A zener diode is a semiconductor diode with unique properties. If a conventional semiconductor, when turned back on, is an insulator, then it performs this function until a certain increase in the value of the applied voltage, after which an avalanche-like reversible breakdown occurs. With a further increase in the reverse current flowing through the zener diode, the voltage continues to remain constant due to a proportional decrease in resistance. In this way, it is possible to achieve a stabilization mode.
In the closed state, a small leakage current first passes through the zener diode. The element behaves like a resistor, the magnitude of the resistance of which is large. During breakdown, the zener diode resistance becomes insignificant. If you continue to increase the voltage at the input, the element starts to heat up and when the current exceeds the permissible value, an irreversible thermal breakdown occurs. If the matter is not brought to it, when the voltage changes from zero to the upper limit of the working area, the properties of the zener diode are preserved.
When the zener diode is directly turned on, the characteristics do not differ from the diode. When a plus is connected to the p-region, and a minus to the n-region, the transition resistance is small and the current flows through it freely. It increases with increasing input voltage.
Zener diode is a special diode, connected mostly in the opposite direction. The element is initially in a closed state. When an electrical breakdown occurs, the voltage stabilizer keeps it constant over a large current range.
A minus is supplied to the anode, and a plus to the cathode. Outside of stabilization (below point 2), overheating occurs and the probability of an element failure occurs.
Specifications
The parameters of the zener diodes are as follows:
- U article - stabilization voltage at rated current I article ;
- I article min - the minimum current onset of electrical breakdown;
- I st max - the maximum permissible current;
- TKN - temperature coefficient.
Unlike a conventional diode, a zener diode is a semiconductor device in which, on the current-voltage characteristic, the areas of electrical and thermal breakdown are quite far from each other.
Associated with the maximum allowable current is a parameter often indicated in the tables - power dissipation:
P max = I st max ∙ U st
The temperature dependence of the zener diode can be either positive TKN or negative. When connecting elements with coefficients of different signs in series, precision zener diodes are created that are independent of heating or cooling.
Switching schemes
A typical circuit of a simple stabilizer consists of a ballast resistance R b and a zener diode shunting the load.
In some cases, stabilization is disturbed.
- Supply to the stabilizer a large voltage from the power source in the presence of a filter capacitor at the output. Inrush current when charging it can cause the failure of the zener diode or the destruction of the resistor R b .
- Load shedding. When a maximum voltage is applied to the input, the current of the zener diode can exceed the permissible one, which will lead to its heating and destruction. It is important to observe the passport area of safe operation.
- The resistance R b is selected small so that at the minimum possible value of the supply voltage and the maximum allowable current at the load, the zener diode is in the working zone of regulation.
Thyristor protection circuits or fuses are used to protect the stabilizer .
The resistor R b is calculated by the formula:
R b = (U pit - U nom ) (I st + I n ).
The current of the zener diode I st is chosen between the permissible maximum and minimum values, depending on the voltage at the input U pit and the load current I n .
Zener diode selection
Elements have a large variation in voltage stabilization. To obtain the exact value of U n , the zener diodes are selected from one batch. There are types with a narrower range of parameters. At high dissipation power, the elements are mounted on radiators.
To calculate the zener diode parameters, initial data are necessary, for example, such:
- U pit = 12-15 V - input voltage;
- U article = 9 V - stabilized voltage;
- R n = 50-100 mA - load.
Parameters are typical for devices with low power consumption.
For a minimum input voltage of 12 V, the current at the load is selected to a maximum of 100 mA. According to Ohm's law, you can find the total load of the circuit:
R ∑ = 12 V / 0.1 A = 120 Ohms.
At the zener diode, the voltage drop is 9 V. For a current of 0.1 A, the equivalent load will be:
R equiv = 9 V / 0.1 A = 90 Ohms.
Now you can determine the resistance of the ballast:
R b = 120 Ohms - 90 Ohms = 30 Ohms.
It is selected from the standard series, where the value coincides with the calculated one.
The maximum current through the zener diode is determined taking into account the disconnection of the load, so that it does not fail if any wire is unsoldered. The voltage drop across the resistor will be:
U R = 15 - 9 = 6 V.
Then the current through the resistor is determined:
I R = 6/30 = 0.2 A.
Since the zener diode is connected to it in series, I c = I R = 0.2 A.
The power dissipation is P = 0.2 ∙ 9 = 1.8 watts.
Based on the parameters obtained, a suitable Zener diode D815V is selected.
Symmetrical Zener Diode
A symmetric diode thyristor is a switching device that conducts alternating current. A feature of its work is the voltage drop to several volts when turned on in the range of 30-50 V. It can be replaced by two counter-current conventional zener diodes. Devices are used as switching elements.
Zener Diode Analog
When it is not possible to select a suitable element, use an analog of a zener diode on transistors. Their advantage is the ability to regulate voltage. For this, DC amplifiers with several stages can be used.
At the input, a voltage divider with a trimming resistor R1 is installed. If the input voltage increases, on the basis of the transistor VT1, it also increases. In this case, the current increases through the transistor VT2, which compensates for the increase in voltage, thereby maintaining it stable at the output.
Zener marking
Glass zener diodes and zener diodes in plastic cases are available. In the first case, 2 digits are applied to them, between which is the letter V. The inscription 9V1 means that U st = 9.1 V.
On the plastic case, the inscriptions are decrypted using a datasheet, where you can also find out other parameters.
The dark ring on the casing indicates the cathode to which the plus is connected.
Conclusion
A zener diode is a diode with special properties. The advantage of zener diodes is a high level of voltage stabilization with a wide range of changes in the operating current, as well as simple wiring diagrams. To stabilize a small voltage, the devices are turned on in the forward direction, and they begin to work like ordinary diodes.