Why diodes across relay coils

It is for this reason that some form of coil suppression is often suggested and used. The most commonly applied method is to place in the circuit a diode in parallel with the coil. This works well, the diode blocks current from flowing when the coil is energized, but when the voltage is removed from the coil, the Back EMF is dissipated through the diode and not into the drive circuit. For most relays and contactors, a common diode used is the well established 1N type or something similar.

Ideally this diode should be physically positioned as close to the relay coil as possible. Durakool has several relays with an integral diode inside the relay cover itself.

It is very common technique in automotive plug in relays such as the Durakool DG56 , DG82 and DG85 types, but also in some industrial relays designed for socket mounting where it would be inconvenient to mount an external diode such as the Durakool DX4. Although it is good at protecting the relay drive circuit, there is a downside to using a diode for Back EMF suppression.

The coil inductance keeps the current flowing after the coil has been de-energized. This induced coil current is enough to keep the relay in the operated state, slow down the contact opening time or cause a bounce of the armature.

A reversed-biased rectifier diode in series with a resistor. A resistor, when conditions permit its use, is often the most economical suppression. A reversed-biased rectifier diode. A resistor-capacitor "snubber". Generally the least economical solution and no longer considered a practical solution. A bifilar wound coil with the second winding used as the suppression device. This is not very practical since it adds significant cost and size to the relay.

Rishab Jain Rishab Jain 31 3 3 bronze badges. Praduman Saini Praduman Saini 1. Discussing electrons is not useful and causes much confusion. Almost all circuit analysis is done using conventional current flowing from positive to negative. Welcome to EE. BobT BobT 3 3 silver badges 8 8 bronze badges.

Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog. Does ES6 make JavaScript frameworks obsolete? Podcast Do polyglots have an edge when it comes to mastering programming Featured on Meta. Now live: A fully responsive profile. Linked 2. If the current flow stops abruptly, the voltage in the coil changes polarity so that the current continues to circulate.

This takes a limited time. The diode must withstand the supply voltage. If the relay is connected to a 24V power source, the diode must support 24 volts plus a safety margin. 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.