RL Time Constant

What it shows:

The growth and decay of current in an RL circuit with a time constant visible in real time.

How it works:

By choosing the values of resistance and inductance, a time constant can be selected with a value in seconds. The time constant τ is given by

τ = L/R

We chose two resistance values, 4.7K and 10K coupled with a 45kH UNILAB 1 induction coil giving time constants of 9.5sec and 4.5sec respectively.

The circuit is set out on a 1.0 × 0.5m plywood board. The actual wiring, as in the RC Time Constant demo, is behind the scenes and the circuit diagram is laid out using colored tape (layout in figure 1). The 45V battery and inductor sit on shelves on the front of the board, but the real resistors are behind, represented by short lengths of plastic pipe; their values marked by colored electrical tape. The two way switch has to be a 'make-before-break' otherwise the back EMF makes it impossible to show the current decay.

The UNILAB inductor has two 20,000 turn coil so that it can function as a transformer. Placing a 1M resistor as a load on the secondary, the induced current in the secondary can also be displayed.

figure 1. RL Circuit layout

RL time constant

Setting it up:

The growth and decay curves are displayed on a storage oscilloscope, whose inputs are connected across the resistors using BNC connectors attached to the board (so we are really displaying the resistor pd against time). We built a screen to clip over the secondary circuit (by plugging into its banana plug holes) so that it is hidden when not being used. We usually use 45V, but any voltage will do.


Very clear presentation of the circuit. The advantage of the storage scope is that you can overlay the charge and discharge traces, overlay two charge curves with different time constants, or compare the gradient of the secondary's pulse with the rate of change of current in the primary.

1 UNILAB Large Inductor 013.401, UNILAB Ltd., Blackburn England BB13BT