relays

Diodes are not used in all relay circuits, but they are used in some. Use Google to search the term "inductive kickback" to see why.

Hello Tauqeer,

You are welcome to the forum.

We wish you that you will contribute to, and learn from this forum.

Few suggestions....

a) If you are asking any question about any problem don't hesitate to give as many inputs as possible about the problem, ( although your first question, was concise and to the point)

b) Use Mr. Google, you will be surprised at the number of questions of this forum which figure in Mr. Google's mind.

c) Be patient, and avoid CAPS while asking questions.

d) If the answers/solutions/links given by the forum members satisfies you and solves your problem, please post it as a reply.

Now, coming to diode..for relays follow this link..

http://www.allaboutcircuits.com/vol_3/chpt_3/9.html

Good luck.

For protection purpose.

Hello taugeer,

Good to see you here at CR4.

If used in Series with the relay coil, ensures AC cannot get through to the coil.

If in series with a resistor, and the combination connected in parallel with the coil as used as mentioned in Posts above, to snub the back EMF, created by the collapsing magnetic field of the relay, which generates a reverse DC pulse into the relay coil windings.

In that case, the diode effectively short-circuits the pulse, preventing the reverse polarity high-Voltage "spike" from damaging electrical/electronic components connected to the supply circuit.

This system while damping or snubbing the reverse DC pulse, also minimises arcing damage to the contacts which which on to operate that Relay or Contactor coil.

Kind Regards....

Are you requesting the details of usage of freewheeling diodes used in dc circuits ?

hi ! tauqeer ,

the diode is used with the relay coil to counterfoil the back emf produce by the coil while switching off. when the supply to the coil is cut off the current in he relay coil produces the voltage which could damage the coil / circuit . the diode present there absorbs this emf .

from aks

Taugeer,

In many applications the relays or motor contactors are controlled from a PLC (Programmable Logic Controller) output. These often require (or should have) protection from the back EMF of the relay coil when it is de-energized. This back EMF often is high enough to burn-out the electronic components inside the PLC's output card. With DC circuits and coils, a diode is connected backwards to short the reverse-polarity spike. With AC circuits and coils, an MOV (Metal-Oxide Varistor) or an RC snubber device would be used. These may be connected directly to the coil's terminals (and some are packaged so they are very easy to use in this way), or may be connected at a terminal strip elsewhere in the control panel.

So you can appreciate the magnitude of the back EMF, consider the older automobiles and their ignition system. They had a coil which was energized at 12-VDC through the distributor. As the distributor rotated from one cylinder's spark plug to the next, the coil was disconnected from the supply, and its collapsing magnetic field would generate a very brief voltage spike as high as many thousand volts which was directed through the distributor to the spark plug. With an 8-cylinder engine running at 4000 rpm, this would happen over 500 times each second!

--JMM

Addendum to the above: In Laymans terms; to avoid or reduce Arcing and relay chatter.