Circuit protection components
Littelfuse
Chicago, IL
773-628-1000
www.littelfuse.com
Circuit protection is an essential part of any electrical or electronic product or system design. As the complexity of the product or system grows, circuit protection design becomes increasingly crucial. As circuitry is increasingly miniaturized, it’s more important than ever to protect it from damaging power surges. For engineers whose work is critical to the safety of a NASA mission, protecting the lives of crew members depends to no small extent on protecting delicate digital circuitry from hazards like electrostatic discharges and lightning-induced surges.
For all but NASA insiders, the Apollo 12 mission to the Moon is often largely forgotten, given its timing between the excitement of Apollo 11’s first manned Moon landing and the rupture of a service module oxygen tank that crippled Apollo 13. But the Apollo 12 mission, launched on November 14, 1969, still serves as a reminder of how seemingly simple incidents can disrupt the incredibly complex set of electrical systems that control a spacecraft.
During the mission, Gary Johnson was an electrical engineer working in the mission evaluation room at NASA’s Manned Spacecraft Center (now Johnson Space Center) in Houston, responsible for the Command/Service module electrical power distribution system. Although no thunderstorms were apparent at the Kennedy Space Center in Florida, conditions were cloudy and rainy. However, almost immediately after liftoff, mission control discovered that there were indeed thunderstorms in the area.
Johnson explained that at the same time the fuel cells disconnected, the momentary low-voltage input to the DC-to-AC inverter tripped the AC undervoltage sensor. This caused the AC Bus 1 and 2 fail lights to illuminate. The transient that affected the silicon-controlled rectifiers in the fuel cell disconnect circuitry affected silicon-controlled rectifiers in the AC overload circuits in the same manner.
A second lightning strike that occurred at 52 seconds after liftoff tumbled the craft’s Inertial Measurement Unit (IMU) gyroscopes. The high dV/dt (delta voltage/delta time) surge from the lightning strikes also caused minor measurement instrumentation failures, including four helium tank quantity measurements, five thermocouples, and four pressure/temperature transducers. Fortunately, the crew was able to reset critical systems because the craft’s battery-powered emergency bus had continued to operate, which allowed them to continue with their mission safely.
Although the design of the spacecraft electrical and circuit protection systems changed little over the course of the Apollo program, NASA began much more extensive monitoring to ensure spacecraft weren’t launched when there was a chance of lightning. Today, Launch Pad 39B at Kennedy Space Center has been modified for the next generation of manned spacecraft, the Orion Multi-Purpose Crew Vehicle. Those modifications include the construction of huge towers that make up a sophisticated lightning protection system.
Most transients induced by nearby lightning strikes result in an electromagnetic disturbance on electrical and communication lines connected to electronic equipment. Fortunately for today’s circuit designers, designing protection against lightning-induced surges typically doesn’t require the construction of enormous towers. For many applications, however, varistor (variable resistor) protective devices offer a better approach to protecting sensitive circuitry from lighting-induced surges. Varistors are voltage-dependent, nonlinear devices that behave electrically similar to back-to-back Zener diodes. When exposed to high-voltage transients, the varistor impedance changes by many orders of magnitude — from a near open circuit to a highly conductive level — thereby clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, protecting vulnerable circuit components.
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