Книга: Introduction to Microprocessors and Microcontrollers

Don’t make the problem worse

In most cases, a piece of equipment or a circuit fails due to a single fault. Two simultaneous but unconnected faults are very rare. There are two popular ways of converting a small problem into a large one. These are static electricity and plugs etc.

Static electricity

When two different materials rub against each other, some negative electrons tend to migrate from one material to the other. This results in a voltage difference between the two materials. The amount of voltages can be very high – several thousand volts. If we walk across a carpet or sit on a plastic covered chair, we can become lethal to an integrated circuit designed for 5 V. Many integrated circuits have antistatic precautions built in but they have limited success. There is a trade-off here in that the better we make the antistatic precautions, the slower the integrated circuit can switch.

We can overcome the problem by reducing the build up of static by allowing it to leak away. In carpets, clothes and furniture we can do this by adding a wax or polish that absorbs and holds a small quantity of moisture. A slight dampness is a very effective way of preventing static problems. For this reason, the weather and air humidity is important. The death rate of integrated circuits tends to vary seasonally! It is not helped by air-conditioned plant where the humidity is low. The effect of static electricity on integrated circuits is difficult to predict. It generally causes small localized failures which can have very peculiar effects.

Better than spraying ourselves with water, we can take a more hightech approach but how far to go in this direction depends on what is at stake. If we are going to handle a couple of cheap AND gates once a week, then only the simplest precaution is worthwhile. However, sitting on a production line plugging in microprocessors will make any precautions economic.

The simplest method is to have a conducting band clipped around your wrist with a lead going off to a ground (earthed) point. These wristbands are made of rubber into which carbon has been amalgamated to allow it to conduct slightly. As well as the wristband we can place a sheet of this rubber on the bench top and ground the bench. Such antistatic workstations are very effective. A word of warning. Do not make your own wrist strap from a length of copper wire. This offers a very low resistance and provides no protection against electrocution in the event of accidentally touching a power line.

At home, just avoid working on a plastic table or chair or wearing clothing made from man-made fibres. Natural materials like cotton, wool and untreated wood naturally absorb some water and are fairly safe. A nice wooden bench coated with polyurethane varnish is effectively a plastic bench and should be avoided.

Problems with plugs

Many plugs used between pieces of equipment have a large number of pins. Pulling one of these out with the power connected is going to disconnect some voltages before others. This can prove fatal for integrated circuits. Either all the supplies must be on, or all should be off so never plug or unplug anything with the power on. For the same reason, never remove or replace an integrated circuit with the power on.

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