Computer power supply capacitors - generic question

[Sal Holland]

I have a generic power supply that had the two main 470uF/200V caps
fail about a year ago (physically leaking, one had the top pretty well
opened.)

I replaced them with new caps from mouser (computer grade caps and
105c rated), and all was working well.

This year, the power supply is again exhibiting problems..upon a cold
boot, the computer will do about two reboots before it finally powers
up.

I was wondering if these voltage ratings on the caps, were too
low..seems like they should have higher rated caps then this I would
think.

On the -12V line, C1 is rated 16V
On the +12V line, C2 is rated 16V
On the +5 or -5V line, C3 is rated 16V
On the +5 line, C4,5 is rated 10V
On the +3.3V line, C6, C10 are rated 10V and 50V respectively
On the +5VSB (+5 Standby I am guessing?), C11,C12, and C13 are all
rated 16V

Reading from the BIOS, the computer shows these voltages:

CPU Core: 1.69v
+3.3 is reading 2.94V
+5V reads 4.99V
+12V reads 11.49V
-12V reads 11.86
-5 reads -5.25
Battery Voltage (should be 3V) reads 2.88V
Standby Voltage (should be +5) reads 4.80V

As you can see, +12 Volts is pretty low, as is the +3.3 volt line. I
suspect these are what is causing the computer to reboot about two
times.

Are the 10 volt ratings of C4,C5,C6 all too low? Seems to me they
should have put some capacitors that had a little higher rating in
order to better handle transient surges, etc?

Are the 16V caps also too low for the +/- 12 volt lines? Would 25V
have been better or perhaps 50V caps? Or is 50V too high?

 

[Art]

Actually the measurements are fairly nominal but as cheap as good power
supplies are now days I'd just replace it. At least try a new one in your
system to see if it rectifies the boot up problem.

 

[Mr TUBEAMPS]

i have fix a few pc supplys,
try replaceing the caps and resistors
that connect to the bases of the two
main switching transistors, you see some
values like 470k around them as well.
check all resistors around them transistors,
you may fine one or two near open or high.
and check for bad soldering joints.
these caps and resistors i call starting
resistors and caps. be cairfull thay are
dangeres!

john

 

[RubbishRat]

You haven't specified the Wattage rating of the PSU or the system you expect
it to power. If the boot up problems are caused by the supply being
borderline then no amount of swapping caps is going to fix it. A new higher
power supply is going to be a lot less trouble and probably the cheapest
solution in the end.
Pete

 

[Don Allen]

Switching power supplies can be very dangerous, so be careful when you
are working on the unit. Given the prices of decent supplies today, I
wouldn't waste your time troubleshooting or replacing caps - just
replace the supply. Several supply manufacturers, such as Antec, etc.
offer excellent supplies for a very low cost. Go to www.newegg.com, or
other good online sources, and find you best price on a 300 to 400 watt
supply.

Don

 

[larrymoencurly]

I have a generic power supply that had the two main 470uF/200V
caps fail about a year ago (physically leaking, one had the
top pretty well opened.)

I replaced them with new caps from mouser (computer grade caps
and 105c rated), and all was working well.

This year, the power supply is again exhibiting problems..upon a
cold boot, the computer will do about two reboots before it
finally powers up.

I was wondering if these voltage ratings on the caps, were too low..

On the +12V line, C2 is rated 16V
On the +5 or -5V line, C3 is rated 16V
On the +5 line, C4,5 is rated 10V
On the +3.3V line, C6, C10 are rated 10V and 50V respectively
On the +5VSB, C11,C12, and C13 are all rated 16V

Reading from the BIOS, the computer shows these voltages:

CPU Core: 1.69v
+3.3 is reading 2.94V
+5V reads 4.99V
+12V reads 11.49V
-12V reads 11.86
-5 reads -5.25
Battery Voltage (should be 3V) reads 2.88V
Standby Voltage (should be +5) reads 4.80V

I've found that BIOS voltages aren't very trustworthy, and I've
personally seen one differ as much as 6% from my digital meter's
reading. Windows software readings are even worse.

Almost all the capacitor problems with mobos and PSUs were with the
low voltage capacitors because only they were the low-ESR types
involved in the counterfeit electrolyte problem that plagued some
Taiwan capacitor brands a few years ago, including Jackcon, JEE,
Luxon, and JGE but not Jamicon or Teapo. See www.badcaps.com and
www.motherboardrepair.com for more details.

I had a cheap PSU where the 16V capacitors for the +12V rail were
getting 50V peak spikes, but I saw nothing that high in a better PSU's
+12V.

 

[RWatson767]

Larry

Re: Computer power supply capacitors - generic question

had a cheap PSU where the 16V capacitors for the +12V rail were getting 50V
peak spikes, but I saw nothing that high in a better PSU's +12V.

From one who has replaced a lot of capacitors.

DC voltage plus signal voltage/ripple times two. In your case DC plus ripple
times two. . Use milspec capacitors. I suspect the computer grade thing is more
marketing hype than anything else.
Bob AZ

 

[Mr TUBEAMPS]

if you buy a new supply, dont throw away the old
one, you can use this supply on other things
like op amps.
i built a 16 input mixer useing a pc power supply.
to run this supply without a computer,
connect a 5watt 100 ohm resistor across
the 5 volt out to ground, its a dummy load.
same for ATX supplys, to fire up an ATX,
ground the SP-ON wire.

john

 

[Norm Dresner]

if you buy a new supply, dont throw away the old
one, you can use this supply on other things
like op amps.
i built a 16 input mixer useing a pc power supply.
to run this supply without a computer,
connect a 5watt 100 ohm resistor across
the 5 volt out to ground, its a dummy load.
same for ATX supplys, to fire up an ATX,
ground the SP-ON wire.

john

Shouldn't a 1/2 Watt 100 ohm resistor be sufficient? 5V / 100 Ohms = 50
ma. 50 ma * 5V = 250 mW, right?

Norm

 

[TCS]

if you buy a new supply, dont throw away the old
one, you can use this supply on other things
like op amps.

that's like using a used bulldozer for a boxgarden.