### Discussion :: Inductors - General Questions (Q.No.18)

Sumeet said: (Oct 4, 2011) | |

In a RL circuit V=v(1-e^-t*R/L) as t = 2*L/R So e^-2*L/R*R/L would be e^-2 as a result V = .86*v. So current i =V/R i.e. 86*20/220 = 78 ma(approx.) |

Pravin Jadhav said: (Sep 10, 2013) | |

Please explain that how l/r term comes? |

Giri said: (Mar 1, 2014) | |

TIME CONSTANT AND CURRENT RELATIONSHIP: 0 0% of max current. 1Ts 63.2% of max current. 2Ts 86.5% of max current. 3Ts 95% of max current. 4Ts 98.2% of max current. 5Ts 100% of max current. First find the maximum possible current through the circuit. Inductor obeys completely a short circuit on DC(ignoring coil resistance). So the impedance of the circuit is only R. Max current is I = V/R. ie,. 20/220 = 0.090909. Question is asked for the two time constant that is '2Ts'.. So the Equivalent current is 86.5% of max current(0.090909) from the above data = (86.5/100)x0.090909 =0.0786363 or 78mA. |

Kavi said: (Apr 25, 2014) | |

How comes time constant and current relationship? |

Adeel Awan said: (Jul 6, 2014) | |

RL circuit V=v(1-e^-t*R/L). as t = 2*L/R. So e^-2*L/R*R/L. e^-2. as a result V = .86*v. So current I =V/R i.e. 86*20/220 = 78 mA(approx.). |

Roopesh said: (Jun 18, 2015) | |

While calculating current, why inductive reactance doesn't come in picture? |

Vinay Raj said: (Nov 28, 2015) | |

@Roopesh. Because it is battery, a DC source. So Xl won't come into a picture. |

Dhaval said: (May 4, 2017) | |

Why taking t=2L/R? This time constant is for series RLC circuit. And there is no capacitor in the circuit. This circuit is RL circuit.for RL circuit time constant is t=L/R. |

Dhaval said: (May 4, 2017) | |

Yes, you gave the right solution, Thanks @Giri. |

Jone said: (Apr 8, 2018) | |

In RL circuit V=v(1-e^R/L t) Given time constant L/R =2 V=20*(1-e^-2) =17.29v. Current through inductor is I = V/R =17.29/220=78mA. |

Haris said: (Jan 11, 2020) | |

Thanks @Giri. |

#### Post your comments here:

Name *:

Email : (optional)

» Your comments will be displayed only after manual approval.