quesrion about inductor in relay

[maie]

question about inductor in relay

so my professor taught us today about relays, and told us this thing-
"when we providing current to the inductor, and its get to 60mA , 70mA, we cant just 'stop' the current immediately".
when we asked him "why? what could happen?" he refuse to answer.
so, why exactly?

 

[D.A.(Tony)Stewart]

Re: question about inductor in relay

so my professor taught us today about relays, and told us this thing-
"when we providing current to the inductor, and its get to 60mA , 70mA, we cant just 'stop' the current immediately".
when we asked him "why? what could happen?" he refuse to answer.
so, why exactly?

The current stores energy in the form of a magnetic field and it cannot be dissipated instantly by open circuit. The back EMF generates a voltage in the reverse direction as it decays the stored energy resulting a large negative spike if pullup up apr a large positive spike if pulled down from the low side.

Thus a clamp diode rated for the same or more current is used in the reverse polarity across the coil to limit the voltage on the switch to the diode forward drop voltage and it conduct the same initial current as it decays to 0 in a short time after the switch opens.

This is essential for semiconductor over voltage protection.

If you used a another relay to,drive an inductor or this 2nd relay you would see the spark when the contact opens.

 

[Audioguru]

Hee, hee. Do you want to feel what an inductor does when you suddenly stop its current?
1) Connect a little 9V relay coil to a 9V alkaline battery.
2) Hold both terminals of the relay coil in one hand (not in separate hands).
3) Disconnect the battery and feel the high voltage spike zap your hand. Ouch! It might make a little spark so keep it away from anything flammable.

Don't ask your wife or girl friend to feel it.

 

[vinodstanur]

In circuits education, Inductors are treated as the bad kid that nobody likes, nobody cares about, and nobody understands, Why

Check our this link, I believe it will clear your doubts about inductor.

 

[juicedry]

Simply that inductors don't like current flow being changed. Much like capacitors try to maintain the same voltage level.

For example you have a coil of wire (inductor) and you pass 1 amp through it. Then you disconnect the flow of current with a switch. Now the inductor tries to keep that current flowing by using the energy stored within it to build up a voltage until that overcomes the opened switch and dissipates the energy stored within the inductor.

- - - Updated - - -

Therefore inductors can be used in DC-DC boost circuit to step the voltage up by switching the current flow on and off. Very useful components.

 

[betwixt]

And Audioguru - you are being cruel :twisted:

Scientific experiments on the effect of back-EMF on a human is against the Geneva Convention. :grin:

Brian.

 

[maie]

so in simple words, the voltage will jump, and the inductor can be molten?

 

[ads-ee]

when we asked him "why? what could happen?" he refuse to answer.
so, why exactly?

Inductors are treated as the bad kid that nobody likes, nobody cares about, and nobody understands

Looks like your professor has the last problem with those "bad kid" inductors. ;-)

 

[D.A.(Tony)Stewart]

Re: question about inductor in relay

replace the word bad Inductor, with Instructor.


Caps like shock absorbers restrict change of potential (voltage)
Chokes like flywheels restrict the change in torque ( current).
Resistors like Friction or brakes in shunt or a clutch in series.

 

[chuckey]

Think of a circuit of an inductor with a resistor across it, when you connect a voltage across it, current flows. This change of current (dI) with time (dT) causes the inductor to generate internally a voltage Vl which equals dI/dT X L (value of inductor). This causes the current exponentially rise to its final value as the magnetic field finally settles to its maximum value. Now if we say that terminal A on the inductor was connected to the positive of the supply, the current running into it caused the magnetic field to increase (from zero to its maximum value), disconnecting the current, causes the field to reduce, so the terminal A now goes negative because the change of magnetic field is now in the opposite direction. The current is now reversed. So now across the resistor the voltage reverses and decrease to zero as the magnetic field decreases.
The reverse voltage generated by the inductor is called "back EMF" and is again dI/dT X L . If there is no resistor across the inductor and the current is switched off, this back EMF can be very high and potentially dangerous to semiconductor devices, so it must always be suppressed.
Frank

 

[HTA]

A free wheeling diode protects the driver output, but also slows down the demagnetization. Thus intelligent driver often use a combination of Z-diode, diode and transistor to minimize the effect by burning the back-EMF energy inside the driver faster(see intelligent solenoid driver: https://www.ichaus.de/wp8_whitepaper_en ).

Enjoy your design work!

 

[Venkadesh_M]

Electronics is very interesting we cant wait until get the answers. Your professor is teaching in a correct way.

 

[maie]

Electronics is very interesting we cant wait until get the answers. Your professor is teaching in a correct way.

didnt have a doubt for a minute.