The big main filter cap

[Rob Northen]

Hello fellow forum brainiacs

I've noticed that it seems vey rarely that the 400v (can be lover) main filter cap goes bad, and when the people give advice here, its almost also not to change this cap.

Question is why ? Its often made by some crap company, where there other products fail.
Is it beacuse there is very low load, current flow through it ?

[jetadm123]

Re: The big main filter cap

Hello fellow forum brainiacs

I've noticed that it seems vey rarely that the 400v (can be lover) main filter cap goes bad, and when the people give advice here, its almost also not to change this cap.

Question is why ? Its often made by some crap company, where there other products fail.
Is it beacuse there is very low load, current flow through it ?

The reason being that the the large cap operates at a lower frequency (think mains frequency) than the smaller caps. The smaller caps operate at a higher frequency, which in turn causes them to run hotter and dry out.

[PCBONEZ]

Re: The big main filter cap

They are right after input rectifier. (The caps are circled, other designs have only one.)
All they do is smooth rectifier output which is low frequency.
That's not that much work.
.

[TheLaw]

Re: The big main filter cap

They can die though, if they are 85*C of an oddball company, and are rather small.

[Rob Northen]

Re: The big main filter cap

The frequency, I see. Makes sense. They will only be subject to the mains, 50/60 Hz
And the smaller ones have to take switching Hz from the switching transformer.

Thanks!

[TheLaw]

Re: The big main filter cap

The frequency, I see. Makes sense. They will only be subject to the mains, 50/60 Hz
And the smaller ones have to take switching Hz from the switching transformer.

Thanks!

Well the smaller ones have to remove much higher frequency noise from the switching transistors and regulators on the secondary side...not the transformer necessarily.