I posted this on YT awhile back. There are some students talking in the background, but once I get going the audios is better.
This videos shows the schematic and demonstrates the operation of a flyback diode with an oscilloscope. The flyback diode will suppress the inductive kickback from the relay when switched into the circuit.
A function generator drives a NPN bipolar junction transistor (BJT) and the frequency is increased to help trigger the scope and demonstrate the application. When the switch for the fly-back diode is open the relay can be heard with the reverse magnetic field applied from the inductance.
When the switch is closed, the flyback diode is in the circuit and the inductor can discharge through the diode with around 700mV max, while without the diode this relay coil peaked at over 100V. Keep in mind the peak occurs when turning the relay off.
K is the reference designater for Relay, and Q is for transistor. This transistor is a switch that is saturated to sink current to the relay coil to energize relay. In other words the NPN 2N3904 provides the ground (current sink). The relay coil is connected to +12V. When energized the coil creates an electromagnetic field to toggle the contacts and control the LEDs.
When energized the relay contacts toggle and the NO (normality open) contacts close, and the NC (normally closed) contacts open. LEDs are used to demonstrate the NO and NC relay contacts.