Resolving Inductance Issues: 0.2H Coil Resistance Calculation

[brenfox]

Homework Statement

Determine the resistance of a 0.2H coil used to replace the inductor if the circuits time constant falls to 0.55ms.

Homework Equations

l/R

The Attempt at a Solution

L/R = 0.2H/R = 0.55ms
Transposing for R... R = 0.2/0.0055 = 36.36Ω is the resistance of the coil?

 

[rude man]

Your English is not good so I don't know exactly what the problem is.

If it's to find the value of R in series with a pure inductor of L = 0.2H to give a time constant of 0.55 ms then your formula is right but you misrepresented 0.55 ms.

 

[brenfox]

Sorry i failed to submit all the question. Here it is: An inductor of negligible resistance and an inductance of 0.2H is connected in series with a 330Ω resistor to a 12 d.c supply. Determine the resistance of a 0.2H coil used to replace the inductor if the circuits time constant falls to 0.55ms.

yep got my powers wrong. So it will be 0.2/0.00055 = 3.6Ω?

 

[rude man]

You have a perfect inductor L of 0.2H in series with R1 = 330 ohms and R2 with unknown resistance (to be solved).

What is the time constant of this configuration as a function of L, R1 and R2? L, R1 and the numerical time constant are known. Solve for R2.

 

[brenfox]

R= 0.2/0.00055 = 363Ω is total resistance.

resistance - 330Ω = 33.6Ω is coil resistance when the circuits time constant falls to 0.55mS?

 

[rude man]

R= 0.2/0.00055 = 363Ω is total resistance.

resistance - 330Ω = 33.6Ω is coil resistance when the circuits time constant falls to 0.55mS?

Right!