THE PROBLEM You landed after a rough flight, got the last rental car, battled the snowstorm, and finally arrived at the Holly Inn. It's nice and warm in there, you can relax, ready for the conference tomorrow. Out of the elevator, of course your room is at the far end of the corridor. No matter, the heating is turned up and you are looking forward to a good night's rest. You make it to the room door feeling very sleepy. You reach out for the door handle when...Zap! It gets you every time! THE CAUSE Static electricity. Everyone knows about it, harmless really, happens when you get out of the car as well. Synthetics are particularly good for generating a good one, along with a dry atmosphere. Thousands of volts - harmless? just a little shock? To you, maybe, to an electronic circuit static discharge is - death. Not necessarily instant, more likely a weakening effect, causing intermittent failures, the most difficult to catch, until the final solid fault. THE EFFECT The problem for an electronic circuit is that they work on 2 or 3 volts at low currents, so when a kilovolt charge turns up, the current which flows is many times more than it is designed for. The effect can be like a fuse, a connection to a microcircuit melts, or is badly damaged. For us there is millions of times less current than would be required to damage any tissue. The voltage doesn't need to be enough to cause a zap, an electronic circuit can be damaged by just handling it, touching metal parts. The potential for damage has been growing since transistors, and, particularly integrated circuits were introduced. Vacuum tube circuits used much higher voltages, such that the circuit was more likely to zap the careless technician than the other way round! THE SOLUTION We have mentioned, in the history of the computer series, the importance of power supplies and air conditioning controls. These are largely there to make sure the electronics can work in its designed range. When the engineer works on these circuits he must take precautions to ensure this range is not exceeded because of static build-up. In the workshop, all work must be performed in a static free environment, components are to be stored in anti-static bags or containers, and anyone working on the equipment must ensure he does not allow static discharge through it. Similarly, when working on the system in the computer room, perhaps removing or replacing a circuit board, or adjusting a circuit, the correct procedures must be followed. THE EQUIPMENT This has become more and more complicated as the potential for damage has grown, with the reduction in size and therefore concentration of circuitry. The replacement cost of a pcb can be several hundred thousand dollars! The front line of defence is the Wrist Strap. This consists of a means of connection to the frame of the machine being worked on, linked to some form of bracelet or band which makes contact with the worker's skin. The link is not a normal piece of wire, but has a resistance of 1 Megohm. This high resistance is to reduce the current when connecting to the frame, so as not to cause problems. The wrist strap itself is usually of some stretch material, with a metal connector for the wire link, and to connect to the wrist. Alternatively, some types have an expanding metal strap, similar to a watch strap. The connector to the frame may be a metal clip, or a plug to fit a receptacle on the machine, for this purpose. In the workshop, all work is carried out on an anti-static mat, made of metalised rubber, so that it is conductive. This mat is grounded, and connected to the frame of the equipment being worked on. Anyone working on the equipment must first connect his wrist strap to the mat via the connectors provided. Another anti-static mat, also grounded, is placed on the floor at the workstation to stand on. In the computer room, a portable workbench, is provided with an anti-static mat, connected to the machine frame, to place components removed or to be installed on. Bags or containers made from conductive material are on hand, and can also be connected to the mat. A mat is placed on the floor where work is being carried out, connected to the frame. The people working on the equipment wear anti-static coats made of conductive material. They also have ankle/heel straps. These fit around the ankle, making contact with the skin, and under the heel of the shoe, to make contact with the mat. Components which have been replaced for suspect problems are returned to the factory for failure analysis. The failed circuit is examined through a microscope to observe the failing area, and to determine if this is a potential problem for other similar circuits. Failures due to electrostatic discharge can also be determined. THE HOME USER Now you know the precautions the professionals take, you can understand the necessity for care to be taken when adding memory, or opening your computer for any other reason. If you have a wrist strap, use it. If not at least touch the frame of the computer before reaching inside. And don't think you don't need to because it works OK when you close it up again. You could be getting the results of your efforts down the track! |
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