Hi Kamran, I've run into this problem myself. Usually, it is because the test conditions you use are different from what the vendor is using. They may be loading the EUT to full capacity and/or using a thermocouple attachment method different from yours. In the case where your product's configuration is not identical to the vendor, you can expect some discrepancy. Compliance agencies such as UL understand that your conditions of applicability are different from the vendor's and thus your data will be unique.
If you wish to minimize the discrepancy, load the EUT to maximum rating and use a similar probing scheme as the vendor. In some cases, magnetic flux can be so great that it induces an undesirable EMF and reading error in which case you might have to run the EUT until thermal equilibrium occurs then, cut power and take a measurement sweep immediately afterward. UL for example, demands that the thermocouple probe be attached to the windings of the coil -with the thermocouple junction in direct contact with the wiring and NOT through core, ferrite or tape insulation (if the conductors are coated there is no need to remove the coating). I was just audited a couple weeks ago for COMPASS FUS and was reminded that the use of cyanoacrylate glue or thermal conductive epoxy to attach a thermocouple to a coil was entirely acceptable. Here's a tip in using thermocouples: It is best to force the junction wires of the thermocouple to fuse into a single point and to avoid using a junction made of twisted bare wires (which forms several thermocouple contact junctions and can cause errors). In the fabrication of the junction, I use a microtorch to fuse type 'J' thermocouples. Twist the wires together, but clip the excess off and leave enough that when you hit it with the torch a 'ball' fuses from the wires and consumes all the twists. This takes a little practice but works much better than using a zapper to 'arc' weld thin 30 to 36ga. wires (IMHO). I then check each channel for accuracy before attachment and test. On most thermocouple types the red wires are negative polarity, white is positive. The thinner the wires, the better ease in accessing tight spots and getting a good contact. Kyle Ehler kyle.eh...@lsil.com <mailto:kyle.eh...@lsil.com> Assistant Design Engineer LSI Logic Corporation 3718 N. Rock Road U.S.A. Wichita, Kansas 67226 Ph. 316 636 8657 Fax 316 636 8889 Fax 316 636 8315 -----Original Message----- From: Kamran Mohajer [SMTP:kmoha...@cisco.com] Sent: Tuesday, August 24, 1999 4:50 PM To: emc-p...@ieee.org Subject: Temperature Measuring of Magnetic Components Hello EMC-PSTCers, I wonder if anyone knows of the method of measuring temperature limits on magnetic components. I happen to get involved in this and found that my results are different than the vendors result by as much as 10-15 degrees on measuring on a same magnetic component. Even applying the thermocouple to different location on a coil seems to give you different results. Is there a method that I should be following to measure temperature with thermocouples methods, not change of resistance, on magnetic parts such as transformers, coils, etc.? Thanks, **************************************************************************** ******* Kamran Mohajer DSL Compliance Lead Cisco Systems, Inc. Phone(408)-525-6121 Fax(408)527-0495 kmoha...@cisco.com **************************************************************************** ******* --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
