Conversations on Electrical Engineering: Electricity - from simple to complex. Part 5.

Batteries, as a rule, are either a separate, structurally completed GALVANIC ELEMENT, or some combination of similar elements, it is in the latter case that one can speak of an ELECTRICAL BATTERY. We mentioned the concept of ELECTRIC POWER (EMF).

We note that the magnitude of the EMF is determined by purely chemical, more precisely, electrochemical properties of the active materials from which the electrodes are made, as well as the properties of the electrolyte, and DOES NOT depend on their (electrode) dimensions! The dependence of EMF on temperature (within reasonable limits) is very low. It is necessary to distinguish such concepts as EMF batteries and its VOLTAGE. Because EMF is such a difference of electrical potentials on the electrodes, which depends only on the chemical properties of the materials used. It is measured with an OPEN external circuit. While the VOLTAGE is measured exclusively at the CLOSED external circuit or, as they say, under load.

This voltage depends on a number of factors, in particular, on the emf of the battery (E), the load current (In) and the so-called INTERNAL RESISTANCE of the battery (Rin) U = E = InRvn. However, one should not think that this is enough to uniquely determine the numerical value of the voltage that the battery develops on the load. This is not so, because RvN is NOT a constant value! It essentially depends on the degree of battery usage, hence, on the degree of its discharge.

There are INITIAL, MEDIUM and FINAL stresses. INTERNAL RESISTANCE (Rvn) depends on the electrolyte used, the material of the electrodes and separators, i.e. Gaskets between the electrodes. Naturally, the Rv is smaller, the better, since the larger the DC current can be at a given voltage on the load.

One of the most important parameters of the battery is also its ELECTRICAL CAPACITY (Q). If the discharge current can be considered constant during the entire discharge time, then the electrical capacitance is the product: Q = IpT, where Ip is the discharge current; T is the discharge time.

If the current intensity (with the discharge of the battery) changes, then use the AVERAGE current value. It represents the arithmetic mean of the currents at the beginning and end of the discharge, respectively. Icp = (In.p. + Ik.p) / 2. In recent years, on the labels of electric batteries, in addition to EMF, it is customary to indicate which discharge current (in other words, what load current) is optimal for this type of battery (element). In addition, indicate the value of the electrical capacity.

All of the above is also characteristic for such a class of electric energy sources, as ACCUMULATORS. At present, they (with a certain exception) are produced in the same standard of housings as electric batteries, but are significantly more expensive.

However, this relative high cost actually turns (with the proper operation of batteries) a colossal win both in terms of economy and in terms of practicality. Since, unlike an electric battery, which exhausts its capacity, turns into a scrap, the batteries can be recharged with an external source of electrical energy.

Modern household batteries, as a rule, allow from 700 to 1000 recharges! Figure 12 shows the dependence of the current on the voltage for a different value of the external resistance. In other words, the FAMILY OF CHARACTERISTICS, clearly demonstrating what a RESISTOR is.

Knowing the specific value of the resistance of the resistor, it is very easy to determine what current will go through this resistor at any arbitrarily given constant voltage applied to its (resistor) terminals. But our story about the simplest DC circuits will be incomplete, if we ignore the fact that in these chains we use (and quite widely) and components whose characteristics are very different from those previously mentioned.

Let's consider (Fig.13) the characteristic <1>. It consists of two completely different parts. The first begins at the point <zero> and ends at the point <A>. It is interesting in that the increase in voltage from 0 to 5.6 V does NOT lead to the appearance of a current! But at the moment of reaching this CRITICAL POINT the picture changes in the most essential way. Because the current flowing through this AWESOME component of the current increases in an avalanche! This is despite the fact that the voltage at the terminals of this component increases by a very small amount.

Thus, with good reason, we can say that with increasing current through this "strange" component from 0 to 30 mA, the voltage drop on it almost does not change! Well, what happens if the current exceeds 30 mA?

It turns out that no fundamental changes will occur in this case. But it must be taken into account that with increasing current, the electric power dissipated in it sharply increases through the component under consideration! But it can not be as large as you want! Therefore, if the maximum permissible power is exceeded, a THERMAL component breakdown occurs. He, more simply, burns! Note that this "amazing" component is very widely used in electrical engineering and electronics.

In reality, it represents a rather complicated SEMICONDUCTOR STRUCTURE and is called STABILITRON. Often the question is asked: does the zener diode obey the Ohm Law or not? If he obeys, what is his resistance? But the resistance of the zener diode is generally not accepted. This parameter in electrical engineering and electronics (concerning zener diodes) is never considered! Instead, it is customary to talk about the stabilizer's DYNAMIC RESISTANCE

(Rdst) Rdst = dU / dI.

