To select a zener diode for the circuit shown in Fig. 3, you need to know the range of input voltages U1 and the range of load changes R N.
Rice. 3. Zener diode connection circuit.
For example, let's calculate the resistance R and select a zener diode for the circuit in Fig. 3 with the following requirements:
So, first, let's calculate the value of resistance R. The minimum input voltage is 11 V. At this voltage, we must provide a current to the load of at least 100 mA (or 0.1 A). Ohm's law allows you to determine the resistance of a resistor:
R C = U1 MIN / I N.MAX = 11 / 0.1 = 110 Ohms That is, the circuit to provide a given current to the load must have a resistance of no more than 110 Ohms.
The voltage drops at the zener diode is 9 V (in our case). Then, at a current of 0.1 A, the equivalent load is: R E = U2 / I N.MAX = 9 / 0.1 = 90 Ohm Then, in order to provide a current of 0.1 A to the load, the quenching resistor must have a resistance: R = R C – R E = 110 – 90 = 20 Ohm Taking into account the fact that the zener diode itself also consumes current, you can choose a slightly lower resistance from the standard E24 series). But, since the zener diode consumes a small current, this value can be neglected in most cases.
Now let's determine the maximum current through the zener diode at the maximum input voltage and the load is off. The calculation must be performed with the load disconnected, since even if your load is always connected, you cannot exclude the possibility that some wiring will become unsoldered and the load will turn off.
So, let's calculate the voltage drop across resistor R at maximum input voltage:
U R.MAX = U1 MAX – U2 = 15 – 9 = 6 VA Now let’s determine the current through resistor R from the same Ohm’s law: I R.MAX = U R.MAX / R = 6 / 20 = 0.3 A = 300 mA Since the resistor R and the zener diode VD are connected in series, the maximum current through the resistor will be equal to the maximum current through the zener diode (with the load off), that is, I R.MAX = I VD.MAX = 0.3 A = 300 mA More is needed calculate power dissipation resistor R. But we will not do this here, since this topic is described in detail in the article Resistors.
But let’s calculate the dissipation power of the zener diode:
P MAX = I VD.MAX * U ST = 0.3 * 9 = 2.7 W = 2700 mW Dissipation power is a very important parameter that is often forgotten to take into account. If it turns out that the power dissipation on the zener diode exceeds the maximum permissible, this will lead to overheating of the zener diode and its failure. Although the current may be within normal limits. Therefore, the power dissipation for both the damping resistor R and the zener diode VD must always be calculated.
It remains to select a zener diode according to the obtained parameters:
U ST = 9 V – rated stabilization voltage
I ST.MAX = 300 mA – maximum permissible current through the zener diode
P MAX = 2700 mW – zener diode dissipation power at I ST.MAX
Using these parameters, we find a suitable zener diode in the reference book. For our purposes, for example, a D815V zener diode is suitable.
It must be said that this calculation is quite rough, since it does not take into account some parameters, such as, for example, temperature errors. However, in most practical cases, the method described here for selecting a zener diode is quite suitable.
Zener diodes of the D815 series have a spread of stabilization voltages. For example, the voltage range of the D815V is 7.4...9.1 V. Therefore, if you need to get the exact voltage across the load (for example, exactly 9 V), you will have to empirically select a zener diode from a batch of several of the same type. If you don’t want to bother with selecting at random, then you can choose zener diodes from another series, for example the KS190 series. True, they are not suitable for our case, since they have a dissipation power of no more than 150 mW. A transistor can be used to increase the output power of the voltage stabilizer. But more about this some other time...
And one more thing. In our case, the dissipation power of the zener diode was quite high. And although according to the characteristics for the D815V the maximum power is 8000 mW, it is recommended to install a zener diode on the radiator, especially if it operates in difficult conditions (high temperature environment, poor ventilation, etc.).
If necessary, below you can perform the calculations described above for your case
Stabilizers are parametric and compensating. The operating principle of parametric ones is that they use the special properties of elements, the parameters of which, namely resistance, change so that stabilization becomes possible.
