I have a small question about a relay.

When we connect a relay (DC circuit) , It is necessary to connect diode parallel to the coil to avoid any inductive effect when disconnet the current flow. Is this correct.

In some situations, a resistor is connected parallel to the coil instade of diode.

Is there any advantage of connecting a resistor more than a diode.
Or is this representing a different situation.

Do we still need to be connect a diode.

Do you understand what I mean.

Your suggestions would be highly appreciated.

Many thanks

 

Yes except that you do not "avoid" the inductive effect but to "take care" of the inductive effect so that it would not damage other circuits.

A parallel resistor is seldom used except that one wants the relay to release quickly. The resistor will waste power all the time while the diode will not. The release time of the relay with the diode is longer than one with a resistor. Another alternative is to use zeners.

For common relays, either diode or resistor, not both.

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L.Chung

 

Release time is much quicker than the diode.

Apart from that, does the resister still provide the protection to other circuirs from inductive affects.

Or the diode provide the better protection.

 

To answer your question, you have to understand the whole mechanism of inductive effect and its effect on other circuits.

When the semiconductor driving the relay trys to interrupt the relay coil current by turning off, because of the inductance of the relay coil, the coil current cannot die down immediately. The result of this is an induced back emf which can rise to many time the coil voltage, usually sufficient to damage the semiconductor or even the coil inter-turns insultation.

The solution to limit the back emf magnitude is to provide a secondary path for the relay coil current to flow when the semiconductor interrupt its current. This is done via the parallel diode, zener diodes or a resistor.

For good protection using a resistor, I would use a value about the same as the relay coil resistance or else the voltage across the relay coil could still rise to a high level.

So diodes or zeners which do not consume power during relay ON are much better choices than resistor.

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L.Chung

 

The answers by eblc1388 are very good. But I would like to add that the disadvantage of using a resistor across the coil to reduce the back EMF is that is consumes energy while the relay is operated. So the solution is to include a diode in series with the resistor.

This has the advantges (ie. over the diode or resitor used alone) - the resistor reduces the release delay and the diode prevents the extra power consumption.

You need to chose the resistor value such that the Vceo limit of the transistor driving the relay is not exceeded.

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Len

 

excelent answers, Thanks everybody.