ZENER Diode - 5 

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 CIRCUITAL DIMENSIONING

The circuit published in figure 9 represents the example more classic than use than a diode  zener, as stabilizing element of the voltage of feeding of a loaded electrical worker.

                                    RZ

Fig. 9 - With reference to this simple and classic stabilizing circuit of voltage, it has been introduced in the text an example of calculation of resistance RZ, obviously in relation to the acquaintance of all the other largenesses electrical workers.

This circuit, than is often repeated in the practical electronics and it acknowledges in many  apparatuses.

A single calculation must be executed in order to compose the project of figure 9, what door to define the exact value of the resistance of zener RZ, notes that are the voltage of entrance available VE, that demand from cargo VU, that it coincides with the voltage of zener the Vz, the minimal current that must circular through zener the Iz and that one that comes absorbed from cargo Ic.

For greater clarity, we list here of continuation the largenesses mentioned notes hour:

VE = input voltage;

Vz = voltage of diode  = VU ;

Iz = current min. in the diode ;

Ic = current in the load

We suppose that the cargo of the circuit of figure 9 demands a voltage of equal feeding VU to 5,6 V with one absorption of current Ic of 0,1 To and that the voltage of feeding of 12 circuit VE is of V. In such case must be used a zener from 5,6 V - 1 W, that is the same employee in the experiments described previously.

In order to determine the value of resistance RZ of the circuit of figure 9, it is necessary to apply the following formula:

RZ = (VE-Vz)/(Iz+Ic)

in which, we repeat it, Iz represents the minimal value of the current that must slide through the cargo and that currency in the measure of a tenth one of the tolerated maximum current from the zener. Therefore:

Indicating with Pz the dissipabile maximum power from zener and with Iz max the coorente principle in Dz it can be written:

Iz max = Pz/Vz

Iz = Iz max /10

Replacing the values it is had:

Iz max = 1 / 5,6  = 0,178 A

Iz = 0,178 : 10 = 0,0178 A

Therefore RZ is worth:

RZ = (12- 5,6 )/(0,0178 + 0.1 )=54.7 ohm

The value found for Rz is of 54.7 ohm that can be approximated to 56 ohm in order to favor the reperibilità trades them

The value of the power of dissipation of Rz is worth:

PRz = 6,4 x 0,117 = 0,76W

A resistor of 1W can be chosen.

Between the hypotheses it begins them for the calculation of resistance RZ, to the diode  zener from 5,6 V has been attributed, po' arbitrarily, to a value of power of 1 W, giacché the powers in game, the circuit of figure 9, could be estimated in inferior measure to 1 W. Hour, however, for those who they love the precision, we can expose the easy calculation that it also concurs to calculate the power to attribute to the zener prechosen.

To the current of cargo Ic it had been attributed the value of 0,1 To, but we suppose that the cargo demands one variable current between this maximum and the minimum of 0,02 To In such case, the absorbed running principle from the diode  zener would have to be of:

0,1 -  0,02 = 0,08 A

but to this it is necessary to add the minimal current previously estimated Iz, that is:

0,08  + 0,017 = 0,097 A

that it represents the true running principle that slides through the zener. Therefore, the power from this dissipated is worth:

Pz = 5,6  x 0,097  = 0,54 W

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