On 3/18/06, Russell McMahon wrote: > Summary: > > I would be interested in knowing if anyone is aware of a formal > specification for fail-Short-Circuit behaviour for zener diodes or any > discussion on the subject. > I guess my experience is a bit different. Anyway, it is just for your reference. SCR/zeners are often used for over-voltage protection. For zener barriers, two or three zeners in parallel forms the over-voltage protection. A fuse will prevent over-current. A resistor will protect the fuse. For EN50020 EEx ia applications, I remember I did a lot of temperature measurement to assure the certification body that the zener will reliably clamping at the specified voltage and not fail open circuit. We need to assume that the current will be 170% of the fuse current (fuse needs to be certified, only a few vendors have the EEx certification), then the zener junction temperature needs to be within the specified temperature range and the power rating needs to be within 2/3 of maximum allowed power rating and the PCB underneath the zener needs to be within the specifications as well. For higher power application, often SCR crowbar circuit is necessary for over voltage protection since zener will not be good enough. In all these cases, the fuse needs to blow if the SCR or zener conducts. The certification body also wants to know the construction of the zener/SCR and fuse as well as the resistor. Anyway, Intrinsic Safety (IS) applications are in general too stringent. Still this may give you some hint. In your case, if you have a fuse with proper rating before the 5V1 zener, I think the zener will be okay. If you have 5A before the zener, I think the vendor will not guarantee that the zener will be okay since the power will be too much for the zener. Regards, Xiaofan -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist