Capacitors are passive devices that store energy in the form of an electric field. This electric field exists between two conducting plates that are separated by an insulating material called a "dielectric." In general, capacitors block steady-state current while allowing alternating currents to pass through. The "ripple current" of a capacitor is simply the root-mean-square (RMS) value of the alternating current that is passing through a capacitor. In real life, capacitors also include a small amount of resistance to alternating current; this is called the ESR (equivalent series resistance). As the ripple current travels through the ESR, heat is generated. A capacitor's ripple current rating expresses the amount of ripple current that a capacitor can safely carry.
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Look in your capacitor's data sheet to determine the ESR. The ESR varies with frequency, so some data sheets will provide different ESR values for different test frequencies. Choose the ESR value that best corresponds with the frequencies that are present in your circuit.
Divide the power dissipation of your capacitor by the ESR, then take the square root of this result to calculate the ripple current. This process is an extension of the basic formula for electric power: Power = (Current^2) x (Resistance). In a capacitor, this formula changes to Power = (Ripple Current)^2 x (ESR). If you have a functioning circuit and you can measure the approximate power dissipation, this formula will give you a rough estimate of the actual ripple current. If you know the maximum power rating of your capacitor, this formula will give you the ripple current rating of the device.
Use the following formula to calculate the ripple current of your electrolytic capacitor if you don't know the power dissipation: Ripple Current = square root ( (dT x A x B) / (ESR) ). B stands for the specific heat conductivity of the capacitor, A stands for the geometric surface area of the capacitor, and dT stands for the difference between the ambient temperature and the capacitor temperature. This formula allows you to calculate the ripple current rating based on the fundamental properties of an electrolytic capacitor.
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