Problem with computer PSU

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Re: Problem with computer PSU

Postby Karlsweldt » Sat Dec 13, 2014 11:25 am

evasive wrote: The main filter caps in that power supply are just enough for providing enough ripple reduction to chop into something decent on the outside so a bit bigger is better in this case.

True, evasive. For a power supply filter capacitor, larger capacitance can be better. But for any circuit coupling, the exact value and voltage must be kept.. or timing cycles will go awry. I did mention "For primary power capacitors, you could use a bit higher capacitance value. Almost all PSU designs have "average" primary filtering needs for the switching transistor. Might be some minor ripple in the DC voltage, but does not affect the desired end.
The only effect with using higher capacitance values might be that the inrush current may be higher for 1/2 second, when the PSU is powered up. If the primary fuse is a fast-blow type, might be need of a 2-second delay type. Same amperage and voltage!
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Re: Problem with computer PSU

Postby rascard2007 » Mon Dec 15, 2014 6:00 am

Thanks Karl and evasive, very useful your explanation, I had read that when U have to replace a blown main filter cap is better to use a little more capacitance, but i don´t now what lttle more is enough or more than enough. let ny note that in trhis case the caps never blown!

on anyway this PSU is the one that I use in my work bench to test other equipment so I think it will not go more tan 25-50 % of the supported load, It´s labelled at 450 Watts (peak 485)

I´m sorry mistyping the brand of the capacitors :oops:
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Re: Problem with computer PSU

Postby Karlsweldt » Mon Dec 15, 2014 6:16 am

It is preferred that capacitors on a motherboard be rated at 105°C, due to higher expected temperatures. But most PSU designs use an 85°C rating, with the thought that there is a lot of cooling air through the unit. But what about the air being exhausted through the PSU from inside the PC case?
Some premium PSU models go with a higher temperature spec on capacitors.
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Re: Problem with computer PSU

Postby evasive » Mon Dec 15, 2014 8:47 am

For normal ATX power supplies that would indeed be fine as the cheaper varieties are gutless wonders with plenty of empty space (where quality supplies would have bigger/better components). :lol:

Except for the +5VSB capacitor that always seems to be in the wrong location (e.g. hottest place in the power supply) and is quickly cooked to a crisp...
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Re: Problem with computer PSU

Postby rascard2007 » Mon Dec 15, 2014 12:28 pm

all of the caps in my little crappy PSU are rated for 85° degrees and yes there is plenty of space inside the PSU.

I will check later where the hell the +5VSB cap is
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Re: Problem with computer PSU

Postby Karlsweldt » Mon Dec 15, 2014 5:17 pm

Here is a link to a "typical" PSU circuit.. http://www.smpspowersupply.com/power-supply.html
And you can view hundreds of PSU circuits and images if you search for computer PSU diagram.
WIKI has a small 'thesis' on computer PSU history and operation.. http://en.wikipedia.org/wiki/Power_supp ... omputer%29
The "chopped" DC from the primary goes into the core transformer at maybe 50 KHz or higher. If the core transformer were to operate at 50 Hz or 60 Hz, it would likely be the size of a cement block, and weigh more than 75 pounds!!!!
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