Three, two, one, blast-off . As the spacecraft shoots off toward space, you're anticipating the discoveries of the latest mission and wondering what the crew members will be experiencing during the flight. The last thing you want to worry about is if your design engineers used the right circuit protection technology. And you shouldn't be worried if your engineering team took the right precautions during the design phase.
Why is circuit protection so important when designing circuits for space applications?
- Lives are at stake. Space vehicles and their supporting systems must be designed to protect the lives of the crew members who will be subjected to the dangers and vulnerabilities of space travel.
- Technology is becoming more complex as circuits get smaller. A large spacecraft is really just a sum of all the little circuits and components that work together to make the vehicle operate. So it's vital to protect each delicate electrical component and digital circuit from hazards like electrostatic discharges and lightning-induced surges. No matter how small, a damaged component or circuit could mean that an entire system will fail, compromising the safety of the entire mission.
A blast from the past
The Apollo 12 mission, launched on Nov. 14, 1969, is an excellent example of how simple incidents can interfere with the highly complex set of electrical systems that control a spacecraft.
At 36 seconds after launch, lightning struck Apollo 12 and caused a massive current to travel through the outer skin of the spacecraft, down to the launch vehicle, and through the rocket flumes to the ground (Figure 1). As a result, launch controllers lost telemetry contact with the crew. The strike triggered the overload circuits, which tripped up the silicon-controlled rectifiers (SCRs) in a box down in the service module where the fuel cells (the spacecraft's primary DC power source) were located. All at once, all three fuel cells were disconnected. At the same time, 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 SCRs in the AC overload circuits in the same way.
As if one lightning strike weren't bad enough, a second one occurred 52 seconds after liftoff. This strike tumbled the craft's inertial measurement unit gyroscopes and caused minor measurement instrumentation failures. Because the craft's battery-powered emergency bus continued to operate, the crew was able to reset critical systems and continue with their mission safely.
What prevented major permanent damage to the systems and the space vehicle? It was the quality of the structural electrical bonding between the launch escape system, command module, service module, spacecraft-lunar module adapter and Saturn V inertial unit. The inertial unit computer in the launch vehicle 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.
Since the Apollo days, NASA has implemented extensive monitoring equipment to ensure that space vehicles aren't launched when there is a chance of lightning. Today, Launch Pad 39B at NASA's Kennedy Space Center has been modified to include a sophisticated lightning protection system of several large towers.
Today's strategies for circuit protection
Fortunately for today's circuit designers, designing protection against lightning-induced surges doesn't usually involve the construction of huge towers. In fact, circuit protection devices are getting smaller and smaller to fit within the miniaturized footprints of their modern applications.
For several decades, Littelfuse has have been providing circuit protection components that have been a critical part of spacecraft and ground control systems. In the 1960s, Littelfuse developed MICRO and PICO® subminiature fuses for NASA, which were used for the Gemini and Apollo space programs. Consider some of the other circuit protection devices provided by Littelfuse today.
High-reliability micro fuses
Littelfuse supplies NASA, other space agencies, and military contractors with 262/268/269 series fast-acting, high-reliability micro fuses with high breaking capacity. They are designed for the protection of electrical, electronic, and communication equipment with printed circuit boards that are used in DC circuits and AC circuits up to 400 Hz, in the most extreme conditions.
The AK series of TVS diodes (shown in Figure 2) were designed for high-energy transient voltage protection applications. These axial-leaded, high-power transient suppressors are optimized for use in AC line protection, as well as demanding AC or DC applications. Using Littelfuse's foldback technology, they offer clamping performance superior to conventional approaches like silicon avalanche diode (SAD) technologies. They provide a clamping voltage that's lower than the avalanche voltage but higher than the rated working voltage. So any voltage rise due to increased current conduction will be kept to a minimum to ensure the best possible level of protection.
AK series TVS diodes also offer the advantages of a no-wearout mechanism, lower leakage rates, and a fast response to surges. And their compact design simplifies board layout. They can be connected in series and/or parallel to create very high-capacity protection solutions.
Littelfuse's LSP10 series surge protection modules (shown in Figure 3) provide transient overvoltage protection for outdoor and commercial LED lighting fixtures. Constructed with Littelfuse-designed thermally protected varistors, they provide robust surge current handling capabilities. A built-in thermal disconnect function provides additional protection from catastrophic failures and fire hazards, even under extreme circumstances (e.g., varistor end-of-life or sustained overvoltage conditions). The LSP10 series is available with either parallel or series connections. With the parallel-connected version, an indicator wire can activate an LED to notify maintenance personnel when to replace the SPD to ensure that the luminaire remains protected. The series-connected version cuts luminaire power off to provide a clearly visible indication that the SPD should be replaced.