With every course of instruction or tutorials there is always a decision to be made regarding the starting point. It must make some assumptions about the readership and their capabilities and experience. The aim of PC-Control is to bring more computer users into the real world of control from the abstract world of pure computing (or even gaming). We are therefore going to assume the readership has no substantial knowledge of electronics. In a lot of cases this will be wrong but we hope that those of you who already have a good working knowledge of electronics will none the less still skim over these sections to an extent. It is always possible to get a new perspective on an old subject looking at it from another direction.


Voltage, Current and Resistance
   No matter what the application in automation and control the end result is attributable to the energy produced in the “device” by the flow of electrical current in it. The electrical current may be large, small, constant, varying, momentary or repetitious, but nonetheless always there. The energy may be converted into many different forms including heat, light, sound and motion depending on the particular application. So, what is electrical current and how is it related to voltage ?


  Consider the following analogy. The water system in your house probably has a “tank” located somewhere high, maybe in the loft. You have pipes that link it to taps (faucets) somewhere lower, say, in your kitchen. When you open a tap the water flows. In this analogy the force pushing the water out of the tap (gravity) is analogous to voltage and the quantity of water flowing per second represents the electrical current. In the electrical circuit equivalent of this we could imagine a battery providing the voltage and wires linking the + and – terminals via a switch. When the switch is closed current flows through the wires. In this case the current is a flow of electrons, invisible to the naked eye but unmistakably present. One word of warning … in general it is not a good idea to connect a wire directly across the terminals of a battery since this provides a path of very low resistance for the current and will very quickly drain the battery and ,depending on the wire used and the capacity of the battery, may result in overheating.. It is analogous to opening the tap fully or, in fact, removing it completely allowing the water to flow un-restricted out of the tank.


  Lets take the analogy a little further by considering a few more variables. If we take the tank of water higher (on the roof perhaps!!) then the gravitational potential (head of water) increases. With the tap fully open again the flow of water (in litres per second) will be greater than previously since the “pushing force” has increased. This corresponds to using a battery with a greater voltage.
  If we keep the tank at the original height and only partially open the tap, then the resulting flow will be reduced. The narrowing of the pipe at the tap orifice represents a restriction to the flow and corresponds, in our analogy, to electrical resistance. Resistance defines the relationship between voltage and current in a given circuit; i.e. the more resistance for a given voltage the lower the resulting current.
  What if we remove the tap completely ? The resulting water flow will be restricted only by the diameter of the pipes linking it to the tank. This would be like removing the switch and connecting a wire directly between the terminals of the battery. It is clear that if we increased the diameter of the pipes we would substantially increase the water flow. The pipe is acting like a distributed restrictor and ,by analogy, the wire linking the battery terminals would have a distributed resistance depending on it’s diameter (for a given material e.g. copper and a given length).


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