)
Another note,Since the rest of the rails aren't loaded,you might be able to load the 12V rail beyond it's ratings a bit..
Nich-
Wow! That PSU article was incredible. I must've downloaded about 30 pics from it (no kidding!) There is some serious technology in some of thehigher end units. I liked the850W model that used theUCC3895 phase-modulated full-bridge controller. Also, the Active PFC'ed units looked pretty cool. The more exotic designs kinda change the "standard" layout of the conventional ATX board layout a bit. BTW, nice slideshow. I would think you could get the -12V you need by doing the necessary mods.
'junkie-
You are right in that if the +5V line is lightly- or un-loaded, then the +12V line can be loaded past its rated output "a bit". Since the majority of half-bridge PSUs use tapped, rather separate windings, the full 20A can be drawn from the +12V line without too much consequence. I like the idea of putting a pot in series with the upper sense resistor to tweak the output voltage, especially since the +12V line was always a little bit low with aa lightly loaded +5V line.
Regarding the separate smaller winding for the -12V line, about the only time I have ever seen this is if it uses the flyback topology.
Wow! That PSU article was incredible. I must've downloaded about 30 pics from it (no kidding!) There is some serious technology in some of thehigher end units. I liked the850W model that used theUCC3895 phase-modulated full-bridge controller. Also, the Active PFC'ed units looked pretty cool. The more exotic designs kinda change the "standard" layout of the conventional ATX board layout a bit. BTW, nice slideshow. I would think you could get the -12V you need by doing the necessary mods.
'junkie-
You are right in that if the +5V line is lightly- or un-loaded, then the +12V line can be loaded past its rated output "a bit". Since the majority of half-bridge PSUs use tapped, rather separate windings, the full 20A can be drawn from the +12V line without too much consequence. I like the idea of putting a pot in series with the upper sense resistor to tweak the output voltage, especially since the +12V line was always a little bit low with aa lightly loaded +5V line.
Regarding the separate smaller winding for the -12V line, about the only time I have ever seen this is if it uses the flyback topology.
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