# Ripple current calculation of electrolytic capacitor in SMPS

#### sprabhujh

Joined Mar 29, 2017
13
Hi friends...! I want to measure the ripple current fed to an electrolytic capacitor. Kindly help me out with the procedure/formula to calculate ripple current.

Regards,
S.Prabhu

#### Papabravo

Joined Feb 24, 2006
20,984
Schematic please. Simulation may be the fastest method of getting the answer you want.

#### #12

Joined Nov 30, 2010
18,224
I saw the formula a long time ago. The current is calculated from nothing but the ripple voltage and the capacitance.
Simple, right?
No. The ripple wave form is nowhere near a sine wave.
But you can get a ballpark figure by pretending it's a sine wave.
If you want better than that, sorry. I can't remember the math.

#### crutschow

Joined Mar 14, 2008
33,976
I think using the capacitive reactance, assuming a sine-wave at the ripple frequency to calculate the current from the pk-pk ripple voltage, should get you a reasonably close answer.

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#### wayneh

Joined Sep 9, 2010
17,493
I want to measure the ripple current...
Kindly help me out with the procedure/formula to calculate ripple current.
Those are very different chores. You've been given some good ways to estimate the ripple current but you could in fact just measure it instead. Doing that might give additional information you didn't expect.

#### crutschow

Joined Mar 14, 2008
33,976
According to my simulations, using the sine-wave calculation approximation should be within about ±10% of the actual value as long as the SMPS duty-cycle is within the range of about 25% to 75%.
Outside that range the calculation error increases rapidly.

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#### sprabhujh

Joined Mar 29, 2017
13
Dear Friends...Thank you all for your ideas and suggestions...Shall i measure the ripple voltage using oscilloscope and divide it by capacitive reactance?

#### MrAl

Joined Jun 17, 2014
11,249
Hi,

Are you really measuring or calculating, or both?

It also depends on the CIRCUIT you are using.
If a switching converter, then if calculating the usual method is to assume a triangular wave not a sine wave and use v=L*di/dt.
If it is a line operated circuit then it's very difficult to calculate because there are so many things that can alter it that you probably dont know, like the line impedance, transformer impedance, so it depends a lot on the actual CIRCUIT and the other components.

To measure you use a current probe, but if you dont have one you are better off inserting a small series resistor and measuring the voltage across it. The value of this resistor must be small enough to not affect the circuit too much, so again it depends on the actual CIRCUIT you are using. A current shunt is sometimes used also.

You can see that we need to know what circuit you are using.