# Calulating time constant

Discussion in 'Homework Help' started by kevinh111, Jan 19, 2012.

1. ### kevinh111 Thread Starter New Member

Jan 19, 2012
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0
Hey guys, have a question on a homework assignment related to RL time constants.

"A constant voltage is applied to a RL circuit at time t=0. The voltage across the inductance is 20 V at 3.46ms and 5 V at 25ms. Obtain R if L = 2H."

Given the equation T(tau/time const.) = L / R, how do I first calculate the time constant of the circuit in order to calculate the resistance?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
If the DC supply is Vd

Then

$V_L(t)=V_de^{\frac{-t}{\tau}}$

at t=3.46ms

$V_L(3.46ms)=V_de^{\frac{-0.00346}{\tau}}=20$

at t=25ms

$V_L(25ms)=V_de^{\frac{-0.025}{\tau}}=5$

Solve the two equations to find Tau

3. ### kevinh111 Thread Starter New Member

Jan 19, 2012
2
0
Awesome, thanks for the quick response.

I am having some difficulties solving for Tau, the answer I obtained is that tau = 0.00465, using mathematica.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
The correct value for the time constant is 15.54ms.

Divide the two equations and take the natural logarithm of the result.