RC filter Ripple Calculation.

Discussion in 'General Electronics Chat' started by dileepchacko, Mar 21, 2011.

1. dileepchacko Thread Starter Active Member

May 13, 2008
102
1
Dear All

How to calculate the ripple voltage of the RC filter. The Input signal is a Square wave signal. Thanks

Regards
DileepChacko

2. t_n_k AAC Fanatic!

Mar 6, 2009
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There's probably no simple rule you can apply. For instance if the RC time constant is very small (<1/5 of T/2) compared with the half period of the square wave cycle then the ripple will be the same as the peak-to-peak value of the square wave. After 5 time constants any transient term in the analysis will have 'vanished'.

For values of RC say >= 1/3 of T/2 (where T= waveform period) then you could come up with a ripple value which is a function of the peak-to-peak input voltage, RC and waveform period T.

3. t_n_k AAC Fanatic!

Mar 6, 2009
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For the latter case I would use the following relationship ...

$V_{ripple}=(2V_{max}) \frac{(1-\epsilon^{-\frac{T}{2RC}})}{(1+\epsilon^{-\frac{T}{2RC}})}$

Where 2*Vmax is the peak-to-peak value of the square wave input.

On reflection I guess this holds for all conditions. If the exponential terms vanish then the ripple is the same as the input.

4. dileepchacko Thread Starter Active Member

May 13, 2008
102
1
Thank you very much for this information, Is the ripple voltage depends on the load current of the circuit. I think the ripple voltage will increase, when the load current increases. Thanks.

5. russ_hensel Distinguished Member

Jan 11, 2009
820
47
The math is not too hard using Q = CV and ohms law ( and calculus ). If that is too much why not simulate with LTSpice ( free ).

6. t_n_k AAC Fanatic!

Mar 6, 2009
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Depends on what you mean by load current.

Do you mean adding a load resistance across the capacitor or reducing the resistance value?

The outcome will be different according to which of the two cases you mean.