Re: Ldi/dt of an inductor

[Hero999]

Because that's what inductors do: they try to keep the current constant.

Think of a flywheel: difficult to get spinning and hard to stop once spinning.

An inductor stores energy in the form of a magnetic field which takes energy to build and releases energy when it collapses.

When there's no current flowing through the coil and its energised, energy is needed to build the field. If the current is suddenly interrupted, the field collapses which induces a high voltage spike which forces the current to carry on flowing by arcing over the switch contact.

 

[pwoomer]

Current in a conductor generates a magnetic field around the conductor.  An inductor is a coil of wire, so when the expanding magnetic field of the current interacts with adjacent coils of conductor, it induces an opposite polarity voltage, which resists the current.  If the current decreases, the magnetic field starts to collapse which induces a voltage which will try to maintain the current.

 

[Kevin Weddle]

An inductor can produce a different result depending on the waveform you apply to it. Sometimes waveforms are not constant and may have periods of DC, AC, or pulsating DC. A diode in parallel with an inductor is sometimes used.

 

[indulis]

And why do you think an inductor is gong to behave differently just becasue you apply a different type of signal (waveform)???

The answer is... it won't... all "the rules" (or math, if you will) that govern their functionality apply all the time!

Why do you think:

An inductor can produce a different result depending on the waveform you apply to it.

Do you know why the diode is "sometimes" placed in parallel and what happens if you don't?

 

[Kevin Weddle]

A diode in parallel with an inductor, like many protection diodes, prevents adverse voltages generated from the inductor from shortening the life of the connected devices.

 

[Hero999]

A diode in parallel with an inductor, like many protection diodes, prevents adverse voltages generated from the inductor from shortening the life of the connected devices.

Unfortunately protection diodes can shorten the life of the relay because the current decays more slowly when it's turned off causing the contacts to open more slowly which increases arcing across the contacts. A resistor in series with the diode (about the same value as the coil resistance as a general rule of thumb) can increase the rate of current decay in the coild.