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pixelphoto 7th July 2012 04:39 AM

Replacing capacitors
 
I intend to replace all or most of the caps on a psu. I realize I can replace them with like specs. And I realize I can replace them with ones of higher specs. For instance for a cap of 16V/470uF specs how much over this could I safely increase to? Would 16V/1000uF be unsafe?

Regards,
Marvin

wintermute 7th July 2012 04:43 AM

You need to give some indication of the purpose of the caps in the PSU. For instance, if it were a regulated PSU increasing the output cap (depending on the regulator) could lead to instability.

If they are reservoir caps then you need to consider what the additional load on the rectifiers and transformer may be at startup (unlikely to be an issue going from 470uF to 1000uF).

Tony.

pixelphoto 7th July 2012 04:59 AM

It is a SMPS. Switching -------- power supply? From a Sony Playstation 1.

Conrad Hoffman 7th July 2012 05:04 AM

Caps in a SMPS are typically chosen by esr, not by value. The value is usually far higher than needed, but is what it is to meet the necessary esr spec at the frequency of operation. I'd be choosing caps by esr, temperature/life ratings and, IMO very important, by manufacturer. You probably don't have enough info about the supply to deviate far from the factory numbers.

marce 7th July 2012 10:33 AM

Second that, SMPS's can be tempremental mistresses, calmand quite one minute, change one little thing and they can squark and scream. As they work at higher frequencies than linear, large output capacitance is not as critical to reduce ripple, but this ripple being of a higher frequency require as stated by Conrad ESR to be taken into consideration. Also you dont want to be adding extra parasitic impedances by changing component sizes, you may introduce unintentional antennas and cause EMC problems.

pixelphoto 7th July 2012 04:14 PM

Thank you all for your informative replies. I know nothing in comparison to you. I'm learning as I go and eliminating as many variables as I can. I think my best move is to replace the old caps with modern versions of similar specs (Nichicon KT and CS series).

Regards,
Marvin

KatieandDad 7th July 2012 04:24 PM

Why are you replacing them - because they are faulty or because you feel like it ?

As has been muted above - most of these commercial SMPSs are designed to a tight spec and predominantly to keep costs to a minimum.

If you replace components inadvertently you can cause it to stop working, or in this case, possibly even cause it to damage the PS.

pixelphoto 7th July 2012 05:00 PM

I'm replacing them because they are between 12 and 15 years old. I believe that is old for a cap. I would think new modern caps of the same or close to specs would improve performance. I'm not overly concerned about loosing a psu as it is my test bed.

KatieandDad 7th July 2012 05:13 PM

Change them one at a time and test after every change.

pixelphoto 7th July 2012 05:22 PM

Thank you KatieandDad. I'll be changing 5 maybe 6 and I will test, you can be sure.

Regards,
Marvin


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