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Quick question: is using a capacitor rated for high voltage (let's say 35 V) in a system that, let's say, supplies 5 V (like for LEDs or what have you) dangerous?
Since it can store up to 35 V, will it like somehow store a bunch and then release it at once, damaging the system, or it is OK to use a higher-rated capacitor than the voltage being supplied?
While not a perfect analogy, think of the voltage on the capacitor similar to the liter capacity of a tank. It will hold "35 V" but you needn't fill it completely. But like @JustJeff said, you'd be wise to ensure the container can hold more than necessary to prevent spills (and in an electrolytic capacitor's case, the electrolyte can expand and quite literally "spill" out).
Note that a better analogy to capacity would be the farad unit, since that's a measure of a capacitor's charge capacity, so don't get that confused with voltage, which is the potential to do work.
No, having a higher rated cap will not somehow store up more voltage than is available in the circuit. You actually want a cap with a slightly higher voltage rating than the highest voltage you expect to put across it. In fact, if you put more voltage on a cap than it is rated for, it is apt to catastrophically fail, i.e., pop or explode.
If a high-voltage electrolytic is used at low-voltage, the actual capacitance might be a lot lower than the stated value.
The voltage rating of a capacitor is a measure of how strong its insulation is. A 35V cap can withstand at least 35 volts applied across it (a higher voltage may cause bad things like a short through the cap and burnup). It has nothing to do with how much voltage the capacitor will store; it can store nothing higher than is input to it. The voltage rating is describing how high its barrier is; electricity shall not pass through it as long as it does not get that high.
I have a staircase with 35 steps. I am standing on the fifth step. What if I fall down? Is it dangerous? Falling down 35 steps can hurt!
Everyone else has explained well, the "pressurised water tank" analogy is very good.
Just to add;
If you look (wikipedia etc.) at how capacitors are constructed, and the factors that determine capacity and voltage-tolerance, that may help explain why the different ratings exist and why using a 1000v capacitor in your 5v circuit may be just as poor an idea as using a 3v one.
Rule of thumb: Always add a bit / round up to the next preferred value for "safety" specs like capacitor voltage, wire current carrying capacity, component power dissipation, etc.
Bear in mind that your 5v circuit is not a perfect 5v, there can be spikes, drops, surges, etc., and powering something from 5v does not guarantee that any part of the circuit will not exceed 5v due to oscillations or whatnot.
We generally spec ~2x the working voltage (so a 12v circuit would have 24v caps, and generally the available rating is 25v so that's what we use), the closer you get to the working voltage the harder the thing is working and the less reliable it will be.
Yes, the voltage is the high end rating of the capacitor but the capacitor is for storing electrons measured in farads or microfarads.
If you forget about the technical jargon, think of it like a battery. Not quite the same but if you have a 24 volt battery supplying a circuit that has a cut off of 19 volts and you only charge it to 12 volts, you have a lot less electrons to supply your circuit than what is needed and chances are the circuit won't work.
A 25 \$\mu\$F capacitor that is rated at 16 volts will have a 25 \$\mu\$F capacitance when operated near the 16 volts but if you substitute a 25 \$\mu\$F capacitor rated at 35 volts you will not have 25 \$\mu\$F capacitance if you only apply 16 volts.
These capacitors have many functions in circuits. One main function is to supply electrons to a circuit when the normal plug in supply has dropped lower than needed such as with alternating current. As the voltage and current reverse, 60 times a second, the level goes from around 170 volts peak down to zero volts and on down to -170 volts and then it repeats. The capacitors filter this drop by supplying the appropriate voltage to keep the circuit smooth. As the voltage rises back up again, it recharges the capacitor.
A leaky capacitor has the effect of a large rated capacitor that leaks and keeps the circuit from working properly. In most cases, you can over rate a capacitor and get away with it. If you double the voltage value of the capacitor but keep the supply voltage low you might want to also double the Farad value. Ex: 25 \$\mu\$F at 16 volts to become 50 \$\mu\$F at 35 volts running on 16 volt supply.
Basic capacitor construction is two plates with leads separated by insulating material...
However, electrolytic capacitor is much more than this. If you open up an electrolytic capacitor, you will find two long strips of plated aluminum separated by paper, all rolled in the cylindrical form. The cylinder is soaked in electrolyte and packaged in an aluminum can.
One most important thing to take note is the real insulation between the strips is not the paper. The porous paper is merely to avoid the strips from directly contacting each other. The real insulation is the chemical layer that forms on the aluminum strips when the capacitor is connected to the DC source in the correct polarity. When connected in wrong polarity the conductive layer forms causing continuous current flow. Temperature quickly rise because the DC resistance of the conductive layer that forms is not very low, causing ohmic power loss.
So, answering the question, the optimum amount of chemical insulating layer forms when the capacitor is operated almost near the rated voltage in correct polarity. Operating a high voltage capacitor at lower dc voltage cause some low continuous current to flow through the capacitor, thus rendering the capacitor not behaving ideally as a capacitor.
The voltage rating of the capacitor is the point at which the dielectric & insulation between the two plates starts to break down and fails.
Consider it like a rope. It will carry so much weight then it snaps, and when it "snaps" it fails catastrophically.
The second point: Do not place very large storage caps AFTER a regulator. They go BEFORE the regulator because a capacitor behaves like a 'short' until charged, not to mention it'll reverse bias the regulator if it loses power.