Below are the characteristics of an ordinary transistor (a) and a silicon zener diode (b):
Current stabilizer
In the first of them, the resistance of the element changes so that, within significant limits of changes in voltage to the elements, the current in it is almost constant. In the other, on the contrary, with significant changes in current, the voltage is almost constant. Therefore, a transistor (or other semiconductor devices with a similar characteristic) can be used to stabilize the current, and a zener diode can be used to stabilize the voltage. Below is a circuit for current stabilization:
To calculate it, first select a stabilizing element CE with a suitable characteristic and current I st (see figure above A). The voltage that will be applied to this element is defined as the average voltage between the beginning and end of stabilization:
In this case, the load will have a voltage of I st R n. Based on these data, the values of Uin that need to be applied to the stabilizer are calculated:
This completes the calculation of the current stabilizer.
Voltage stabilizer
The voltage stabilizer shown in the diagram below is calculated similarly:
Based on the given value of U st, a suitable zener diode is selected and I min and I max are determined from its characteristics. Using these data, the current I st = (I min + I max)/2 is calculated. The total current I in is equal to I st + U st / R n. to provide support at the load U st = I st R n when the voltage in the network decreases, the voltage supplied at the input U in is chosen 20 percent higher than U st. This excess will be used on the ballast resistor R b, the value of which will be found using the formula:
To determine the quality of the stabilizer, a stabilization coefficient has been introduced, equal to the ratio of the relative deviations of the input voltage to the relative deviations of the voltage at the load:
At K st = 1, there is no stabilization. The more Kst differs from unity, the more effective the stabilization.
For parametric stabilizers, the stabilization coefficient is not very large. For high-quality stabilization, so-called compensation stabilizers are used. The stabilizing element in them are ordinary transistors, which are automatically controlled so that their collector voltage changes and compensates for the deviation of the incoming voltage.
Parametric voltage regulators are still used to power low-power electronic products, so it is necessary to be able to calculate them.
Often, when repeating finished structures, the operating conditions of which differ from those recommended by the developer, it is necessary to analyze the operation of the parametric voltage stabilizer to clarify the resistance value of the ballast resistor.
These problems were solved using a Microsoft Excel file developed by the author. Two options for calculating a parametric voltage stabilizer and a calculation for analyzing the operating conditions of a zener diode in a finished circuit are presented.
The objects of calculation and analysis in the examples are parametric stabilizers of two well-known designs of audio frequency power amplifiers. This is from Interlavka and from Andrey Zelenin A.
Basic relations for calculating a parametric stabilizer using a zener diode
In Fig. Figure 1 shows a schematic diagram of a parametric stabilizer: Uin – input unstabilized voltage, Uout=Ust – output stabilized voltage, Ist – current through the zener diode, In – load current, R 0 – ballast (limiting, quenching) resistor.Uin=Ust+(In+Ist)R 0 =Ust+IR 0, (1)
I=In+Ist – current flowing through the ballast resistor R0.
Rice. 1. Diagram of a parametric voltage stabilizer using a zener diode
As can be seen from Fig. 1, a parametric stabilizer based on a silicon zener diode is a voltage divider consisting of a ballast resistor R0 with a linear current-voltage characteristic (VC) and a zener diode VD1, which can be considered as a resistor with a sharply nonlinear I–V characteristic.
When the voltage Uin changes, the current through the divider changes, leading to a change in the voltage drop across the resistor R0, and the voltage across the zener diode, therefore, across the load Rн practically does not change.
A small change in voltage across the load in the range from Ust min to Ust max corresponds to a change in the current through the zener diode from Ist min to Ist max. Moreover, the minimum current through the zener diode corresponds to the minimum input voltage and maximum load current, which is achieved with the resistance of the ballast resistor
R 0 =(Uin min-Ust min)/(In max+Ist min). (2)
In turn, the maximum current through the zener diode will flow at a minimum load current and maximum input voltage.
