As I recall, capacitors are rated by the amount of charge they store. (Although, in theory, they are never 100% charged; it is asympototic.) The stored charge is measured in basic units of Farads (F) - with the more practical unit being microFarads (uF).
Nonetheless, wouldn’t it be more useful to know what voltage the capacitor can provide? Granted, the voltage will drop off as the charge is dispensed, but isn’t there some way to talk about apples to apples regarding the voltage any capacitor can provide?
I mean, if HVAC can have terms such as “ton of refrigeration” and “standard air” - both of which are industry-standardized terms based on some qualifying parameters…couldn’t the electronics people, or NIST perhaps, give us some conventional standard by which to equate uF to volts (V) or millivolts (mV), etc?
Or, maybe there’s some simple formula I missed…? Thanks!
There is charge in the capacitor, measured in coulombs. One ampere flowing for one second delivers one coulomb.
The capacitor will have a voltage across it proportional to the number of coulombs held inside. One farad of capacitance stores one coulomb per volt, two farads stores two coulombs per volt.
The capacitor connected to a voltage source through a finite resistance approaches that voltage asymptotically, but you can put 100% of some charge into the capacitor in a finite time (unless you exceed the voltage limit).
The energy in joules is the charge times the square of the voltage, over two (IIRC). For a given dielectric, such as polypropylene, the energy a capacitor can store is proportional to the quantity of dielectric.
From a more hands-on standpoint, I’ve never seen a cap that didn’t have it’s capacitance and voltage printed on it (except tiny ones where there isn’t space).
Looks like most of the capacitors on Mouser have voltage ratings.
Oh. Re-reading the third paragraph, it lloks like you’re trying to convert between farads and volts, which isn’t possible without knowing other things, as Napier points out.
Since we’re on the topic, perhaps someone could also explain why a fist-sized capacitor rated at 10 volts could hold half a farad, while one in the thousands of volts of the same size would only hold µF or pF?
Dielectric strength. A dielectric (insulator) to hold back only 10 volts can be MUCH thinner (and therefore take up less total volume) than one needed to hold back several killovolts.
When you see a voltage number associated with a capacitor it generally means the maximum voltage that the manufacturer says can be across the terminals of the capacitor. If you go higher you can damage the capacitor.
There is no real relationship between the capacitance value of the capacitor and the voltage rating. The voltage rating comes from the dielectric material and its thickness. You can make high voltage capacitors with low capacitance values or you can make high voltage capacitors with high capacitance values. The size will be different as QED discusses.
In general the capacitance value is the one people care about. You have a capacitance that you need then you find a capacitor that can be used at the maximum voltage you know will be across it.
Listen to gaspacho! The numbers printed on a capacitor, or the color code in some cases, tell you the size of the capacitor in microfarads, or picofarads as the case mey be and the maximum applied voltage you can use without damage to the part.
You are confusing things. Capacitors don’t “provide voltage.” And uF can’t be equated to volts except through a formula.
The voltage across a capacitance is equal to the time integral of the current into it divided by the capacitance in Farads.
For the moment take the current as equal to a steady state sinusoid, Icos(wt). The integral of this is Isin(wt)/w. Dividing by the capacitance gives the voltage as:
V = Isin(wt)/wC
We can go another step. Sin(wt) = cos(wt - 90[sup]o[/sup]) which can replace sin(wt) in the Voltage formula.
V = Icos(wt - 90[sup]o[/sup])/wC
This says that the capacitor voltage is equal to the current multiplied by the factor 1/wC and shifted back by 90[sup]o[/sup]. The factor 1/wC is the magnitude of the reactance of a capacitor to a steady state alternating current and w = 2pi * the frequency of the current.