Why is diode across relay coil

This spike results from the collapsing of the magnetic field around the coil. The movement of field across the coil produce very voltage spike which can damage electronic components. This is when clamping diode comes into play. By installing C diode in parallel with coil, a bypass is created for the electrons during the time circuit is open or current through coil stops. As others have mentioned, the diode is connected "anti-parrallel" , i.

It's primary purpose is to limit the voltage appearing across the relay when it is turned off, which in turn limits the voltage across the switching element bipolar transistor or MOSFET or relay contacts which protects the switching transistor from failing due to overvoltage. The diode only works in DC circuits, an alternative is needed for AC circuits e.

It might be an advantage for a hydraulic solenoid, as the valve will close slowly without water hammer. If an alternative higher supply voltage is available, then the diode could be connected to this; for example powering the relay from 12v, and having the diode go to 24v means the switch on and switch off times would be roughly similar, and the stored energy is dumped into the 24v supply. If the coil is to be switched repeatedly then a lot of energy will be wasted with the simple diode circuit, a slightly more complex arrangement can recover most of the energy.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why is there a diode connected in parallel to a relay coil? Ask Question. Asked 7 years, 9 months ago. Active 1 year, 3 months ago. Viewed k times. Apostle Apostle 2 2 gold badges 5 5 silver badges 9 9 bronze badges. It's called a freewheeling diode.

This answer covers the details and that's why I put this as a comment only : electronics. In short, the diode takes the energy stored in the relay's coil when you switch the current off. Engineers sometimes have the tendency to over complicate a problem instead of focusing on simple logical solutions.

It turns out that this was all done in vain. We were pretty amused to find out that this mayhem was caused by a faulty fuel indicator that let his fuel tank run empty. In electronics, what might seem like a large problem can sometimes have an easy solution.

For example, when designing PCBs that have mechanical relays, you can avoid large voltage spikes and provide relay noise suppression by adding a flyback diode to your circuit. However, when you have placed a flyback diode on a relay for flyback protection and your controller still keeps resetting, you might need to consider other sources of electrical noise. 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. The speed at which 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 i.

This causes a huge voltage potential to build up on 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. 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. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. 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. 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.

This takes a limited time. The diode must withstand the supply voltage.