It is easy to find the operating conditions of the stabilizer:
ΔUin=ΔUst+R 0 (ΔIst-ΔIn), (3)
where ΔUin=Uin max-Uin min, ΔUst= Ust max-Ust min, ΔIst=Ist max-Ist min, ΔIn= In max-In min.
For simplicity, let us set ΔUst = 0 and analyze expression (3).
The load current range cannot be greater than the zener diode current range, since in this case the right-hand side of the expression becomes negative and the circuit will not work as a voltage regulator.
If the change in load current is insignificant, the expression for the operating condition of the stabilizer is simplified:
ΔUin= ΔIstR 0. (4)
The efficiency of a parametric stabilizer is determined from the expression:
Efficiency=Ust In /(Uin (In + Ist)=1/(Nst(1+ Ist/In)), (5)
where Nst=Uin/Ust – stabilizer transmission coefficient; usually Nst=1.4…2.
From expression (5) it follows that the lower the transfer coefficient of the stabilizer and the lower the ratio of the current through the zener diode to the load current, the higher the efficiency.
The main parameter of a voltage stabilizer, by which its performance quality is assessed, is the stabilization coefficient:
Kst=(ΔUin/Uin)/(ΔUout/Uout)= R 0 Ust/rdUin=R 0 /Nst-d=Kfefficiency, (6)
where rd is the dynamic resistance of the zener diode; Kf – filtration coefficient.
The first option for calculating a parametric stabilizer
We will carry out this for the case when the supply voltage is unstable and the load resistance is relatively constant.
The initial data for the calculation are: Uout, In, ΔIn, Uin, ΔUin.
To obtain the required output voltage, according to the reference book, select a zener diode with the parameters: Ust = Uout, Ist max, Ist min, rd.
We calculate the required input voltage based on the extreme optimal transfer coefficients of the stabilizer Nst = 1.4...2, which can also be selected by the user in any required range Nst:
Ist р=0.5(Ist min+Ist max)>In.
Let's calculate the resistance of the ballast resistor:
R 0 =(Uin - Ust)/(Ist p+ In).
Let us calculate the power of the ballast resistor with a double margin:
Po=2(Ist p+ In) 2 R 0 .
Let's check the selected mode of operation of the stabilizer.
The calculation is correct if, with a simultaneous change in Uin by the amount ΔUin and In by the amount ΔIn, the zener diode current does not go beyond the limits of Ist max and Ist min:
Ist r max=(Uin+ ΔUin- Ust)/(R 0 -(In- ΔIn))<0,8 Iст max;
Ist r min=(Uin- Ust)/(R0-(In+ ΔIn))>1.2 Ist min.
This takes into account the 20% margin required for reliable operation of the zener diode. The maximum operating value of the current through the zener diode, accepted in the calculation, is no more than 0.8 from the reference Ist max, due to considerations of the operational reliability of the device, so that the power dissipated by the zener diode is below the maximum. To guarantee the required stabilization coefficient, the minimum operating value of the current through the zener diode Ist p min is taken to be 1.2 times greater than Ist min.
If the obtained current values Ist p max and Ist p min are outside the permissible values, then it is necessary to select a different value for Ist p, change the resistance R 0 or replace the zener diode.
We will also calculate the stabilizer parameters that determine its quality and efficiency - stabilization coefficient Kst = (ΔUin/Uin)/(ΔUout/Uout)= R 0 /(rdNst),
efficiency factor efficiency=Ust In /(Uin (In + Ist))=1/(Nst(1+ Ist/In)),
and filtration coefficient Kf=Kst/efficiency.
Calculation example No. 1
Let's calculate a parametric voltage stabilizer with the following characteristics: stabilized load voltage Un=9 V; load current In = 10 mA; change in load current ΔIn=2 mA; change in input voltage ΔUin=10%.Let's choose a zener diode of type D814B, for which Ust= Un=9 V; rd=10 Ohm; Ist max=36 mA; Ist min=3 mA.
