Why Put a Diode Across a Relay Coil?
When the power supply is connected to the relay, the inductance coil’s voltage builds up to match that of the power source. The speed at which the current can change in an inductor is limited by its time constant. In this case, the time it takes to minimize current flow through the coil is longer than the time it takes for the power supply to be removed. Upon disconnection, the inductive load in the coil reverses its polarity in an attempt to keep current flowing according to its dissipation curve (% of maximum current flow with respect to time). This causes a huge voltage potential to build upon the open junctions of the component that controls the relay.
This voltage built up is called flyback voltage. It can result in an electrical arc and damage the components controlling the relay. It can also introduce electrical noise that can couple into adjacent signals or power connections and cause microcontrollers to crash or reset. If you have an electronics control panel that resets each time a relay is de-energized, it’s highly possible you have an issue with flyback voltage.
To mitigate this issue, a diode is connected with reverse polarity to the power supply. Placing a diode across a relay coil passes the back electromagnetic field and its current through the diode when the relay is energized as the back EMF drives the flyback protection diode in forwarding bias. When the power supply is removed, the voltage polarity on the coil is inverted, and a current loop forms between the relay coil and protection diode; the diode again becomes forward biased. The freewheeling diode allows current to pass with minimal resistance and prevents flyback voltage from building up, hence the name flyback diode.
What Is a Flyback Diode and Why You Need It
If you’ve been creating printed circuit boards that feature mechanical relays, then you probably have heard of flyback diode. A flyback diode or freewheeling diode is placed with reverse polarity from the power supply and in parallel to the relay’s inductance coil. The use of a diode in a relay circuit prevents huge voltage spikes from arising when the power supply is disconnected. A flyback diode is sometimes called flywheel diode, freewheeling diode, relay diode, or snubber diode as a flyback diode circuit is a type of snubber circuit.
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