2A DIRECT CURRENT CIRCUITS AND MEASUREMENTSLaws of ElectricityV = IRP = IVChemistry 331Chapter 2 Electrical Components and Circuits The purpose of this chapter is to discuss basic direct current (dc) circuit components in preparation for the two following chapters that deal with integrated circuits and microcomputers in instruments for chemical analysis. 2A DIRECT CURRENT CIRCUITS AND MEASUREMENTS Some basic direct current circuits and how they are used in making current, voltage, and resistance measurements will be considered. The general definition of a circuit is a closed path that may be followed by an electric current. Laws of Electricity Ohm’s law describes the relationship among potential, resistance and current in a resistive series circuit. In a series circuit, all circuit elements are connected in sequence along a unique path, head to tail, as are the battery and three resistors shown in Figure 2-1. Ohm’s Law may be written as:V = IR Where V is the potential difference in volts between two points in a circuit, R is the resistance between the two points in ohms, and I is the resulting current in amperes. Kirchhoff’s current law states that the algebraic sum of currents around any point in a circuit is zero. Kirchhoff’s voltage law states that the algebraic sum of the voltages around a closed electrical loop is zero.The power law states that the power in watts dissipated in a resistive element is given by the product of the current in amperes and the potential difference across the resistance in volts:P = IVAnd substituting Ohm’s law gives:P = I2R = V2/RBasic Direct Current Circuits Two types of basic dc circuits will be described; series resistive circuits and parallel resistive circuits. Series Circuits Figure 2-1 shows a basic series circuit, which consists of a battery, a switch, and three resistors in series. Figure 2-1 (Principles of Instrumental Analysis)The current is the same at all points in a series circuit, that is: I = I1 = I2 = I3 = I4Application of Kirchhoff’s voltage law to the circuit in Figure 2-1 yields:V = V1 + V2 + V3The total resistance, Rs, of a series circuit is equal to the sum of the resistances of the individual components. Rs = R1 + R2 + R3Parallel Circuits Figure 2-2 shows a parallel dc circuit. Figure 2-2 (Principles of Instrumental Analysis) Applying Kirchhoff’s current law, we obtain:It = I1 + I2 + I3Applying Kirchhoff’s voltage law to this circuit gives three independent equations. V = I1R1V = I2R2V = I3R3Substitution and division by V gives:1/ Rp = 1/R1 + 1/R2 + 1/R3Since the conductance, G, of a resistor, R, is given by G = 1/R:Gp = G1 + G2 + G3Conductances are additive in a parallel circuit rather than the resistance. In conclusion, the most important things to remember about the differences between resistors in series and parallel are as follows:Resistors in series have the same current and Resistors in parallel have the same voltage. 2B SEMICONDUCTOR DIODES A diode is a nonlinear device that has greater conductance in one direction than in another. Useful diodes are manufactured by forming adjacent n-type and p-type regions within a single germanium or silicon crystal: the interface between these regions is termed a pn junction. Basically, they are composed of an electron rich and electron deficient area of which one is capable of dropping a voltage over it. The interface will actas an insulator until it reaches a threshold and becomes a conductor. Figure 2-3a is a cross section of one type of pn junction, which is formed by diffusing an excess of a p-type impurity, such as indium, into a minute silicon chip that has been doped with an n-type impurity, such as antimony. A junction of this kind permits movement of holes from the p region into the n region and movement of electrons in the in the reverse direction. As holes and electrons diffuse in the opposite direction, a region is created that is depleted of mobile charge carriers and thus has very high resistance. This region is referred to as the depletion region. Because there is a separation of charge across the depletion region, a potential difference develops across the region that causes amigration of holes and electrons in the opposite direction. The current that results from the diffusion of holes and electrons is balanced by the current produced by migration of the carriers in the electric field, thus there is no net current. The magnitude of potential difference across the depleted region depends upon the composition of the materials used in the pn junction. For silicon diodes, the potential difference is about 0.6V, and for germanium, it is about 0.3V. When a positive potential is applied across a pn junction, there is little resistance to current in the direction of the p-type to the n-type material. Onthe other hand, the pn junction offers a high resistance to the flow of holes in the oppositedirection and is called a current rectifier. Figure 2-3b illustrates the symbol for a diode. The arrow points in the direction of low resistance to positive current. The triangular portion of the diode symbol may be imagined to point in the direction of current in a conducting diode. Figure 2-3c shows the mechanism of conduction of charge when the p region is made positive with respect to the n region by application of a potential; this process is called forward biasing. The holes in the p region and the excess electrons in the n region move under the influence of the electric field toward the junction, where they combine and annihilate each other. The negative terminal of the battery injects new electrons into the n region, which can then continue the conduction process; the positive terminal extracts electrons from the p region, creating new holes that are free to migrate towards the pn junction. Figure 2-3d shows when the diode is reverse-biased and the majority carriers in each region drift away from the junction to form the depletion layer, which contains few charges. Only the small concentration of minority carriers present in each region drifts toward the junction and creates a current.Figure 2-3 (Principles of Instrumental Analysis)Transistors The transistor is the basic semiconductor amplifying and switching device. This device provides an output signal whose magnitude is significantly greater than the signal at the input. Several types of transistors are available. Two of the