We enter the above information into the corresponding cells of the source data (highlighted with a light blue fill) of the “First calculation option” sheet of the Microsoft Excel table “Calculation and analysis of the operation of a parametric voltage stabilizer.xlsx” and immediately obtain the calculation results in the calculation cells, highlighted with light brown filling:
input voltage Uin=15.0 V; resistance of the ballast resistor R 0 =240 Ohm, power of the ballast resistor with a double reserve Po=0.3 W; Kst=15.0, efficiency=24%, Kf=62.5 (see Fig. 2).
Rice. 2. Print from the screen of calculation example No. 1
We choose a resistor with a resistance of 240 Ohms and a power of 0.5 W.
Let us assume that at the input of the stabilizer there are ripples of alternating voltage with an amplitude Upin = 0.1 V = 100 mV. The ripple amplitude at the output of the stabilizer will be Upst = Upin/Kph=100/62.5=1.6 mV.
Calculation example No. 2
Let's calculate a parametric stabilizer for supply voltages Up=Uin=±25 V; ±35 V and ±45 V.The calculation will be performed for a parametric stabilizer of positive polarity (R5, VD1, C2), since another stabilizer of negative polarity (R6, VD2, C4) differs only in the direction of switching on the zener diode.
Let's prepare the initial data: stabilized load voltage Un=12 V, load current In=(12-0.5)/R2=11.5/10=1.15 mA, ΔIn=0.115 mA, change in input voltage ΔUin=10 %.
Let's choose a zener diode BZX55C12, which has the following parameters: Ust= Un=12 V; rd=20 Ohm; Ist max=32 mA; Ist min=5 mA.
The calculation results are shown in Fig. 3; for Up=±25 V R5=R6=1.3 kOhm (0.25 W); for Up=±35 V R5=R6=2.4 kOhm (0.5 W); for Up=±45 V R5=R6=3.6 kOhm (1 W).
Rice. 3. Calculation of parametric stabilizers for the “Green Lanzar” amplifier
The second option for calculating the parametric stabilizer
uses the limit values of the load current In min and In max as initial data, which, when In min = 0, makes it possible to provide for the idle mode of the stabilizer. For constant load choose In max = In min.
So, the initial data are: stabilized load voltage Uout, load currents In min, In max, rated input voltage Uin and its deviations ΔUin n and ΔUin in.
The zener diode parameters are the same as in the previous calculation: Ust = Uout, Ist max, Ist min, rd.
We calculate the maximum and minimum values of the zener diode operating current:
Ist p max=0.8 Ist max,
Ist p min=1.2 Ist min.
If the stabilizer must operate in idle mode (In min=0), select Ist p min=Ist min.
We check the suitability of the zener diode selected for stabilization voltage within the specified limits of load current and supply voltage:
(Ist p max+ In min)(1- ΔUin n)-(Ist min+ In max)(1+ ΔUin in)>0,
where ΔUin n=(Uin-Uin min)/ Uin, ΔUin in=(Uin max-Uin)/ Uin.
If the inequality does not hold, you need:
use a more powerful zener diode;
set to smaller values ΔUin n and ΔUin in;
reduce In max or increase In min.
The rated voltage Uin, which the rectifier must provide, is calculated using the formula:
Uin= Ust [(Ist p max+I n min)- (Ist p min+ I n max)]/[(Ist p max+I n min)(1- ΔUinn)- (Ist p min+I n max) (1+ΔUin in)].
Ballast resistor resistance:
R 0 = Uin(ΔUin in+ΔUin n)/[(Ist p max+ In min)- (Ist p min+ In max)].
We also calculate the power of the resistor with a double margin:
Po=2(Uin(1+ ΔUin n) - Ust) 2 /R 0 .
Using the formulas given in the first version of the calculation, we find Kst, efficiency and Kf.
Calculation example No. 3
Let's calculate a parametric voltage stabilizer with the following characteristics: stabilized load voltage Un=9 V; current In min =0, In max =10 mA; change in input ΔUin n=10%, ΔUin v=15%.Let's choose a zener diode of type D814B, for which Ust = Un; rd=10 Ohm; Ist max=36 mA, Ist min=3 mA.
