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Old 04-26-2011, 11:41 AM   #1
adict2jane
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Default Replacing badcaps with new, higher capacitance

Hello,
First post!

I am relatively new at modding electronic circuits in general. I have a quick question.

I need to replace a bunch of blown 6.3V 2200uf and one 6.3V 1800uf caps on a Dell optiplex 745 mobo. I am going to get a bunch of 2200uf rated caps, can I just use a 2200uf cap to replace the 1800uf? I am pretty sure I can but just want to make sure.

thanks!
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Old 04-26-2011, 03:52 PM   #2
rhan101277
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Default Re: Replacing badcaps with new, higher capacitance

I don't see why not, just don't reduce the voltage.
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Old 04-26-2011, 07:20 PM   #3
delaware74b
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Default Re: Replacing badcaps with new, higher capacitance

If you're in US or Canada, I'd recommend getting caps from the following suppliers: Badcaps.net, Digikey.com, Mouser.com.

Do NOT purchase caps from retailers such as RadioShack, etc. They sell general purpose caps and will not perform correctly in any computer, especially the Dell and HP ovens.

Please follow the site's recommendations of attaching photos. I recommend you attach clear-focused pictures no larger than 2000x2000 pixels. In order for us to help, good photos of caps in question, along with brand, series, capacitance, voltage, date code, dimensions, board model and revision are a must. For example: Rubycon MBZ, 1800 uF, 6.3 volts, 10 x 16mm, etc on a Dell Optiplex 745 board (form factor here), v 1.1
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Old 04-26-2011, 08:48 PM   #4
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Default Re: Replacing badcaps with new, higher capacitance

The value is so close I guess it wouldn't matter, personally I would replace with exactly the same capacity - unless I knew for certain it didn't matter or it was advised to increase capacity for stability\reliability or something.
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Old 04-26-2011, 09:06 PM   #5
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Default Re: Replacing badcaps with new, higher capacitance

[Repeat] Personally I like to replace with exactly the same capacity.
That said..
The uF on caps is +/-20% and 120% of 1800uF is 2160uF.
- Plus uF tends to go down as caps age.

Probably [and in theory] not a problem..
.. But you have to ask yourself:
"if they were ordering 2200uF in droves anyway why would they complicate the assembly line by use a single 1800uF there if the uF didn't matter?"
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Old 05-21-2011, 09:11 AM   #6
adict2jane
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Default Re: Replacing badcaps with new, higher capacitance

Hmm...
Ok, thanks for your reply everyone! Sorry its been a while, life is crazy...
Maybe I was thinking about capacitors in the wrong way, it's been a while, haha.

I was viewing a capacitor like a battery and the Farad rating as a measure of capacity. A 2200uf cap has the same exact properties as a 1800uf cap at the same voltage except it can store more energy (which would simply be an excess and never used in this case). Is this wrong?
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Old 05-21-2011, 09:41 AM   #7
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Default Re: Replacing badcaps with new, higher capacitance

No it's not wrong but that's far from the only thing that caps do.

uF affects how long it takes to charge the cap and that is sometimes used to control how long it takes a voltage to get to some point where it 'trips' another circuit on or off.
[For instance when starting a PC if voltage is not to 80% of the nominal value within about 50 milliseconds a safety feature cuts-in that turns the power supply back off.]
Huge uF caps could keep the system from coming on because they take so long to charge that the safety clock times out and trips the PSU off every time.
In that case to uF would probably have to be really huge but there are all kinds of smaller control circuits that do similar things.

uF can also affect frequencies and there are several timing circuits on a motherboard.
Look closely at a mobo.
You'll at least one [and probably several] quartz crystals [usually in ob-long metal cans] that help set the frequency for timers [oscillators, clocks].

Frequency also comes into play when filters use an inductor in combination with a cap. The values [uF and uH] are chosen to optimize filtering at some frequency.
If you change one value without changing the other then the filter will be less efficient at removing the expected noise [ripple] frequency.

If you don't want to trace out a circuit and do the math to find out what the cap does it's best to stick to the original uF value.
.

And that's just the concerns about uF.

ESR and Ripple are important too and that's a whole 'nuther discussion.
Motherboard caps are low ESR [and high Ripple] caps - and GP electronics places like Radio Shack & Fry's Electronics don't have them.

Last edited by PCBONEZ; 05-21-2011 at 10:02 AM..
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