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Electronics => Repair => Topic started by: Heyyy on March 23, 2019, 05:25:20 pm

Title: ATX PSU repair
Post by: Heyyy on March 23, 2019, 05:25:20 pm

Hi,
I have an ATX 450w psu that has bad caps(the bigg 200v ones that probably form the half bridge together with the chocke). I have some spares but they are half the nominal value of the original ones.I put them and the psu seems to work fine but the ripple levveles are high :200mv high and switching transients are way higher.
I did the mesurements under slight load at bootup.Is this fix safe or i should relly struggle to find caps of the right value that fit? I don't think i'll push it beyond 200W.My other psu is a 500w model and uses 470u in series....what my new caps are.The old ones were a pair of 820u in series.
Output voltages seem to be in speck or 10mV out.
PS:The new caps are 200V each so the voltage rationg is ok.My concern is about the capacitance.

Title: Re: ATX PSU repair
Post by: url on March 23, 2019, 05:31:41 pm

Hi

I think you have answered your own question - the ripple levels are high.

Plus - it should be repaired back to how it was designed... the correct caps must be around somewhere at the right price.


David

Title: Re: ATX PSU repair
Post by: Heyyy on March 23, 2019, 05:39:30 pm

I have them.But they don't fit.The only form factor i found for their original value are short and too bulky.Does anyone know any tricks to make them fit?

Title: Re: ATX PSU repair
Post by: coromonadalix on March 23, 2019, 08:46:54 pm

I would not try to fit them if the size dont match .. my 2 cents

add longer leads to the caps ??

Title: Re: ATX PSU repair
Post by: Heyyy on March 23, 2019, 09:44:19 pm

I'll try the leads method and use longer leads ,but it's really cramped in there.
Also ATX supplys what topology do they use?Since i saw 2 power switches and 2 caps + 1 chocke i belive it is a half bridge type?
This way i'll know more about what cap value does to the output characteristics.

Title: Re: ATX PSU repair
Post by: PKTKS on March 25, 2019, 05:14:00 pm

Yes usually a half bridge  but  cheap models also use a switch
110/220 which short cuts the virtual ground to the bridge rectifier
making it considerable different from the usual half bridge split cap.

If you are using 110 mode you have a different layout on the split
cap half bridge virtual ground.

In other words the full bridge rectifier with split cap virtual ground
is valid only on 220 inputs. On 110 a by pass switch cuts that.

Not considering more expensive models w/ PFC supervisors....

Paul