After entering the initial data into the “Second calculation option” table sheet, we obtain the following results (Fig. 4):
Uin=14 V, R 0 =221 Ohm, Po=0.45 W, Kst=14.2.
Rice. 4. Screenshot of the parametric stabilizer in idle mode
We choose a resistor with a resistance of 220 Ohms and a power of 0.5 W.
Analysis of the operation of a parametric stabilizer
The initial data of the analysis are as follows: Un, In, ΔIn, ΔUin, R 0 .Also, the analysis requires the parameters of the zener diode: Ust = Un, rd, Ist max and Ist min.
The analysis comes down to calculating the operating current of the zener diode Ist p=(Uin-Ust)/R 0 -In; transmission coefficient Nst = Uin/Ust; power Po of the ballast resistor, stabilization coefficient Kst, efficiency and filtration coefficient Kf.
It is important to check the operating mode of the zener diode in the stabilizer circuit, which is performed using formulas similar to those given in the first calculation option.
Analysis example #1
Let's analyze the values of ballast resistors R3 and R4 of the compensation voltage stabilizers of the Lanzar amplifier, depending on the supply voltage used.The declared range of amplifier supply voltages is from Up=±30 V to ±65 V, while on schematic diagram the resistance of the ballast resistors is indicated R 0 =R3=R4=2.2 kOhm (1 W).
In another publication, it is recommended to select the resistance value of ballast resistors depending on the supply voltage of the amplifier using the formula R 0 = (Up-15)/I, where I = 8...10 mA. Table 1 shows the calculation using the specified formula for the amplifier supply voltage range in 5 V increments.
Initial data for analysis: stabilized load voltage Un=15 V, load current In=(15-0.5)/R5=14.5/6.8=2.13 mA, ΔIn=0.213 mA, change in input voltage ΔUin=10%.
Let's choose a zener diode 1N4744A, which has the following parameters: Ust= Un=15 V; rd=14 Ohm; Ist max=61 mA; Ist min=5 mA.
An analysis of the operation of parametric stabilizers in the Lanzar amplifier showed that the minimum stabilizer current Ist p min was selected at the limit with a margin of only 3...14% instead of the required 20% (Fig. 5).
Rice. 5. Operating modes of stabilizers in the Lanzar amplifier depending on the selected supply voltage
Using the Microsoft Excel spreadsheet data analysis tool “Parameter Selection,” we will clarify the resistance of the ballast resistors. To do this, let's go to the cell with the formula for Ist p min (cell C26) and select from the menu Data -> « What-if analysis»-> Parameter selection.
Set it in a cell C26 value 6.0 (margin 20% of Ist min), changing the value of the cell in which the resistance of the ballast resistor is entered ( $C$15).
We get R 0 = 1.438 kOhm. Let's enter into this cell the nearest resistance value from the standard series R 0 =1.3 kOhm.
Having carried out the indicated operation in the table for all values of supply voltages, we obtain the following result (Fig. 6).
Rice. 6. Clarification of operating modes of parametric stabilizers of the Lanzar amplifier
The results of the analysis are also summarized in Table 2.
The power of resistors for amplifier supply voltages from ±30 V to ±40 V is 0.5 W, for other voltages – 1 W.
Bottom line
It is necessary to calculate even such a simple device as a parametric voltage stabilizer. Choosing the value of the ballast resistor “by eye” can cause design errors that will not be immediately noticed.Before assembling the design you like, it is advisable to analyze and, if necessary, clarify the operating mode of the zener diode of the parametric stabilizer using the proposed spreadsheets in Microsoft Excel.
Sufficient for many electrical circuits and circuits simple block power supply that does not have a stabilized voltage output. Such sources most often include a low-voltage transformer, a diode bridge rectifier, and a capacitor acting as a filter.
The voltage at the output of the power supply depends on the number of turns of the secondary coil of the transformer. Typically, the household network voltage has mediocre stability, and the network does not produce the required 220 volts. The voltage value can fluctuate in the range from 200 to 235 V. This means that the voltage at the output of the transformer will also not be stable, and instead of the standard 12 V, the result will be from 10 to 14 volts.
