# How do I calculate LC circuit time-constant?

#### Rajan_1993

Joined Dec 24, 2014
6
Is that correct thing to say that LC circuit also has time-constant as RC circuit and if so how do I find the time constant for LC circuit? If possible can anybody explain by some sort of example. I would really appreciate that. Please guide me on this.

#### OBW0549

Joined Mar 2, 2015
3,566
Is that correct thing to say that LC circuit also has time-constant as RC circuit
No.

and if so how do I find the time constant for LC circuit? If possible can anybody explain by some sort of example. I would really appreciate that. Please guide me on this.
An LC circuit (either series or parallel) has a resonant frequency, equal to f = 1/(2⋅π⋅√(LC)), where f is in Hz, L is in Henries, and C is in Farads.

#### Dodgydave

Joined Jun 22, 2012
10,698

#### crutschow

Joined Mar 14, 2008
31,554
But an LR circuit does have a time-constant equal to L/R.

• MrAl

#### DickCappels

Joined Aug 21, 2008
9,627
Are you interested in the risetime of a high or low pass filter?

Edit: Can you draw a picture to illustrate your question?

Last edited:

#### MrAl

Joined Jun 17, 2014
9,784
Is that correct thing to say that LC circuit also has time-constant as RC circuit and if so how do I find the time constant for LC circuit? If possible can anybody explain by some sort of example. I would really appreciate that. Please guide me on this.

Hi,

The time constant in a series RC circuit is R*C.
The time constant in a series RL circuit is L/R.
So they are a little different, but represent the time it takes to change by A*(1-e^(-1)) which is about 0.632 times the maximum change. So for a circuit that changes by 2 from start time to some long time period, for one time constant we'd see a change of about 1.264 in whatever units.
Often approximated to 63 percent of max.
Also often approximated to be a straight line rather then the slight curve it really has.

#### Lyonspride

Joined Jan 6, 2014
137
Don't forget that the capacitors ESR plays a part too. This is where theory can get screwed over by reality, as theory tends to assume perfect component properties.

#### Rajan_1993

Joined Dec 24, 2014
6
Are you interested in the risetime of a high or low pass filter?

Edit: Can you draw a picture to illustrate your question?
We have designed a schematic and we are trying to determine an impedance of the circuit. That circuit contains some switches and R, L, C and an MCU.

#### MrAl

Joined Jun 17, 2014
9,784
We have designed a schematic and we are trying to determine an impedance of the circuit. That circuit contains some switches and R, L, C and an MCU.
Hi,

Great, let's celebrate Now how about showing the circuit so we can calculate the impedance.