Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!ucbvax!hplabs!hpfcdc!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: real inductors
Message-ID: <11170003@hpfcdj.HP.COM>
Date: 1 May 89 18:30:53 GMT
References: <5290@brunix.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 55

>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?

Use the RMS ("root-mean-square", also known as "effective") current; this
is 0.707 the peak current for a sine wave, or 0.5 times the peak for a
50%-duty-cycle square wave.

>2.What happens when you overdrive an inductor?
>I mean regarding modeling its behavior.  Does its resistance
>go up and its inductance go down? 

By "overdrive", I assume that you mean "run excess current through the
inductor".  If this is the case, there are two things that can happen:

1. The inductor will saturate.  The value you give is fairly low (100 uH), so
   I assume this is an air-core inductor.  If so, saturation shouldn't be
   a problem.  For future reference, though, remember that iron or ferrite
   cores can saturate if you run excessive current.  This means that the
   material has as much of a magnetic field in it as it will allow; further
   increases in current do not result in additional energy being stored in
   the magnetic field, and so the device is no longer inductive.  This means
   that a saturated inductor is essentially a short circuit (no inductive
   reactance), except for what little DC resistance is present.  This
   effect can quickly cause the failure of parts in the circuit.  By the way
   when looking into the possibility of saturation, consider the PEAK current
   to be handled (not the RMS); an inductor which saturates only at the
   peaks still isn't doing the job (unless it's in one of those rare designs
   where it's SUPPOSED to saturate - assume otherwise unless you have
   good reason for believing it's supposed to saturate!)

2. The part will overheat.  This is the more likely problem with the little
   air-core inductors, and can result in the failure of the part (the 
   "inductor" becomes a "fuse").  In most materials, the resistance does
   increase as the temperature goes up, but if you're running into a
   problem due to this effect, you're probably very close to the "meltdown"
   stage!


>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?

The inductance is not likely to change to any significant degree over time
with this type of inductor.  However, the insulation is likely to break
down, etc., if run too hot for extended periods.  The general advice as far
as running any component beyond (or even near) its ratings is DON'T!


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
{the known universe}!hplabs!hpfcla!myers |  sentient life-form on this planet.

(part is 100uH with a Idc Max of 160mA)
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