Wiring diagramsThe difference between a schematic diagram and a wiring diagram is the amount ofdetail included. In a schematic diagram, the interconnection of the components isshown, but the actual values of the components are not necessarily indicated.You might see a diagram of a two-transistor audio amplifier, for example, with re-sistors and capacitors and coils and transistors, but without any data concerning thevalues or ratings of the components. This is a schematic diagram, but not a true wiringdiagram. It gives the scheme for the circuit, but you can’t wire the circuit and make itwork, because there isn’t enough information.Suppose you want to build the circuit. You go to an electronics store to get theparts. What sizes of resistors should you buy? How about capacitors? What type of tran-sistor will work best? Do you need to wind the coils yourself, or can you get them readymade? Are there test points or other special terminals that should be installed for thebenefit of the technicians who might have to repair the amplifier? How many wattsshould the potentiometers be able to handle? All these things are indicated in a wiringdiagram, a jazzed-up schematic. You might have seen this kind of diagram in the back ofthe instruction manual for a hi-fi amp or an FM stereo tuner or a television set. Wiringdiagrams are especially useful and necessary when you must service or repair an elec-tronic device.Voltage/current/resistance circuitMost dc circuits can be ultimately boiled down to three major components: a voltagesource, a set of conductors, and a resistance. This is shown in the schematic diagram ofFig. 4-5. The voltage or EMF source is called E; the current in the conductor is called I;the resistance is called R. The standard units for these components are the volt, the am-pere, and the ohm respectively.68 Basic dc circuits4-5Simple dc circuit. Thevoltage is E, the current isI, and the resistance is R.You already know that there is a relationship among these three quantities. If oneof them changes, then one or both of the others will also change. If you make the resis-tance smaller, the current will get larger. If you make the EMF source smaller, the cur-rent will decrease. If the current in the circuit increases, the voltage across the resistorwill increase. There is a simple arithmetic relationship between these three quantities.