Zener diode - what is it and what is it for?

2024 Author: Howard Calhoun | [email protected]. Last modified: 2024-01-02 13:48

A zener diode is a semiconductor diode with unique properties. If an ordinary semiconductor is an insulator when switched back on, then it performs this function until a certain increase in 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 stabilization mode.

In the closed state, a small leakage current first passes through the zener diode. The element behaves like a resistor, the resistance value of which is large. During breakdown, the resistance of the zener diode becomes negligible. If we continue to increase the voltage at the input, the element begins to heat up and when the current exceeds the permissible value, an irreversiblethermal breakdown. 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 a zener diode is directly turned on, the characteristics are the same as a diode. When the plus is connected to the p-region, and the minus is connected to the n-region, the transition resistance is small and the current flows freely through it. It rises as the input voltage increases.

A zener diode is a special diode connected mostly in the opposite direction. The element is first in the closed state. In the event of an electrical breakdown, the voltage zener diode maintains it constant over a wide current range.

A minus is applied to the anode, and a plus to the cathode. Beyond stabilization (below point 2), overheating occurs and the probability of element failure increases.

Features

Zener parameters are as follows:

• U_st - stabilization voltage at rated current I_st;
• I_{st min - minimum electrical breakdown start current;}
• I_{st max - maximum allowable current;}
• TKN - temperature coefficient.

Unlike a conventional diode, a zener diode is a semiconductor device in which the electrical and thermal breakdown regions are quite far apart on the current-voltage characteristic.

Associated with the maximum allowable current is a parameter often specified intables - power dissipation:

P_max=I_{st max}∙ U_st.

The temperature dependence of the zener diode can be either positive or negative. By connecting elements in series with coefficients of different signs, precision zener diodes are created that do not depend on heating or cooling.

Inclusion schemes

Typical circuit of a simple stabilizer, consists of a ballast resistance R_{b and a zener diode that shunts the load.}

In some cases, there is a violation of stabilization.

1. Submission to the stabilizer of a large voltage from the power source in the presence of a filtering capacitor at the output. Current surges when charging it can cause failure of the zener diode or destruction of the resistor R_b.
2. Load shutdown. When the maximum voltage is applied to the input, the current of the zener diode may exceed the allowable one, which will lead to its heating and destruction. Here it is important to observe the safe work passport area.
3. Resistance R_{b is selected small so that at the minimum possible supply voltage and the maximum allowable current at the load, the zener diode is in the working regulation zone.}

Thyristor protection circuits or fuses are used to protect the stabilizer.

Resistor R_{b is calculated by the formula:}

R_b=(U_pit - U_nom)(I st _{+ I}n_).

Currentzener diode I_st is selected between the allowable maximum and minimum values, depending on the input voltage U_pit and load current I_n.

Zener Selection

Elements have a large spread in stabilization voltage. To get the exact value of U_{n, zener diodes are selected from the same batch. There are types with a narrower range of parameters. With high power dissipation, the elements are installed on radiators.}

To calculate the parameters of the zener diode, initial data are required, for example, these:

• U_{pit=12-15 V - input voltage;}
• U_{st=9 V - stabilized voltage;}
• R_{n=50-100 mA - load.}

The parameters are typical for devices with low power consumption.

For a minimum input voltage of 12 V, the current on the load is selected to the maximum - 100 mA. According to Ohm's law, you can find the total load of the circuit:

R_{∑=12 V / 0.1 A=120 Ohm.}

At the zener diode, the voltage drop is 9 V. For a current of 0.1 A, the equivalent load will be:

R_{eq=9 V / 0.1 A=90 Ohm.}

Now you can determine the resistance of the ballast:

R_{b=120 Ohm - 90 Ohm=30 Ohm.}

It is selected from the standard row, where the value is the same as 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 B.}

Then the current through the resistor is determined:

I_{R=6/30=0, 2 A.}

Since the zener diode is connected in series with it, I_c=I_{R=0.2 A.}

Power dissipation will be P=0.2∙9=1.8 W.

According to the parameters obtained, a suitable Zener diode D815V is selected.

Symmetrical zener diode

Symmetrical diode thyristor is a switching device that conducts alternating current. A feature of its work is a voltage drop of up to several volts when turned on in the range of 30-50 V. It can be replaced by two counter-connected conventional zener diodes. Devices are used as switching elements.

Analogue of a zener diode

When it is not possible to find a suitable element, they use an analogue of a zener diode on transistors. Their advantage is the possibility of voltage regulation. Multi-stage DC amplifiers can be used for this.

A voltage divider with a trimming resistor R1 is installed at the input. If the input voltage increases, on the basis of transistor VT1 it also increases. At the same time, the current through the transistor VT2 increases, which compensates for the increase in voltage, thereby maintaining it stable at the output.

Marking of zener diodes

Glass zener diodes and zener diodes in plastic cases are produced. In the first case, 2 numbers are applied to them, between which the letter V is located. The inscription 9V1 means thatU_{st=9, 1 V.}

On the plastic case, the inscriptions are deciphered using a datasheet, where you can also find out other parameters.

The dark ring on the case 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 operating current changes, as well as simple connection schemes. To stabilize a small voltage, the devices are switched on in the forward direction, and they begin to work like ordinary diodes.