Heat is the obvious source of the problem. But what causes the heat, how hot does the ferrite get, and what is the rate of change of temperature?
Clearly, permanent magnetization of the ferrite is simply irrelevant here, since by definition, the soft ferrites used to make beads and cores cannot be permanently magnetized. The flip side of that is that hard ferrites (which CAN be permanently magnetized, and are used to make magnets) are useless for making ferrite beads and cores. Self heating due to an imposed alternating magnetic field will continue to generate heat until the material's Curie point is reached. At this point, the ferrite looses its magnetic properties, and no further heat is generated due to hysteresis losses. But with many ferrites, the Curie point is over 200°C. It seems possible that this temperature level could cause cracking, IF the rate of change of temperature was extreme. Temperature alone will not cause either cracking, or changes in material properties. After all, ferrites are made by a firing process that is FAR hotter than a mere few hundred degrees! But in the manufacturing process, temperature is ramped up and down slowly, to eliminate any possibility of thermal shock. So you are probably right that observed effects are due to the ferrite becoming cracked. The reason for cracking cannot be due to temperature by itself, but rather to an extreme rate of change of temperature. Excess energy applied too fast, for the volume of ferrite used, is the cause of the excessive rate of change of temperature. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Robert Macy [mailto:m...@california.com] Sent: June 6, 2002 11:06 AM To: Robert Wilson; emc-p...@majordomo.ieee.org Subject: Re: ferrite transient voltage/current response Then why would a 10 A surge change their characteristics? Unless it cracks it? - Robert - -----Original Message----- From: Robert Wilson <robert_wil...@tirsys.com> To: Robert Macy <m...@california.com>; emc-p...@majordomo.ieee.org <emc-p...@majordomo.ieee.org> List-Post: emc-pstc@listserv.ieee.org Date: Thursday, June 06, 2002 8:51 AM Subject: RE: ferrite transient voltage/current response Soft Ferrites cannot be permanently magnetized. This is precisely why they are used as beads and cores. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Robert Macy [mailto:m...@california.com] Sent: June 5, 2002 11:20 PM To: don_borow...@selinc.com; emc-p...@majordomo.ieee.org Cc: shbe...@rockwellcollins.com Subject: Re: ferrite transient voltage/current response He may have magnetized it. Degaussing with one of those Radio Shack thingies would probably brought it back. Can he try it again? - Robert - Robert A. Macy, PE m...@california.com 408 286 3985 fx 408 297 9121 AJM International Electronics Consultants 619 North First St, San Jose, CA 95112 -----Original Message----- From: don_borow...@selinc.com <don_borow...@selinc.com> To: emc-p...@majordomo.ieee.org <emc-p...@majordomo.ieee.org> Cc: shbe...@rockwellcollins.com <shbe...@rockwellcollins.com> List-Post: emc-pstc@listserv.ieee.org Date: Wednesday, June 05, 2002 2:52 PM Subject: RE: ferrite transient voltage/current response > > > >While I was at Agilent in Spokane, one of the engineers or technicians claimed >that he had changed the RF characteristics of a 6-hole ferrite bead (wound with >2 1/2 turns) used on a power supply trace to a noisy assembly. The normal >current was about 1 amp, but he accidently shorted the power supply voltage >after the inductor. This caused a current spike as the power supply filter >capacitor discharged (and then the supply current limited at about 10 amps). >After this, there was a problem with RF leakage from the assembly. Replacing the >inductor fixed the problem. Apparently the effect was repeatable. > >I didn't observe this personally, so I can't guarantee it. > >Don Borowski >Schweitzer Engineering Labs > > >Sorry that I wasn't clear; I typically try to keep my questions general so > not to get too detailed about the specific application. And thanks to Bob, > Chris and Mike who have responded ... putting it into Chris's words ... I > was just trying to find out if ferrites had ratings to prevent them from > "j > ust plain blowing the ferrite to smithereens". Also, I was looking for a > shortcut if someone else had faced this question rather than reading > through all of the vendor web sites. > > I understand and have used ferrites quite often for typical EMI >suppression; the ferrites typically being rated for the application >currents, voltages, etc. In this case, the program is trying to protect a >power supply input from the DO-160 waveform 5B pin injected lightning pulse >of 300 volts open circuit & 300A short circuit. If the Gas Discharge Tube >is located past (closer to the supply which was done for packaging >limitations) than the "T" EMI filter, a question was raised as to whether >the ferrite properties would be altered by the lightning pulse. Most of >the standard literature on the use of ferrites does not address these types >of transients. > > >Susan Beard > > > > > > > >"Robert Wilson" <robert_wil...@tirsys.com>@majordomo.ieee.org on 06/04/2002 >02:16:48 PM > >Please respond to "Robert Wilson" <robert_wil...@tirsys.com> > >Sent by: owner-emc-p...@majordomo.ieee.org > > >To: <shbe...@rockwellcollins.com>, <emc-p...@ieee.org> >cc: > >Subject: RE: ferrite transient voltage/current response > > > >Your question is not all that clear. It appears to imply that transients >have an affect on the ferrite beads, but it is the other way around >(maybe that is what you meant). But in general, small ferrite beads have >little effect, except at very high frequencies (hundreds of MHz), unless >they are no longer "beads" (i.e. they are very large). > >Have a look at the various magnetics vendors data sheets and app notes. > >Magnetics Inc: www.mag-inc.com >Fair-Rite Inc: www.fair-rite.com (whoever came up with THAT name should >be shot! >Steward Inc: www.steward.com >Ferroxcube: www.ferroxcube.com >Epcos (was Siemens): www.epcos.com > > >Bob Wilson >TIR Systems Ltd. >Vancouver. > >-----Original Message----- >From: shbe...@rockwellcollins.com [mailto:shbe...@rockwellcollins.com] >Sent: June 4, 2002 8:57 AM >To: emc-p...@ieee.org >Subject: ferrite transient voltage/current response > > >Could someone point me to some good App Note information on the response >of >and affect on ferrite beads to transient voltage & current waveforms? >The >waveforms are based on the indirect lightning pulses specified in >Section >22 of DO-160. > >Thanks in advance, >Susan Beard > > >This e-mail may contain SEL confidential information. The opinions expressed >are not necessarily those of SEL. Any unauthorized disclosure, distribution or >other use is prohibited. If you received this e-mail in error, please notify >the sender, permanently delete it, and destroy any printout. Thank you. > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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