RL Circuit | UCSC Physics Demonstration Room

Equipment:

RC/RL demo circuit
Oscilloscope
Function generator
Video camera (for a large class)

Demo:

This demonstration shows how an RL circuit responds to positive and negative square pulses. The input signal and the voltage across the inductor are displayed simultaneously on the oscilloscope.

Explanation:

The circuit is driven by a transfer function which relates the input and output of a linear time invariant(LTI) system with zero initial conditions. The impulse response of the circuit depends on the transfer function and may be found by taking the inverse Laplace Transform of the transfer function.

The impulse response for the inductor voltage is given by:

$h_{L} = \delta(t) - \frac{R}{L}e^{-t\frac{R}{L}}u(t)$ where $u(t)$ is the Heaviside Step Function.

The impulse response for the resistor voltage is given by:

$h_{R}(t) = \frac{R}{L}e^{-t\frac{R}{L}}u(t)$

From these equations, it’s easy to see that if the square impulse, $u(t)$ is positive, this response of the resistor will be positive and the response of the inductor will be negative. The opposite will be true if the impulse is negative.

Notes:

Use the switch (K) to toggle the circuit between RC and RL modes.
The time constant of the circuit can be changed by adjusting the variable resistor (R).