# RL circuit time constant question

1. Jun 10, 2012

### supra_tt

Hi,

Was hoping someone could help me.

I have the following RL circuit:

20V input
8.2kohm resistor and 50mH Inductor in series.

Determine time constant:

I answered with: T = L/R 50mH / 8.2kohm

= 6.09 uS

Is this correct?

The question I am stuck on is:

Determine the value of VL after 1.5 time constants?

I have the formula, VL = E ( e - t / T)

I cant work out what to put for the t? I tried putting in:

20 (e - 1.5 / (6.09uS) but cant be right?

If someone could help I would be much appreciated

Thanks!

2. Jun 10, 2012

### ehild

1.5 time constant means that that t/T=1.5. Substitute 1.5 for t/T into the formula of VL, but do not forget using parentheses.

ehild

3. Jun 10, 2012

### supra_tt

Thanks for the quick response!

I am getting VL = 4.46V, I was wondering if you could please confirm this?

I am a little confused because the teacher has written the answer as VL = 15.54V

Does this mean I just need to take 4.46 from 20 to get the answer?

Thanks again

4. Jun 10, 2012

### ehild

VL(t) is different if the source is switched on or off at t=0. You calculated VL after the battery is switched off. In case of switching on, VL=E(1-e-t/T).

ehild