# Transient Response of RL circuits

In an RL circuit V=RI+L(dI⁄dt) and solving this will give I=I0×e− (R⁄L)t. The coefficient of the exponential term R/L can be extracted from the graph of voltage across the inductor. The resistance of the inductor coil should be included in the calculations, R=Rext+R*L.

# Objective

Capture and analyse the voltage across an inductor when a voltage step is applied to it via a series resistor. By measuring the voltage across the inductor as a function of time, we can extract the R/L ratio.

# Procedure

- Use the 3000 turn coil as inductor
- Click on 0->5V STEP and 5->0V step Buttons to plot the graphs
- Adjust the horizontal scale, if required, and repeat.
- Calculate the value of inductance
- Insert an iron core into the inductor and repeat

# Discussion

The transient response of the RL circuit is shown in figure. The exponential curve is fitted to extract the L/R value. The resistance of the coil is measured by comparing it with the known external resistance under DC conditions. A2 is connected to OD1 for a more accurate measurement of the coil resistance. From this the program calculates the value of inductance and displays it.

The applied voltages are above zero, but the graph went to negative voltages. Why ?

What was the current before doing the 5->0 step ? What is back EMF ?

Repeat with two coils in series, by (a) placing them far away (b) placing one over the other and (c) after changing the orientation. The effect of mutual inductance can be seen.