# Inductance, volt drop and time constant? Help

1. Aug 30, 2014

### ilovescience85

Good evening all, I am looking for some clarity on a question I am answering at the moment.

An inductor of negligible resistance and an inductance of 0.2H is connected in series with a 330ohm resistor to a 12v d.c. Supply determine:

A - the time constant of the circuit
B - the voltage drop across the inductor after two time constants
C - the voltage drop across the inductor after three time constants
D - the resistance of a 0.2H coil used to replace the inductor if the circuits time constant falls to 0.55s

Attempt so far
I = V/R = 12/330 = 36.36mA
A - t=L/R = 0.2/330 = 606microseconds
B - I'm trying to use Ve exp -Rt/L but it does seem to be giving me any answers that make sense?!
C - same as B
D - R= L/t = 0.2/0.00055 = 363.64 ohms

Any help would be greatly appreciated.

Thanks

2. Aug 30, 2014

### Staff: Mentor

What do you get?

At (D), your value for t in the formula does not match the value given in the problem statement. I guess that should read 0.55ms?

Last edited: Aug 30, 2014
3. Sep 1, 2014

### ilovescience85

Hi sorry for the delay i had a busy weekend.

When using the above equation i get as follows;

B - 12 exp -(330 *0.0012)/0.2 = 12 exp -1.98 = 1.66v
C - 12 exp -(330 *0.0018)/0.2 = 12 exp -2.97 = 0.62v

Regarding D yes sorry that should have read 0.55ms

4. Sep 1, 2014

### Staff: Mentor

Apart from rounding errors, it is correct. Note that you can directly plug in -2 and -3 in the exponents, without calculating the time.