Capacitors????

[scooby-doo]

Hi

Just a couple of questions regarding capacitors

Most of you know that capacitors are a common fault, I just want to understand a couple of things

Why is it ok to replace 25v caps with 35v caps?

uf Rating...how much movement have you got on the uf rating, for example lets say I have a 1200uf cap could I replace this with a 1000uf capacitor or a 2000uf cap, or do you need what it says on the tin?

And do capacitors solely fail,  is there another component on the board somewhere that is starting to overload the capacitor for it to fail?


Thanks for any advice on this one, have looked at Wiki but I am Lee Layman and get lost in translation.

[sparky]

hi
its best to use same value cap's but the higher voltage
is ok
dave

[Turnip]

Hi,

Agree with Sparky. Electrolytic caps usually fail because of internal heating causing them to either dry up or swell - either causes a rise in impedance and more heating. 
Reason for failure is high ripple current in power supplies of low'ish secondary voltages but high current.
Must admit I try to use the biggest low ESR cap that will fit the board - Chris.

[scooby-doo]

Cheers Sparky

So why didnt the manufacturer fit 35v instead of 25v is this a cost issue, and if the manufacturer did fit 35v in the first place would this of prolonged the life of the TV.

[sparky]

hi
prob they wont tv to fail
dave

[brainstorm]

Hi

I think it is a question of cost, with price erosion causing falling TV prices every year they have to save money, and this can sometimes cause premature failure of components when they get it wrong

Brian

[scooby-doo]

Right OK, I am a little bit with you Sparky, in the fast moving industry of electronics and progression why would any company in the right mind want a tv to last as long as my Grans CRT, it would put you lot out of jobs.

Moving onto the uf I know you have a 20% tolerance, but is it more important to stick bang on what comes out of the set or do some of you throw caution to the wind and lets say fit a 1500uf where a 1200uf has been.

[Turnip]

Hi,

Gran's tv probably had a 150v HT line (if a modern ancestor - failing that some 250v plus for some older gals)

Ripple current was much lower so rarely a problem.

As to capacitance value of replacement - as big as will fit the board at voltage of original - stand to be corrected but don't think increased working voltage generally equals lower ESR.

Final thought - without those dodgy caps some of us in the trade would be awful slim  - Chris.

[Ohm Supplies]

As a general rule yes, its ok to use higher voltage caps as long as they physically fit (if they’re next to a heatsink your asking for problems)
Try to stick to the same values unless you have a good reason not to, it can cause problems further down the line with the repair or the set in later life.
With the repair of PSU’s I’d try to use 105degs caps, even if 85degs are fitted, some manufacturers use 85 and these can give problems, I’d also go for ones around 2000-3000 hours span if you want a bit of life on the repair.
Most caps engineers see are swollen (top or bottom) due to the electrolyte turning from a liquid to gas, cheaper caps tend to be more prone to this and as the caps that give problems tend to be the same ones in the same boards no matter what the brand used is then it does lead you to think that it’s a number of reasons that cases the fault

if you want to read some more info. on failing caps you could try the badcaps forum, although after reading you might think that capacitors are gettign the blame for everything from failing sets to why mars bars aren't as big as they used to be ( and don't even get me started on the size of a packet of Monster Munch)

regards
Andy

[Turnip]

Hi Andy,

Can you justify your second paragraph ?

(Try to stick to the same values unless you have a good reason not to, it can cause problems further down the line with the repair or the set in later life.)

Just curious - Chris

[brainstorm]

I have noticed that we didn't get the problems with capacitors when the snickers bar was called 'Marathon'

They changed the name to Snickers and all of sudden we are flooded with Samsung TV's with bulging capacitors!

Coincedence? I think not!!

Brian

[Ohm Supplies]

Brian, you might have a point there.

my point about the correct values, I should say the correct capacitance, although you can use a higher rated capacitor in most cases (and the set will work and sets will work with low capacitors before they give up) I have found that if you use caps that are a lot higher in value (if we’re talking about electrolytics) then its more of a practical consideration, they’re more expensive and often to large although as I’ve said you can in most cases get away with it.

With the case of a 820uf being a slightly odd value engineers will often sub a 1000uf taking time to wedge it under the heatsink or stuffing them out of the top. If a customer sees it then you can expect a good ear bashing, its not great practise, so I'd say yes, "try" to stick to the correct value.

regards
Andy

[Ohm Supplies]

Brian
Did you.get the email I sent?

Andy

[brainstorm]

Hi Andy

I emailed your email address on this site, so it looks like you didnt get my email and I'm not getting yours!!

Regarding the capacitor debate, I always fit the exact capactance value, but regarding the voltage I try to make sure the working voltage is not dramatically higher than the original or this can effect the ESR

ie fit a 16v in place of a 10v but don't go sticking a 250v one in!!

And I didnt realise anyone was still using 85 degree caps!!

Brian

[Ohm Supplies]

Brian,

Look in a vestel psu next time you have one. Sites will often sell lots of caps in a "bulk buy" at a cheap price and you'll find they're often 85degs

Yes, a degree of common sence is needed with caps. you can upset how a cct. works by changing the cap values although size probably prevents you from doing this.

Ye, I got your email and replied to it, you should have 4 emails from me (try looking in your trash).
I'll resend from a different email account

regards
Andy