In a message dated 9/12/2002, Scott Douglas writes: > We use gas tubes in our products for protection against lightning and/or > surge. We now have an edict to get them out of all of our products. So what > does one use to provide similar protection without using a gas tube?
Hi Scott: Well, the answer depends on what sort of signal lines you are trying to protect and the level to which you are trying to protect them. The main attributes of gas tubes are their low capacitance (good for high speed signal lines) and their ability to carry huge surge currents (good for cases where there is no other primary protection device). There are many disadvantages associated with gas tubes, such as the fail-open mode that you mention and their slow response. There are some other, more esoteric phenomena associated with gas tubes, such as their tendency to have a "glow voltage" in the transition region where a plasma forms and they get red hot. In general, I avoid gas tubes whenever possible, due to the disadvantages listed above. If your lightning surges are limited to less than a few hundred amps, you may be able to use an appropriate Teccor sidactor. However, sidactors have greater capacitance, and this capacitance can be nonlinear in certain configurations, so you have to be careful that you don't compromise signal integrity. In summary, the correct answer to your question would require a more detailed understanding of the requirements of your particular application. If you are willing to provide more information, feel free to post to the group or contact me offline. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