Operation of the stabilizer circuit
Electrical devices that are not sensitive to small changes in supply voltage can use a conventional power supply. And more capricious devices will no longer be able to work without a stable power supply, and may simply burn out. Therefore, there is a need for an auxiliary output voltage equalization circuit.
Let's consider a circuit that equalizes DC voltage using a transistor and a zener diode, which plays the role of the main element, determines, and equalizes the voltage at the output of the power supply.
Let's move on to a specific consideration electrical diagram a conventional stabilizer to equalize DC voltage.
- There is a step-down transformer with a variable output voltage of 12V.
- This voltage is supplied to the input of the circuit, and more specifically, to the diode rectifier bridge, as well as a filter made on a capacitor.
- The rectifier, made on the basis of a diode bridge, converts alternating current into direct current, but an abrupt voltage value is obtained.
- Semiconductor diodes must operate at the highest current with a reserve of 25%. This current can be generated by the power supply.
- The reverse voltage should not drop less than the output voltage.
- The capacitor, which plays the role of a kind of filter, evens out these power fluctuations, converting the voltage waveform into an almost ideal graph shape. The capacitance of the capacitor should be in the range of 1-10 thousand microfarads. The voltage must also be higher than the input value.
We must not forget about the following effect: after the electrolytic capacitor (filter) and the diode rectifier bridge, the alternating voltage increases by about 18%. This means that the result is not 12 V at the output, but about 14.5 V.
Zener diode action
The next stage of work is the operation of the zener diode to stabilize the DC voltage in the stabilizer design. It is the main functional link. We must not forget that zener diodes can, within certain limits, maintain stability at a certain constant voltage when connected in reverse. If you apply voltage to the zener diode from zero to a stable value, it will increase.
When it reaches a stable level, it will remain constant, with a slight increase. This will increase the strength of the current passing through it.
In the considered circuit of a conventional stabilizer, whose output voltage should be 12 V, the zener diode is defined for a voltage value of 12.6 V, since 0.6 V will be a voltage loss at the emitter-base transition of the transistor. The output voltage on the device will be exactly 12 V. And since we set the zener diode to a value of 13 V, the output of the unit will be approximately 12.4 volts.
The zener diode requires a current limit to protect it from excessive heating. Judging by the diagram, this function is performed by resistance R1. It is connected in series with a zener diode VD2. Another capacitor, which acts as a filter, is connected in parallel with the zener diode. It must equalize the resulting voltage pulses. Although you can completely do without it.
The diagram shows transistor VT1 connected to a common collector. Such circuits are characterized by a significant current increase, but there is no voltage gain. It follows that the output of the transistor produces a constant voltage present at the input. Since the emitter junction absorbs 0.6 V, the output of the transistor is only 12.4 V.
In order for the transistor to open, a resistor is needed to create a bias. This function is performed by resistance R1. If you change its value, you can change the output current of the transistor, and, consequently, the output current of the stabilizer. As an experiment, you can connect a 47 kOhm variable resistor instead of resistor R1. By adjusting it, you can change the output current of the power supply.
At the end of the voltage stabilizer circuit, another small electrolytic capacitor C3 is connected, which equalizes the voltage pulses at the output of the stabilized device. Soldered to it parallel circuit resistor R2, which closes the emitter VT1 to the negative pole of the circuit.
Conclusion
This circuit is the simplest, includes the smallest number of elements, and creates a stable output voltage. This stabilizer is quite sufficient for the operation of many electrical devices. Such a transistor and a zener diode are designed for a maximum current of 8 A. This means that for such a current a cooling radiator is needed to remove heat from the semiconductors.
Zener diodes, transistors and stabilizers are most often used. They have reduced efficiency, so they are used only in low-power circuits. Most often they are used as main voltage sources in compensation circuits of voltage stabilizers. Such parametric stabilizers are bridge, multi-stage and single-stage. This is the most simple circuits stabilizers built on the basis of a zener diode and other semiconductor elements.
