dad@cs.brown.edu (David A. Durfee) (05/01/89)
I have inherited a circuit which uses an inductor. Its a dc voltage converter and the inductor is switched across 5v by a FET so that a higer voltage can be generated across the inductor. The inductor catalog shows a maximum DC current for the part. Here are my questions (since I don't know anything about inductors): 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? 2.What happens when you overdrive an inductor? I mean regarding modeling its behavior. Does its resistance go up and its inductance go down? 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? (part is 100uH with a Idc Max of 160mA)
myers@hpfcdj.HP.COM (Bob Myers) (05/02/89)
>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) ----------
jeffw@midas.STS.TEK.COM (Jeff Winslow) (05/06/89)
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