# Response time of RC filter

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#### UshaMaram

##### Junior Member level 3
rc filter

how to calculate the response time of RC low pass filter,the frequency of the filter is 100Hz,R=15.9kΩ C=0.1µF

#### biff44

rc filter response time

I always use t = 2.2RC

#### UshaMaram

##### Junior Member level 3
rc filter response

But t=2.2RC is a time constant which is not equal to response time of a filter

#### wpchan05

##### Full Member level 4
step response of rc filter

UshaMaram said:
But t=2.2RC is a time constant which is not equal to response time of a filter
I think what biff44 said is based on a 1s order response (eg y = Aexp(-t/RC) )

time1=-RC  ln(0.1A/A)
time2=-RC  ln(0.9A/A)
response time = time1 - time2 = RC  ln (9) ~ 2.2RC

#### biff44

2.2rc time constant

Yeah...what he said!

#### LvW

group delay calculation of rc filter

UshaMaram said:
how to calculate the response time of RC low pass filter,the frequency of the filter is 100Hz,R=15.9kΩ C=0.1µF
I suppose you ask for a step response, don´t you ?
In this case, a practical value is T(response)≈(6...7)R*C.
(In theory, the output reaches the input at infinity).

#### FvM

##### Super Moderator
Staff member
how to calculate response time for rc circuit

7*RC is setting time to 0.1% residual error for a first order low pass. Can be a meaningful parameter, but I never found it designated "response time". Unfortunately, wpchan05 didn't manage to specify the requested parameter exactly. I don't see other candidates than tr(10%,90%), which is 2.2*RC, as said, or possibly group delay, which is 1.0*RC for f << fg.

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