Content:Low-current circuits with loads of 20 mA or less use a low-efficiency device known as a parametric voltage regulator. The design of these devices includes transistors, stabilizers and zener diodes. They are used primarily in compensating stabilizing devices as reference voltage sources. Depending on technical characteristics, parametric stabilizers can be single-stage, multi-stage and bridge.
The zener diode, which is part of the design, resembles a reverse-connected diode. However, reverse voltage breakdown, characteristic of a zener diode, is the basis for its normal functioning. This property is widely used for various circuits in which it is necessary to create a voltage limitation of the input signal. Parametric stabilizers are high-speed devices; they protect sensitive areas of circuits from impulse noise. The use of these elements in modern circuits has become an indicator of their high quality, ensuring stable operation of the equipment in various modes.
Parametric stabilizer circuit
The basis of a parametric stabilizer is a zener diode switching circuit, which is also used in other types of stabilizers as a source of reference voltage.
The standard circuit consists of, which, in turn, includes a ballast resistor R1 and a zener diode VD. The load resistance RH is connected in parallel with the zener diode. This design stabilizes the output voltage with changing supply voltage Up and load current In.
The circuit operates in the following order. The voltage increasing at the input of the stabilizer causes an increase in the current passing through resistor R1 and the zener diode VD. The voltage of the zener diode remains unchanged due to its current-voltage characteristic. Accordingly, the voltage across the load resistance does not change. As a result, all changed voltage will be supplied to resistor R1. The operating principle of the circuit makes it possible to calculate all the necessary parameters.
Calculation of a parametric stabilizer
The quality of operation of a voltage stabilizer is assessed by its stabilization coefficient, determined by the formula: KstU= (ΔUin/Uin) / (ΔUout/Uout). Next, the calculation of the parametric voltage stabilizer on the zener diode is carried out in accordance with the resistance of the ballast resistor Ro and the type of zener diode used.
To calculate the zener diode, the following electrical parameters are used: Ist.max - maximum current of the zener diode in the working section of the current-voltage characteristic; Ist.min - minimum current of the zener diode in the working section of the current-voltage characteristic; Rd - differential resistance in the working section of the current-voltage characteristic. The calculation procedure can be considered using a specific example. The initial data will be as follows: Uout= 9 V; In = 10 mA; ΔIн= ± 2 mA; ΔUin= ± 10%Uin.
First of all, the zener diode of the D814B brand is selected in the directory, the parameters of which are: Ust = 9 V; Ist.max = 36 mA; Ist.min= 3 mA; Rd= 10 Ohm. After this, the input voltage is calculated using the formula: Uin=nstUout, in which nst is the transfer coefficient of the stabilizer. The operation of the stabilizing device will be most effective when nst is 1.4-2.0. If nst = 1.6, then Uin = 1.6 x 9 = 14.4V.
The next step is to calculate the resistance of the ballast resistor (Ro). For this, the following formula is used: Ro= (Uin-Uout) / (Ist+In). The current value Ist is selected according to the principle: Ist ≥ In. In the case of a simultaneous change in Uin by the value ΔUin and In by the value ΔIn, the zener diode current should not exceed the values of Ist.max and Ist.min. In this regard, Ist is taken as the average permissible value in this range and is 0.015A.
Thus, the resistance of the ballast resistor will be equal to: Ro = (14.4 - 9) / (0.015 + 0.01) = 216 Ohms. The closest standard resistance will be 220 ohms. In order to select the desired type of resistor, you need to calculate the power dissipated on its body. Using the formula P = I2Ro, we obtain the value P = (25·10-3)2x220 = 0.138 W. That is, the standard power dissipation of the resistor will be 0.25W. Therefore, an MLT-0.25-220 Ohm ± 10% resistor is best suited for the circuit.
