Hi Bob, Some old designs did use carbon composition resistors as a "fuse". The problem with this design is that the resistor would open whenever overcurrent conditions occured. That was good for safety but bad for customer satisfaction. Customers often didn't see or notice the overcurrent condition, but they did notice when thier device wouldn't work any more (due to the burned resistor). Most customers wouldn't want to take the unit apart and solder in a new resistor; most customers aren't qualified to do that anyway
Another drawback to this is compliance testing. Sometimes, if the properly rated fuse is used, some of the testing can be skipped. Otherwise, if a resistor is used, some labs would want to run an overcurrent test on a number of resistors to see if it "fuses" reliably without creating a hazard. However, now that resettable fuses (polyswitch type devices) are available; a design can be made that survives overcurrent conditions and keeps on working after the overcurrent has occured. There are three problems with these devices: First, they are somewhat expensive. Second, there are only a couple of companies that make them; but the list is growing. Third, if your design is a small quantity product, you may not be able to buy the minimum amount. The last time I looked, 2500 was the minimum order and the distributor wouldn't break reels. I found that surprising for a part that costs a buck to two bucks each. There is more than one way to do this either by using resistors, fuses, polyswitches and/or combinations of these devices. It always boils down to function, cost and availability. Hope this helps, Chris Maxwell | Design Engineer - Optical Division email [email protected] | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | > -----Original Message----- > From: Bob Patel [SMTP:[email protected]] > Sent: Tuesday, January 22, 2002 5:25 PM > To: EMC > Subject: Use of resistor on the front end of E1design > > > Hi! I had a question regarding the use of a resistor > between TIP/RING of E1 interface before the protection > circuitry. > 1)on the E1 interface there is protection circuitry > but the designer wants to put befoer this circuitry > i.e. on line side a 120ohm resistor between TIP/RING. > The resistor mfgr. gurantees that it will pass all the > surge & immunity tests. Then the obvious question is > why not use the resistor as your protection element > rather than the fuse & sidactor. > 2)Although this is E1 it is being sold as an > "intraoffice" product and so I assume that there will > some protection circuity sitting external to this > interface and in that case the 120ohm resistor will > not be a problem. > Any comments are welcome. > Thanks in advance. > Bob > > > __________________________________________________ > Do You Yahoo!? > Send FREE video emails in Yahoo! Mail! > http://promo.yahoo.com/videomail/ > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: [email protected] > Dave Heald [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the > old messages are imported into the new server. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
