Why diode is mandatory in relay?

[@valveman12]

A very well done demo of why a diode is required to direct a collapsing magnetic field to prevent circuit damage. The use of LEDs to make your point was genius.

[@psdaengr911]

In the (g)olden days before transistors, relays and switches controlled everything, and "low voltage" control was often 24VAC, sometimes rectified to 12V with a bridge. With nothing installed across the coils of the relays, switches contacts would become welded shut from the back EMF. Initially caps were used to shut the back current, but when the price of diodes came down they became the standard

[@opera5714]

This is the basis of boost converters, having the inductor increase the voltage. The down side is a diode slows down the relay opening by two to three times allowing more arcing of the relays contacts. Since you only need to protect the switching device, a resistor can be placed in series with said diode to increase the clamping voltage or a zener. RC networks can also be effective.

[@abdraoufx]

Oh, i was working on access door system before and i found this diod and didn't know what it's for.

[@Royd20]

@ Tech Ideas

[@leerzeichen1910]

very useful, thanks. how do we know what diode do we need there? since, for example, for a fan we need very much amps? what kind of diode should we use if there's a 500A draw?

[@isettech]

The diode provides a path for back EMF current to flow delaying release of the relay. For high speed relays, sometimes a resistor of the same resistance of the coil results in decay times = to the attack rate. On release the emf voltage is the same as the applied voltage was, so no damaging spike. Sometimes a zener diode is used instead of the resistor. This is sometimes used in protection circuits for fast response. The string of LED's in the video dissipate the coil energy quicker than the shunt diode as the higher reverse voltage changes the current quicker. This can be seen on a scope. For most applications just the diode for protection is required.

[@honeyko7422]

I always wondered why a diode was required. What rated/type of diode would I need for a 5v and a 12v circuit?

[@firstmaje2271]

Since the led and the diode are connected in parallel so the reverse voltage choose to be short by the diode. I would assume it would flow through both but half and half?

[@countryside8122]

It was also added to prevent what used to be called relay chatter. Had to do with voltage on the coil.

[@jhovaniebenliro1098]

The main purpose of diode in this connection is to protect additional circuits/power source.

[@r.d.machinery3749]

When I wired this up to test it I noticed that the LED pulses faintly sometimes when the switch is closed as well as when it opens again. I suppose this could be due to the contacts in the switch occasionally bouncing when it is pressed.

[@javilarg]

Thank you, it’s a very good graphical way to explain it.

[@IkmalRusli]

Hi I 've used solid state relay for my car dome light trigger, as we know car dome light is negative trigger. so I connect pin 3 to continuous +12v and pin 4 to switched ground negative. When the pin 4 swithed to ground the relay work as expected it joint/closed pin 1 and 2, however once the pin 4 ground is switched off the relay is still closed. Any explanation or solution to this issue ?

[@Gringo_In_Chile]

A snubber is a device used to suppress "snub" the “inductive flybackback” from motors, solenoids, relay coils, or any electromagnetic device energized with an inductive load is abruptly interrupted (switched off). When the flow of current is suddenly eliminated, A sharp rise in voltage across the switching contacts polls can create a high voltage electrical spike within the device. If this voltage spike is higher than what the internal electronic components can tolerate, (max voltage rating) permanent damage may result.

[@Jesse-wm6sz]

Hi, how would you know what diode to use in a circuit?

[@nisri1567]

Clever! You did very nice, clearly demonstrativ job. No words at all, but worth than 1000000 words. Keep doing it.

[@bgable7707]

EXCELLENT practical example to visually demonstrate the need and use of a "Flyback Diode".

[@simoncheung2427]

i don’t understand . can i use a and 2v Led red light instead of a 0.7V diode? besides, why a 0.7v diode can withstand a reverse voltage which is more than 10 v.

[@peterduxbury927]

What about the use of a double-pole switch, which also breaks the circuit between the sensitive load (your five LEDs) to prevent any back-emf from damaging a switching transistor or similar?? In this way, there would not be a voltage drop across the relay.