After completing all the calculations, you need to check whether the operating mode of the zener diode is selected correctly in the general circuit of the parametric stabilizer. First, its minimum current is determined: Ist.min = (Uin-ΔUin-Uout) / Ro - (In+ΔIn), with real parameters the value Ist.min = (14.4 - 1.44 - 9) x 103/ 220 is obtained - (10 + 2) = 6 mA. The same actions are performed to determine the maximum current: Ist.max = (Uin+ΔUin-Uout) / Ro - (In-ΔIn). In accordance with the initial data, the maximum current will be: Ist.max = (14.4 + 1.44 - 9) · 103/ 220 - (10 - 2) = 23 mA. If the obtained values of the minimum and maximum current are outside the permissible limits, then in this case it is necessary to change Ist or Ro. In some cases, the zener diode needs to be replaced.
Parametric voltage stabilizer on zener diode
For any radio-electronic circuit, a power source is required. They can be direct and alternating current, stabilized and unstabilized, and linear, resonant and quasi-resonant. This variety makes it possible to select power supplies for different circuits.
In the simplest electronic circuits, where high stability of the supply voltage or high output power is not required, linear voltage sources are most often used, characterized by reliability, simplicity and low cost. Their components are parametric voltage and current stabilizers, the design of which includes an element having a nonlinear current-voltage characteristic. A typical representative of such elements is a zener diode.
This element belongs to a special group of diodes operating in the reverse branch mode of the current-voltage characteristic in the breakdown region. When the diode is turned on in the forward direction from the anode to the cathode (from plus to minus) with a voltage Uthr, electric current begins to flow freely through it. If the reverse direction from minus to plus is turned on, then only a current Irev, amounting to only a few μA, passes through the diode. An increase in the reverse voltage on the diode to a certain level will lead to its electrical breakdown. If the current is sufficient, the diode fails due to thermal breakdown. Operation of a diode in the breakdown region is possible if the current passing through the diode is limited. In various diodes, the breakdown voltage can range from 50 to 200V.
Unlike diodes, the current-voltage characteristic of a zener diode has a higher linearity under conditions of constant breakdown voltage. Thus, to stabilize the voltage using this device, the reverse branch of the current-voltage characteristic is used. In the section of the direct branch, the operation of the zener diode occurs in exactly the same way as with a conventional diode.
In accordance with its current-voltage characteristic, the zener diode has the following parameters:
- Stabilization voltage - Ust. Depends on the voltage on the zener diode during the flow of current Ist. The stabilization range of modern zener diodes ranges from 0.7 to 200 volts.
- Maximum allowed D.C. stabilization - Ist.max. It is limited by the maximum permissible power dissipation Pmax, which, in turn, is closely related to the ambient temperature.
- Minimum stabilization current - Ist.min. Depends on the minimum value of current passing through the zener diode. At this current, the device’s functionality should be fully preserved. The current-voltage characteristic of the zener diode between the parameters Ist.max and Ist.min has the most linear configuration, and the change in the stabilization voltage is very insignificant.
- Differential resistance of the zener diode - rst. This value is defined as the ratio of the increase in stabilization voltage on the device to the small increase in stabilization current that caused this voltage (ΔUCT/ ΔiCT).
Parametric transistor stabilizer
The operation of a parametric stabilizer using transistors is almost no different from a similar device using a zener diode. In each circuit, the voltage at the outputs remains stable, since their current-voltage characteristics affect areas with a voltage drop that is weakly dependent on current. That is, as in other parametric stabilizers, stable current and voltage indicators are achieved due to the internal properties of the components.
The voltage drop across the load will be the same as the difference between the voltage drop of the zener diode and r-n transition transistor. The voltage drop in both cases weakly depends on the current, from which we can conclude that the output voltage is also constant.
Normal operation of the stabilizer is characterized by the presence of voltage in the range from Ust.max to Ust.min. To do this, it is necessary that the current passing through the zener diode be in the range from Ist.max to Ist.min. Thus, the maximum current flow through the zener diode will occur under conditions of minimum transistor base current and maximum input voltage. Therefore, a transistor stabilizer has significant advantages over a conventional device, since the value of the output current can vary over a wide range.
