Inductive kick from relay

[JohnCU]

I've been at a noise suppression conference all week for nuclear instrumentation and at the end, the guy brought up something about an inductive kick from a relay.

he said in one of his classes he asked about this question and someone said well, you have your standard V=L*(di/dt) so current can't change instantaneously which causes the arcing, but then he said no, that's wrong, the collapsing magnetic field from the inductance causes the voltage which causes the arc, but he's just explaining the same phenomenoma with two different definitions, both of which seem like they could be derived from the other. i'm not sure, this guy was really weird. he basically questioned everything i had learned in my EE curriculum, he had a crazy explanation for everything. i think he even questioned maxwell at one point.

couldn't you argue both for the inductive kick in relay?

 

[Modelworks]

Sounds like that guy has been shocked one time too many.

 

[Mark R]

They are just different ways of saying the same thing.

The magnetic field collapse is just the fact that I is falling, as B is proportional to I. The point is that the energy for the 'kick' is stored in the magnetic field, so as the magnetic field collapses, it releases its energy, which is what sustains the arc. This is therefore a more explanatory way of expressing it, whereas V = L di/dt is more analytical. However, the explanations are equivalent.

 

[wwswimming]

maybe another way to put it, if you're using a transistor to control
the current through an inductive relay, or a solenoid, when the
transistor switches off, a transient voltage develops, sometimes
in the hundreds of volts.

putting a diode across the inductor can help, though the diode
needs to switch on fast enough to dissipate the energy, before
the voltage on the transistor exceeds a breakdown voltage.

 

[Analog]

Its the same explanation. It will create a reverse voltage because the magnetic field is collapsing. A flyback diode is generally used to protect the switching device from reverse breakdown. This diode is sometimes called a freewheeling diode in a switching power supply.

 

[newschool]

Exact. If not diode is used, the transistor will be damaged more and more each time the relay is open-circuit, and finially it will short.