Twelve millionths of a calorie – does that sound dangerous? We’re talking about 0.05 joules from a CRT. A single food calorie doesn’t usually inspire terror (even if you’re on a diet), and that’s 4,200 joules. That’s four thousand, two hundred joules. On the way to the answer, it might be helpful to know that a single food calorie (which is actually 1,000 “scientific” calories) is equivalent to 4,200 joules. But what’s a joule? And could 0.05 of them harm the family jewels? Okay, we’ve got an estimate of the total peak energy (and it’s an overestimate at that, for a compact CRT). If we plug in 10kV and 1nF while paying attention to the correct powers of ten, we get an energy of 0.05J (50 millijoules). This equation spits out energy in joules with capacitance in farads and voltage in volts. The equation for the energy stored in a capacitor is 0.5 x C x V^2. That way, even if the glass is really 3mm thick, or the dielectric constant is actually 5, we’re still overestimating the capacitance. Again, to keep the numbers easy (and conservative), we’ll round upward to 1000pF (or one nanofarad). If we guess that the glass is 5mm thick and has a relative dielectric constant of 4, you can estimate the capacitance as several hundred picofarads. An easy one is to pretend that the CRT is a half-sphere (even if it isn’t, exactly).
To estimate its value, you can use any number of methods (estimating it is much more fun than just looking it up or measuring it). Since the glass is thick, it has a high breakdown voltage, just perfect for building a high-voltage capacitor. The glass is nice and thick in the first place (remember, atmospheric pressure is almost 15 pounds per square inch, so the glass of a CRT has a couple of thousand pounds bearing down on it). There is a conductive coating on the inside and outside that forms the plates of a parallel plate capacitor. The capacitor in a CRT’s high voltage (HV) system is actually part of the CRT itself. So for a compact Mac’s 9″ CRT, we expect about 10kV (let’s keep the numbers round what we’re doing doesn’t justify more precision than that).
As a crude rule of thumb, you can assume that a typical CRT high voltage supply will provide roughly 1kV (kilovolt) per inch of diagonal measure. To estimate the stored energy, we need to know both the voltage and capacitance. Working on a unit that’s plugged in is most certainly extremely dangerous, with a real danger of lethal harm, even if the power switch is in the off position. Throughout this article, we are assuming that the unit is not only turned off, but that the power cord is unplugged from the wall. Then we’ll try to get calibrated on what that means, and then we’ll evaluate what the risks might be. So here’s what we’re going to do: First, we’ll estimate the total energy stored in a CRT. It unnecessarily terrifies people who’d otherwise fix their broken Macs, consigning perfectly fine computers to an ignominious end as toxic waste in a landfill. If you don’t want to read the whole thing, here’s an executive summary:įear of death by a CRT’s discharge is irrational and misdirects people’s attention away from a potentially much more dangerous (and exposed) part of a CRT-based Mac’s circuitry. It’s important to counter the misconceptions spread by well-meaning (but essentially ignorant) people about the potential of a CRT discharge to do lethal harm. Would you accept advice from a gardener about brain surgery, or might you do a little more research? It turns out that most of these warnings are delivered by people with little or no expertise in electronics. If you try to delve deeper into the subject, though, you find mainly strong opinions and assertions delivered with great passion, but precious few facts (do a Google search and see for yourself the ratio of emotion to knowledge). The warnings are so numerous and frightening, in fact, that it’s easy to believe that a CRT can vaporize humans and entire herds of cattle with a single discharge. When someone says “kilovolts”, you usually hear “ killovolts”, right? And if you fire off a quick search on the Web, you find dire warnings everywhere that reinforce your natural fears.
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