Running shoes, hosiery, and nylon shirts can be death for electronic circuits. Taken together, they can generate an electrostatic discharge (ESD) of 30,000V for the briefest of moments, sending handheld devices or laptop computers into a response resembling cardiac arrest. "You walk up to a computer and bam!" Bob Capdevielle, senior applications engineer for Littelfuse Inc., a maker of circuit protection devices, told us. "Suddenly, everything resets."
Still, problems associated with over-voltage and over-current remain an afterthought for most engineers. With their duties expanding and with design cycles compressing, most engineers relegate circuit protection to the end of the to-do list. "These days, engineers have to design the core functionality of their devices as quickly as possible," Jim Colby, manager of technology and business development for Littelfuse, said in an interview. "They have to get the form factor done, get the software done, get the prototype built, and prove out the concept. Then they have time to think about circuit protection."
Polyfuse LoRho SMD Resettable PPTC Resistors provide over-current protection and automatic reset for handhelds.
(Source: Littelfuse Inc.)
More than ever, though, that approach is creating problems for product designers. Cellphones, computers, and music players are getting smaller. Moreover, they're running on tiny voltages that are more susceptible to ESD, distant lightning strikes, motor switching, and stray currents from process machinery. "It's usually 10,000 or 15,000 volts," Capdevielle said. "But it can get really high. We're getting calls from people asking about 30,000 volt parts."
The unfortunate result of leaving such matters to the last minute is that design functionality suffers. Engineers can't find room for circuit protection devices on their printed circuit boards. They end up re-spinning the boards and losing valuable development time. Worse, they hurriedly choose the wrong protection device, resulting in functional failures, poor reliability, safety issues, shock, or even fire.
For those who face the gloomy prospect of such problems, however, there's hope. Following are the expert recommendations of engineers whose professional lives revolve around the subjects of over-current protection and shock immunity.