Why diode in relay

The purpose of the diode is to allow the current flowing through the coil to continue circulating when the relay is deactivated. No current passes through the diode because it is inversely polarized. As there is no way for the current to circulate, a diode is placed parallel to the coil. In this way, the current circulates through the diode and voltage peaks are prevented from damaging other components of the circuit. The coils have a very interesting feature.

If the current flow stops abruptly, the voltage in the coil changes polarity so that the current continues to circulate. For example: A 12V supply relay with a coil resistance of passes a current of 30mA. This is OK for a timer IC maximum output current mA , but it is too much for most ICs and they will require a transistor to amplify the current.

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode eg 1N is connected 'backwards' across the relay coil to provide this protection. Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off.

The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil and diode so the magnetic field dies away quickly rather than instantly.

This prevents the induced voltage becoming high enough to cause damage to transistors and ICs. Protection diode for a relay. Rapid Electronics: 1N diode. Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch. Reed relays generally have higher coil resistances than standard relays for example and a wide range of supply voltages V for example.

They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents mA maximum for example.

Rapid Electronics: reed relays. Like relays, transistors can be used as an electrically operated switch. 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. 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 electro magnetic field and its current through the diode when the relay is energized as the back EMF drives the flyback protection diode in forward 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.

Tiny flyback diodes prevent huge flyback voltage from damaging your components. The placement of a flyback protection diode is rather simple; it should be placed directly across the relay's coil.

A schematic for a freewheeling diode circuit in a relay is shown below. In this schematic, the resistor R in parallel with the flyback diode wiring represents the coil's intrinsic DC resistance. Flyback diode wiring in a relay circuit. Note that the placement of the diode does not prevent a voltage spike from travelling to some downstream load. Instead, it provides a path with low resistance that reroutes the current, thus the voltage spike at the downstream load will be much lower.

Using a simple 1N diode is sufficient to suppress large voltage spikes in a 24VDC relay with a diode protection circuit. The current path in the diode depends on whether the switch in the relay is closed or opened. As the switch is initially closed, the inductor load generates a back electro magnetic field as its transient response, and the voltage slowly rises to the supply voltage value.

Once the switch is opened, the back electro magnetic field created by the inductor switches direction and points towards ground, creating a transient response that slowly dies off. Thanks to the low resistance loop created by the freewheeling diode in forward bias, current is diverted through the diode rather than creating a large voltage spike elsewhere in the circuit. Current flow through the flyback diode wiring in a relay circuit. You might have thought that placing flyback diodes in a relay circuit will solve all your electrical noise issues.

This was despite the fact that I used every relay with a diode protection circuit. The humidity controller was connected to external mechanical relays that controlled industrial heating elements. This routine project turned into a witch hunt for the problem causing the controller to reset.

After hours of trying out various power supplies, cables, grounding methods and electromagnetic interference EMI foil, it finally dawned upon me that perhaps it was the external mechanical relays that were causing the problem. True to my suspicion, none of the external relays installed by the third party had a flyback diode circuit connected in parallel to their inductance coils. The resulting flyback voltages caused electrical interference over the connecting cable and into the humidity controller, thus causing the system reset.