2
\$\begingroup\$

I have a EVGA 500W PSU with an APFC which is causing spike noises, presumably from the APFC or main switching as they both run in sync controlled by the CM68000TX combo apfc switching PWM chip

Noise (about 700mV every 16uS) comes in when APFC turns on, even on the 5VSb line. (5VSb line is smooth until PSU turns APFC and everything else on, noise on all 12v, 5, 3.3 lines etc.) Also when I load the PSU with e.g. a 10Ohm resistor on the 5V line, the Power Good output goes low for about 270mS. Voltage levels averages are kept somewhat ok, but I believe the PG is rebooting the PC "randomly"

Now I have measured the original 400V 270uF and is giving me less than 240uF so I have in hand two 250V 470uF caps, can I connect these in series for testing purposes?

I'm a bit concerned if it being 200 extra uF would cause any issue, or may be something else I don't know.

\$\endgroup\$

1 Answer 1

2
\$\begingroup\$

If 2 x 470uF capacitors are connected in series, the result is a 235uF capacitor, but only if both are identical the maximal voltage of each one will be exactly 50%. If the capacitance is not exactly the same, the voltage will be unbalanced and could exceed 250V for one of them. Furthermore, for SMPS, the filter capacitors need to be suited for the high current peaks and resulting dissipated heat from the series equivalent resistor. Often parallel capacitors are used in those devices, but never series capacitors. In other words: those 2 capacitors as substitute most likely would not be an improvement.

But to get the right capacitor, the next recycling yard may have some old TVs, PCs, monitors or other devices with SMPS, where some 400V capacitors could be salvaged for free. Caveat: A few years back, those capacitors were prone for failing, testing before use is essential.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks, good reminder that capacitance doesn't add like resistors \$\endgroup\$ Commented Jun 1, 2020 at 20:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.