Question regarding capacitors?

So I'm trying to repair an LCD monitor with some blown caps but the problem is I can't find the exact ones in town and I'd like to do it without having to order parts. My question is if I have the right voltage does the farads matter as much? I understand too little could be a problem but is too much a problem? I'm not an electrical engineer but if I understand it right farads is similar to amps in that it can hold more charge but doesn't necessarily mean it's discharging that much. I could be completely wrong though. Any insight would be appreciated.

Edit: I am able to find the proper voltage ratings needed but the farads are a bit below or over the ones I'm replacing by about microfarads either way.

I don't see how a bit high would hurt anything, personally.

Most of the time, the blown caps are found in power supply circuits where the capacitance doesn't matter much, but ESR is important. Often times, the caps have twice or more the capacitance than is really needed in the circuit, simply because a physically larger capacitor has an inherently lower ESR, and the larger caps were cheaper than smaller ultra-low ESR types.

So it really depends what sort of capacitors you have available as replacements.

ESR matters in some applications, especially power filtering like Old Thrashbarg said. That's where you'll typically find blown electrolytics.

That said, your conception of farads isn't quite right. More can be VERY bad in lots of applications. Saying that the capacitance of a capacitor (the "farads") doesn't matter much if you're using something bigger is like saying resistance doesn't matter as long as you use a big enough one. Big problems.

In most power filtering applications, a slightly smaller or larger cap is fine. In other types of filters, coupling, DC blocking, etc., changing the capacitance of a component can do some very bad things.

If I were you, I would just grab the right caps from Mouser. Being within uFs doesn't mean much without knowing the original value. It's probably fairly safe to use a different value as long as you're no more than, say, 10-20% off.

The working voltage is what doesn't matter much. As long as it's higher than the working voltage of the cap you're replacing, you're fine. This parameter is simply the amount of voltage that you can put across the cap before the electrons break the dielectric (thus making a "leaky" cap).

For many applications the farads does matter, as Johan said. If the cap is used in a filter circuit, then the resonance frequency of the filter changes. This could be very bad if that circuit is part of say the tuner unit of your TV. For amplifiers, more farads can often be slightly beneficial; smaller ones are used simply to cut costs. Coupling caps also aren't too picky on the farad rating.

Therefore, since you don't really know the application of that cap, the best thing for you to do is buy caps rated the same as the ones you're pulling out. Keep in mind, the blown caps could have been casualties of another component(s) failing. Other components could be casualties of the the caps blowing. You may replace the caps just to find out something else is fried, so inspect the PCB very closely and look very well for signs of more damage, then hope that there's no further damage that's not visible.

Also too, you'll probably save money buying electronics from sites like digikey and mouser, as opposed to buying locally.

To add to Templi's post with a few minor changes:

The working voltage or rated voltage on the cap you are replacing matters and is very important. If you are expected to filter say VCC/VEE of 15V and you throw in a 5.6V cap in there, it will explode almost instantly. For example on motherboards, you can say replace a 5.6V CPU-VRM cap with a 4V or heck even a 2.5V cap because the VRM voltage is capped to say 2.0V. However, that is cutting it a bit too close for comfort.

The goal in a VRM bypass circuit is to ensure that ripple current is shunted to the ground plane of the PCB. The caps used there are conductive polymer because they offer least ESR in a tiny package. To obtain an equivalent ESR (say 7mOhm) from a traditional electrolytic cap, you would have to go for a higher voltage and a higher capacitance part.

I presume you are looking to replace the 16V bypass on the soft-start circuit? (These are the ones that almost always fail). Do you have pics?

To obtain an equivalent ESR (say 7mOhm) from a traditional electrolytic cap, you would have to go for a higher voltage and a higher capacitance part.

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You mean a lower voltage and a higher capacitance? I usually see ESR go down when voltage goes down.

Also, in Templi's defense, he did say:

As long as it's higher than the working voltage of the cap you're replacing, you're fine.

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Which is an ok general rule.

johan851 said:

You mean a lower voltage and a higher capacitance? I usually see ESR go down when voltage goes down.
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Not necessarily. ESR in a cap arises in many areas:

• Quality of welded leads
• Drying of the Electrolytic fluid.

The latter situation has more of an influence. So in a physically larger cap, the effects of the electrolyte degenerating will have less impact. The chemistry is very complicated but I can speculate a bit if you wish.

I remember referring to this work a long time ago...
http://www.amazon.com/Demystifying-...=sr_1_4?ie=UTF8&s=books&qid=1276187579&sr=1-4