Stabilitrone - this is what is it and what is it for?

The Zener diode is a semiconductor diode with unique properties. If a conventional semiconductor is an insulator when it is turned on again, it performs this function until a certain increase in the applied voltage is made, after which an avalanche-like reversible breakdown occurs. With further increase of the reverse current flowing through the zener diode, the voltage continues to remain constant due to a proportional decrease in the resistance. In this way, it is possible to achieve a stabilization regime.

In the closed state, a small leakage current passes through the zener diode first. The element behaves like a resistor whose magnitude of resistance is great. In the breakdown, the resistance of the zener diode becomes insignificant. If we continue to increase the voltage at the input, the element begins to warm up and if the current exceeds the permissible value, an irreversible thermal breakdown occurs. If the matter is not brought to him, when the voltage changes from zero to the upper limit of the working area, the properties of the zener diode remain.

When the zener diode is turned on directly, the characteristics do not differ from the diode. When the plus is connected to the p-region, and the minus to the n-region, the resistance of the transition is small and the current flows freely through it. It increases with increasing input voltage.

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

The anode is fed negative, and the cathode - plus. Outside the stabilization (below point 2), overheating occurs and the probability of an element failing is increased.

Characteristics

The zener diode parameters are as follows:

• U _ст - voltage of stabilization at rated current I _ст ;
• I _{ст min} - the minimum current of the beginning of electrical breakdown;
• I _{ст max} - the maximum permissible current;
• ТКН - temperature coefficient.

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

The maximum permissible current is associated with the parameter, often indicated in the tables - the power dissipation:

P _max = I _{ст max} ∙ U _st .

The dependence of the zener diode on temperature can be either with positive TCR or negative. When the elements are connected in series with different coefficients, precision zener diodes are created, independent of heating or cooling.

Schemes of inclusion

Typical scheme of a simple stabilizer, consists of a ballast resistance R _b and a zener diode, shunting the load.

In some cases, there is a violation of stabilization.

1. Supply to the stabilizer of a large voltage from the power supply in the presence of a filter capacitor at the output. Current spikes when charging it can cause a zener diode to malfunction or a resistor R _{b is} destroyed.
2. Load disconnection. When the maximum voltage is applied to the input, the current of the zener diode can exceed the permissible value, which will lead to its heating and destruction. It is important to observe the passport area of safe operation.
3. Resistance R _{b is} chosen small, so that at the lowest possible value of the supply voltage and the maximum permissible current on the load, the zener diode is in the working control 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 ) (Ist + _IH ).

The current of the Zener diode I is selected 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 wide spread in the stabilization voltage. To obtain the exact value of UH, the zener diodes are selected from the same batch. There are types with a narrower range of parameters. At high dissipation power, the elements are mounted on radiators.

For the calculation of the zener diode parameters, the initial data are needed, for example, the following:

• U _pit = 12-15 V - input voltage;
• U _ст = 9 V - the stabilized voltage;
• R _n = 50-100 mA is the 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 of 100 mA. According to Ohm's law, one can find the total load of the chain:

R _Σ = 12 V / 0.1 A = 120 Ω.

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

R _eq = 9 V / 0.1 A = 90 Ω.

Now you can determine the ballast resistance:

R _b = 120 ohms - 90 ohms = 30 ohms.

It is selected from the standard series, where the value coincides with the calculated value.

The maximum current through the zener diode is determined taking into account the load cut-off, so that it does not fail in the event that any wire disappears. 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 will be P = 0.2 ∙ 9 = 1.8 W.

According to the parameters obtained, a suitable Zener diode D815B is selected.

Symmetrical zener diode

A symmetrical diode thyristor is a switching device that conducts alternating current. A feature of his work is a voltage drop of up to several volts when switched 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.

Analog of a zener diode

When it is not possible to select a suitable element, use an analog zener diode on transistors. Their advantage is the possibility of voltage regulation. For this, DC amplifiers with several stages can be used.

At the input, a voltage divider with trimmer R1 is installed. If the input voltage increases, it also increases on the basis of the transistor VT1. This increases the current through the transistor VT2, which compensates for the increase in voltage, thereby maintaining its stable output.

Marking of zener diodes

Available are glass zener diodes and zener diodes in plastic cases. In the first case, 2 digits are placed on them, between which the letter V is located. The inscription 9V1 means that U _ст = 9,1 V.

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

The dark ring on the body designates the cathode, to which the plus is connected.

Conclusion

The zener diode is a diode with special properties. The advantage of zener diodes is a high level of voltage stabilization for a wide range of operating current variation, as well as simple connection schemes. To stabilize the low voltage devices are turned in the forward direction, and they start to work like ordinary diodes.