Usually Rdst does not exceed several tens of ohms. This is for very mediocre specimens, for good zener diodes a few ohms or even less. In Fig. 13. The characteristic <2> is also given. Its fracture point <C> corresponds to 9.5 V.

This means that this zener diode has a STABILIZATION OF 9.5 V. It should be noted that the REAL characteristics of zener diodes differ by a more smooth break (Fig. 14). Therefore, real zener diodes are characterized not only by the stabilization voltage, but also by the MINIMUM and MAXIMUM stabilization current. In Fig.14. Ist.min = 5 mA, Ist.max = 28 mA.

Next, let's look at how the Stabilitron is used to stabilize the voltage and why this is so necessary.

The above values ​​of the currents I2min and I2max, respectively equal to 6 and 14.6 mA, knowingly do not achieve in this example the passport values ​​of the minimum and maximum stabilization currents for the zener used (in this case, KC168).

This means that having accepted the Ust unchanged, we were completely right and did not in the least make a mistake against the truth. Therefore, the zener diode allows to maintain an unchangeable voltage on the load in the event that the load resistance is not constant. And now imagine another situation, which in practice is very common. Suppose that the supply voltage is UNSTABLE and varies between 12 and 18 V. What will happen in this case at point "A" at Rmin = 680 Ohm?

1) URb = Umin - Ust = 12 - 6.8 = 5.2 V; I1 = 5.2 / 510 = 10.2 mA; Iн = 6.8 / 680 = 10 mA; I2 = I1 - In = 10.2 - 10 = 0.2 mA!

2) URb = Umax - Ust = 18 - 6.8 = 11.2 V; I1 = 11200/510 = 22 mA; Iн = 10 mA; I2 = 22 - 10 = 12 mA.

And what will happen if R n.max = 5 kOhm? At U min = 12 V we have U Rb = 5.2 V; I1 = 5.2 / 510 = 10.2 mA; Iн = 6.8 / 5 = 1.4 mA; I2 = 10.2 - 1.4 = 8.8 mA.

At U max = 18 V we have U Rb = 18 - 6.8 = 11.2 V; I1 = 11200/510 = 22 mA; Iн = 6.8 / 5 = 1.4 mA; I2 = 22 - 1.4 = 20.6 mA.

In this case, it can be argued that at Umin = 12V and Rmin = 680 ohm, the stabilization mode disturbance is observed, since the zener diode current is less than 3 mA. In all other cases, i.e. At Rnmax = 5 kOhm, and also at Umax = 18

The zener diode does not go beyond the passport mode of voltage stabilization. From the violation of stabilization can get rid of, if, for example, Rb = 390 Ohm.

Making simple calculations, it is easy to verify that AS when changing the supply voltage, and when changing Rn (within the above limits, of course) the use of a zener diode allows to maintain the voltage on the load UNEXPECT and equal Ust. As for the disadvantages of the above-mentioned SIMPLE voltage stabilization scheme, it is worthwhile to list them:

1) the load current is ALWAYS comparable in magnitude to the current flowing through the zener diode. This means that the efficiency of the simplest stabilizer does not reach 40%;

2) the permissible range of the load current does not exceed, as a rule, 2-3 times. Any zener diode is characterized by such an important parameter as TKN - temperature coefficient of voltage. It is specified in the technical parameters for all types of zener diodes.

TKN shows how much the stabilizing voltage of a given type of zener diode changes with an unchanging current if the ambient temperature changes over a range of temperatures. This range depends primarily on whether the zener diode is intended for domestic, industrial or special electronics.

Usually, the TCH is expressed as the percentage of the maximum permissible temperature change of the stabilization voltage dUst to the stabilization voltage UT.on TCN = dUst / Ust. Nom. Depending on the size of the TCH, the zener diode is divided into conventional and precision ones.

In the simplest stabilization scheme (Figure 15), there is no point in using precision Zener diodes. First of all, because, as was shown above, the basis for the operation of such a circuit is precisely the change in the zener diode current. But in this case we can not speak of any fixed value of TKN. Hence another drawback of the simplest scheme: there is not sufficient voltage stability at point "A". The "padding" of this voltage reaches several tens of millivolts, which, as will be shown later, is unacceptable for feeding most radio engineering circuits. Note that circuits like the simplest are called PARAMETRIC voltage regulators.

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