# Electrical Science - LR Time Constant & Voltage Drops

• hopkid
In summary, the conversation discusses the use of an inductor and resistor in a series circuit with a 12V d.c. supply. The time constant of the circuit is determined to be 0.606 ms, and the voltage drop across the inductor and resistor after two and three time constants is still being calculated. The resistance of a 0.2 H coil used to replace the inductor is also being determined, with a value of 33.64 Ohms. The conversation also mentioned finding an equation for the voltage on the inductor, which can be found in the provided link.
hopkid
Electrical Science -- LR Time Constant & Voltage Drops

1. An inductor of negligible resistance and an inductance of 0.2 H is
connected in series with a 330 Ω 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 resistor after three time constants
(d) the resistance of a 0.2 H coil used to replace the inductor if the
circuit’s time constant falls to 0.55 ms.

Relevant Equations:
a) T=L/R
b)?
c)?
d)T=L/R

Attempt:

a) T=L/R
T= 0.2/330
T= 0.000606
T= 0.606 ms
*Quite confident on this one.

b&c) I'm really struggling trying to ascertain what equations I need to calculate these two answers and that is where I need the help.

d) T=L/R
R=L/T
R= 0.2/0.00055
R= 363.63 Ohms
Resistance of coil = 363.63 - 330 = 33.64 Ohms
*Quite confident on this one too.

If someone has the time to confirm my answers for a) & d) look ok, and to help with the equations and path I need to take to calculate b) & c) it would be much appreciated.

Thanks, all sorted now... I appreciate your response.

## 1. What is LR time constant and how is it calculated?

The LR time constant is a measure of the time it takes for the current in a circuit to reach 63.2% of its maximum value. It is calculated by multiplying the inductance (L) of the circuit in henries by the resistance (R) in ohms.

## 2. What factors affect the LR time constant?

The LR time constant is affected by the inductance and resistance of the circuit, as well as the voltage and frequency of the power source. It also depends on the type of material used for the conductor and the physical layout of the circuit.

## 3. What is the relationship between LR time constant and voltage drops?

The LR time constant is directly related to the voltage drop across a circuit. As the current in a circuit changes, the voltage drop across the inductor also changes. This can be calculated using the formula V= L(di/dt), where V is the voltage drop, L is the inductance, and di/dt is the rate of change of current.

## 4. How can the LR time constant be used in practical applications?

The LR time constant is commonly used in electronic circuits to control the timing of events or to smooth out sudden changes in current. For example, it is used in filters to block certain frequencies and in motor control circuits to regulate the speed of a motor.

## 5. How does the LR time constant compare to the RC time constant?

The LR time constant is similar to the RC time constant, but it involves an inductor rather than a capacitor. The main difference is that the LR time constant measures how long it takes for the current to reach a certain level, whereas the RC time constant measures how long it takes for the capacitor to charge or discharge to a certain voltage.

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