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