Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!ames!lll-winken!uunet!tektronix!zephyr!midas!jeffw
From: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Newsgroups: sci.electronics
Subject: Re: real inductors
Message-ID: <4352@midas.STS.TEK.COM>
Date: 5 May 89 23:55:56 GMT
References: <5290@brunix.UUCP>
Reply-To: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Distribution: usa
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 43

In article <5290@brunix.UUCP> dad@cs.brown.edu (David A. Durfee) writes:
>I have inherited a circuit which uses an inductor...
>1. I'm driving this thing with a square wave and don't see
>any current rating for AC.  Should I simply take the average
>current and compare that to the max DC rating?

There are two maximum currents of interest. One is the maximum peak
current, which is where the inductor saturates unacceptably (begins to
lose too much inductance to be useful). The other is the maximum RMS current,
which is how much you can put through it before the windings heat up too
much. Exceeding either one makes it a lot easier to exceed the other. But
wait, there's more...

>2.What happens when you overdrive an inductor?

If it has a ferro/ferrimagnetic core, as yours undoubtedly does, it
saturates (at the current peaks), and the current (both peak and RMS)
through it rapidly increases as long as the voltage across it is maintained.
In your circuit, the nice ramp current waveform begins to grow ugly fins
at its high points. 

>I mean regarding modeling its behavior.  Does its resistance
>go up and its inductance go down? 

The inductance goes down. The resistance will go up if the inductor heats up,
due to the tempco of the winding. If you heat the thing up enough, the core
material will get over its Curie point and then you may as well consider it
an air-core inductor, for any current.

>3. MOST IMPORTANT- How will the part degrade over time if
>operated beyond its rating?  Will it heat up and break down
>its insulation?  Will its inductance value change over time?

I don't know if the inductance value will change significantly over time -
but if its temperature when you operate it is pretty close to its
temperature under normal maximum conditions, ie if you are lucky, you
shouldn't have much to worry about. Better compare the FET's temperature, too.

The average current through the inductor is not very informative when it has
a large AC component compared to its DC component. RMS and peak is what you
need to know.

							Jeff Winslow