This video explains the necessity of placing a flyback diode across the coil of a DC relay. The presenter uses a neon bulb, which requires a high voltage to illuminate, to visually demonstrate the large reverse-polarity voltage spike (inductive kickback) generated when current to the relay's coil is interrupted. Through a series of experiments, it shows that while a simple transistor switch might appear to function without a diode, it is often absorbing this damaging spike itself, leading to eventual failure. This is demonstrated by comparing a low-breakdown voltage transistor (2N3904), which prevents the neon bulb from flashing by breaking down first, with a high-breakdown voltage transistor (MPSA42), which allows the spike to occur. The diode provides a safe path for this energy to dissipate, protecting switching components. The video also explains how this spike can damage the relay's own coil windings over time, even when using a mechanical switch.
Why a Diode Across a Relay Coil is Essential
0:00 The Problem: The video addresses the common question of why a diode is placed across a relay's coil, as its function isn't immediately obvious from basic operation.
3:54 Demonstrating the Voltage Spike: When power is disconnected from a relay coil that lacks a protective diode, a high-voltage spike is generated. This is visualized using a neon bulb (which requires ~70-90V to light up) that flashes upon disconnection.
5:09 Reverse Polarity Spike: The generated voltage spike has the opposite polarity of the supply voltage. This is a fundamental property of inductor kickback and is key to how the protection diode works.
8:40 Transistor as a Switch: A transistor can be used to switch the relay on and off. Initially, it appears to work fine without a protection diode, which is misleading.
12:22 The Hidden Damage: A low-breakdown transistor (2N3904) absorbs the spike itself, breaking down internally before the voltage rises high enough to light the neon bulb. This degrades and eventually destroys the transistor. A higher-breakdown transistor (MPSA42) survives, allowing the full spike to be observed with the neon bulb.
15:40 The Diode's Function: The diode is placed in reverse-bias during normal operation. When the reverse-polarity spike occurs, the diode becomes forward-biased and safely shorts out the spike's energy, dissipating it within the coil/diode loop and protecting the switching circuit.
17:08 Protecting the Relay Itself: The high voltage spike can also break down the insulation on the relay's own coil windings over time. This causes an internal short and eventual coil failure, even when using a simple mechanical switch. It's a common cause of mysterious relay failures.