There are many ways to protect from overcurrent conditions. The oldest is still the best.
Episode Summary:
Development of low cost, mass-produced semiconductors has transformed one of the oldest and most important functions in electrical circuits: overcurrent protection. Modern systems may use algorithms to actively manage current flow, and even report on power or signal quality through the protected circuit. But Jim Anderton believes that there is still a role for the oldest and simplest overcurrent protection device: the fuse.
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Transcript of this week’s show:
This is a fuse. It’s an ATO type, commonly seen in the auto industry, although it is being replaced with smaller equivalents today. If you drive too much current through a circuit, any circuit, it will generate heat. If the overcurrent condition exists, the heat will build until something fails. At the dawn of the age of electricity, before the turn of the last century, there was a simple way to prevent things from burning up: let something intentionally burn up. Inserting a short segment of small gauge wire in a circuit created an intentional weak point, containing the damaging effects of overcurrent to a controlled, safe location. Fuses became industry-standard, and until the advent of circuit breakers decades later, it was the second most well understood electrical device on the planet, after the light bulb. When the lights went out, you checked the fuses. If it were burned out, you could see it visually and replacement was something you didn’t need an electrician to do. This fuse protected a 24 V AC circuit that energizes the coil on a single pole double throw contactor that controls current to an air conditioning compressor. Contactors themselves were an innovation, allowing safe, low cost, low voltage control currents to manage distinctly unsafe, high-voltage, high-power circuits. This 50-cent device protects an expensive control board, and just as importantly, its failure is both visible to the naked eye, and testable because the blade contacts are exposed at the top for in circuit testing. Would a circuit breaker be better? It would certainly be easier to reset than replacing this fuse, but it has several disadvantages: it’s more complex, more expensive and is itself a potential point of failure. Its very convenience makes it possible to ignore transients or occasional shorts by resetting at the push of a button, not exactly conducive to good MRO. If you want to get really sophisticated, we could control current with something like an SCR and a microcontroller using a current sensing feedback loop to throttle the current, as necessary. In this home HVAC application that could make that $400 board a $1000 board, and since that 24 V thermostat power transformer fails once every 20 years, there’s a real chance that the protective devices could fail before the device they are intended to protect. Fuses are good because they are simple and cheap. So why am talking about this? Because the increasing prevalence of processor-controlled devices and the increasing reliability of those devices is starting to shift focus from complex system failures, back to simpler, traditional ones, in both consumer and industrial equipment. 20 years ago, I would instinctively suspect an 8088 processor in a motherboard as the culprit in an automation application for example. With today’s much more sophisticated systems however, a semiconductor power device, or a short circuit or open ground are far more likely to cause a circuit failure. That doesn’t require a PhD to cope with. And for anything over about 5 A, these devices make troubleshooting literally a snap. I hope they never go away.
