Circuit Protection Can't Be an Afterthought

Don't wait too long. Colby told us:

Circuit protection can bite you if you do it too late in the project. You could put yourself in a situation where the space is not available for your ESD device. You end up settling for a non-optimal location, where the device won't function the way it's supposed to.

The best time to start thinking about such matters is after you've picked out the chip set and begun laying out the circuit board, Colby said. Doing it in that way, ESD ratings are available and designers know how robust or how sensitive the chips are. "Some of these chips are running at 1.5 volts and you don't have to do a lot to upset them," Capdevielle said. "The circuitry is more complicated and more sensitive than we sometimes realize."

Understand failure modes. "Everybody understands a fuse, but over-voltage may not be so obvious, and people might not realize the consequences," Capdevielle said. The consequences, however, do exist, even if they are not as catastrophic as those of over-current. Over-voltage has incapacitated the Hubble Telescope, shut down refineries, killed smartphones, and stopped roller coasters mid-ride. In some portable medical devices, over-voltages can even be life threatening, according to Capdevielle.

Sources of excessive current and voltage include lightning, ESD, motors, arc welders, and the aforementioned running shoes and hosiery, among others. "People understand lightning but they may not know it travels across the ground," Capdevielle said. "It can create huge glitches in power lines a mile away."

Define the threats. To accurately predict a product's circuit protection needs, the design engineer must first be able to imagine how it will be used. "You have to know where the product might end up," Capdevielle said. "You have to understand its environment and what might be adjacent to it. A device will be more susceptible to a factory setting than to an office."

Once the designer understands the environment, he or she can begin making accommodations. "You should start with the connection points," Capdevielle said. "You should have an over-voltage device, not three inches away, but as close as possible to the connector."