Substitute for 400v 1uF Electrolytic?

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bigal_scorpio

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Hi to all,

I am trying to repair my sons PC power supply.

The cause of the breakdown is the usual swelled caps - 4 of which are 470uF 16v which I had on hand and the other culprit is a 400v 1uF that I can't find anything even close in my electrolytic box.

The only capacitor I have that is 1uF and rated 400v is the large poly type MKS.

Since this is a complex SMPS I cannot be certain what the capacitor is doing and wondered what if any harm would be done by using the MKS until I can get a correct one?

Failing that is there any way of using multiple electrolytics as a substitute?

Al
 
Those sort of values are normally start-up capacitors, check if it's in the primary or secondary side of the PSU.

If it's in the primary, then it's most probably a start-up cap, you could try the MKS one - but personally I've always kept suitable electrolytics in stock at work for PSU repairs (in old TV's etc.)
 
Hi Nigel,

Nice to hear from you mate.

The cap is on the output side of the PSU but this is one of the 1000w ones and has two massivly full boards inside. Its all heatinks and components, very little free space and this cap is on the input of the second (Output Board) and come directly from a bank of 390uF 420v electrolytics. The last time I saw caps as meaty as these was in a Denon Amp!

I think that the cap must be for the decoupling between the stages but it seems a bit big for that at 1uF?

Anyway I have found some at Maplin that I can pick up tomorrow. That should stop the whining noise (my son)

Al
 

It seems weird that it's 400V, there's normally only low voltages on the secondary side - when you get it working measure the voltage across it, then you can be depressed that you didn't need a 400V cap at all
 
If theres a bunch of 1u 400v caps in the design that run at that voltage and only one 1u that runs at alow voltage, maybe the manufacturer chose to have them all 400v to avoid stocking more parts and avoid the risk of the wron cap being fitted.
 
dr pepper said:
If theres a bunch of 1u 400v caps in the design that run at that voltage and only one 1u that runs at alow voltage, maybe the manufacturer chose to have them all 400v to avoid stocking more parts and avoid the risk of the wron cap being fitted.
Click to expand...

Not a good idea, high voltage caps like those have really crappy ESR.
 
Yes very true.
I just repaired a wall wart, the only prtotection it has on the mains is a fusible resistor, or incendiary fuse as I call them, in fact I'm not sure its meant to be fusible.
 
Hi Guys,

Just fitted the new caps and tried it. Seems I may have invented a quicker way of releasing the magic smoke than usual!

Dead short somewhere in the PSU with quite an impressive explosion. Strange thing is that there are no visible signs of anything wrong and I can't find any shorts!

Time to call it a day I think, just get another one. My wifes nerves won't stand another switch on.

Al
 
Nigel Goodwin said:
Not a good idea, high voltage caps like those have really crappy ESR.
Click to expand...

Guys,
Your use of acronyms with no explanation of their meaning lead me to look what ESR (equivalent series resistance) is and I found the answer to why they use what they use.(I think) This is a link to a paper by Mark Gebbia, Illinois Capacitor, Inc that explains a few things, and in passing why the higher than needed voltage caps in the power supplies.

I do love to swim where the water is over my head. Thanks for the education.
TBob




https://www.electro-tech-online.com/custompdfs/2012/11/low_ESR_fact_or_fiction.pdf
 
TBobTrasman said:
Your use of acronyms with no explanation of their meaning lead me to look what ESR (equivalent series resistance) is and I found the answer to why they use what they use.
Click to expand...

ESR is EXTREMELY well known, it's the major cause of failure in electronic equipment over the last 20-30 years, with ESR meters an essential part of any service engineers equipment.
 
Agreed I've a couple, one that goes with me is self made.
TV engineers all have an esr box.
 
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