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Diode Direction at Relay's Inductor's leg

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nabilishes

New Member
I'm creating a circuit for a project which uses a transistor and relay.

For security measurements i must put a diode at the Relay's Inductor's leg to avoid Back EMF from flowing back to the Transistor. The direction of Diode is normally opposite the direction of flow of current. I just want to know the reasons for it? why is the direction of diode is installed in that direction?
 
because when the circuit is shut off the current will surge back from ground and the diode protects the relay, a similar situation takes place with motors
 

MikeMl

Well-Known Member
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Ross Craney

New Member
because when the circuit is shut off the current will surge back from ground and the diode protects the relay, a similar situation takes place with motors

Would have to be the most imaginative (incorrect) answer I have ever seen. The relay would hardly need protection from its self generated back emf.
 

crutschow

Well-Known Member
Most Helpful Member
Would have to be the most imaginative (incorrect) answer I have ever seen. The relay would hardly need protection from its self generated back emf.
It is possible (although not likely) for the relay coil inductance to generate enough of an inductive spike to punch through the winding insulation.
 

Ross Craney

New Member
It is possible (although not likely) for the relay coil inductance to generate enough of an inductive spike to punch through the winding insulation.

Yes , possible. But lets address the original question & initial answer. The diode is essentially there to protect driver circuitry from the back emf generated by the collapsing magenetic field of the relay coil.
 

mbarazeen

Member
i think it may be the answer as you expected. you cannot change the current through an inductor instantly. when you switch off the relay you are trying to break it from a value to zero, thats the reason it gives high voltage.

now by providing a diode when you open the supply the diode will take care of the current imediately from the same value and keep the circuit as closed to the coil until it dischages its energy slowly.
 

Sceadwian

Banned
crustchow, it's theoretically possible but extremely unlikely, the transistor is likely going to avalanche and provide a current path before the insulation on the relay's switching coil.

The diode type can be important if the switching is abrupt the voltage spike can be quiet high and sudden and diodes take a moment to switch from blocking to conduction states due to some capacitance at the PN junction, if a diode is used in an application that it isn't suited for the transistor or whatever other part you're atempting to protect will still see a very brief high voltage spike.
 

Vizier87

Active Member
Protection? Surge? When driving a relay with a transistor, the electromagnetized core/coil takes a few seconds before the the relay kicks back after you switch off the transistor's forward active mode, which is undesirable when you want to switch a device with precision.

The diode just makes sure this doesn't happen, so when the base current of the transistor goes off, the remaining current in the coil immediately flows into the diode, demagnetizing the coil.

That's why the 'tick-tick' sound of the relay comes out if you activate the transistor briefly when current goes in the base to collector (if npn) for a fraction of a second.

Otherwise, without diodes, the relay goes 'tick........................tick'.
 

Sceadwian

Banned
I'm sorry, what are you talking about Vizier? The physical delay and noises the relay actuation coil makes is irrelevent. Inductive kickback is almost instantaneous, on the order of a few hundred nano seconds depending on the RC time constant of the overall circuit. The diode keeps the voltage on that coil from reaching multiple dozens or hundreds of volts. The tick tick you're referring to could possibly be from the arc created if you don't have the diode. But your explanation doesn't make sense electrically/mechanically.
 

Vizier87

Active Member
It's what I experienced. The relay stays switched on when you drive it with a transistor (I used 2N2222) without having a freewheeling diode for a while caused by the residual magnetization, as the inductive current decays. The diode makes sure when the transistor goes from ON to OFF, the remaining inductive current in the coil immediately relieves itself into the snubber diode.

Or it could be that I am mistaken, and I need to open my Boylestad textbook.
 

Sceadwian

Banned
That description is right, what you were saying with the ticktock didn't make sense to me though =)
 
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