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.

The Apollo 12 crew (left to right): Charles Conrad Jr., Richard F. Gordon Jr., and Alan L. Bean.
Littelfuse circuit protection components have been an essential part of spacecraft and ground control systems for decades. For example, in the 1960s, Littelfuse developed MICRO and PICO sub-miniature fuses for NASA, which were mission-critical components of the Gemini and Apollo space programs. Today, Littelfuse continues to supply NASA, other space agencies, and military contractors with 262/268/269 Series micro fuses, with high breaking capacity.

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.

An artist’s rendition of a new lightning protection system under construction at NASA’s Kennedy Space Center Launch Pad 39B.
At 36 seconds after launch, lightning struck Apollo 12, causing a massive current to travel down through the outer skin of the spacecraft, to the launch vehicle, and down through the rocket flumes to the ground. Launch controllers immediately lost telemetry contact with the crew. Johnson recalled, “At that time, the circuit overload devices that NASA used were motor switches with a sensor attached to them. The sensor would sense current, and if it exceeded an extremely high value, that sensor turned on a silicon-controlled rectifier (SCR) that would power the off-side of that motor switch and drive the circuit open. It turns out that if you have a very high voltage spike over a very short period of time, it will actually cause the device to turn on. What happened is the lightning strike triggered these overload circuits, which would trip up those SCRs that were located in a box down in the Service Module, down below the Command Module, where the fuel cells are. And so all at once, it disconnected all three fuel cells. Those are the primary DC power source to the spacecraft. So right away the crew got alarmed about that disconnect, and received an undervoltage because all of the space- craft loads were shifted over.”

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.

Because they have no wear-out mechanism over multiple surge events, Littelfuse AK Series TVS Diodes recover fully, unlike traditional surge protection device (SPD) modules that begin to degrade after each surge event.
Johnson credited the quality of the structural electrical bonding among the launch escape system, command module, service module, spacecraft-lunar module adapter, and Saturn V Inertial Unit (IU) with preventing major permanent damage to the systems and vehicle. The inertial unit computer in the launch vehicle’s assumed ascent guidance and control because the command module computer, which would normally have provided backup for ascent, went offline after the first lightning strike. Having the redundant computer for ascent guidance and control located in the launch vehicle rather than the command module prevented the need to abort the flight.

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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NASA Tech Briefs Magazine

This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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