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Select Zener Diode, Working Principle and Test Method

Jul 30 2024

Zener diodes, when used as a voltage regulator, are play a role in regulating voltage in the circuit. Different part number of Zener diodes have different voltage regulator values. In the normal circumstances, the Zener diode works in reverse breakdown, and its main function is to regulate the voltage. In the electronics circuit, the cathode of the Zener diode is usually connected in series with a current limiting resistor, which is turned on when the reverse voltage reaches the breakdown voltage of the Zener diode. If the voltage regulation value of a Zener diode is 2.5V, it will regulate the input voltage higher than 2.5V in the circuit and output it to 2.5V, and when the input voltage is lower than 2.5V, it will not work, it means the circuit is in an open circuit state.

Zener Voltage Regulator Working Principle

When the Zener diode is broken down, the voltage across it will remain basically unchanged. When the power supply voltage fluctuates, the Zener diode can keep the voltage across the load unchanged to achieve voltage regulation. What is a breakdown? When the reverse voltage approaches the critical value, the reverse current suddenly increases, which is called breakdown.

zener diode working principle

Zener Voltage Regulator

From the volt-ampere characteristic curve, it can be seen that the sudden increase in the current on the left side is the breakdown current. The working area of the Zener diode is generally on the left side of the curve, using the reverse breakdown of the PN junction.

Parameters to Consider When Select a Zener Diode

The Zener voltage Vz is the regulate voltage value generated at both ends when the Zener diode passes the rated current. It should be noted that even for the same model, the value may be different due to differences in the manufacturing process. In addition, when working, it will be slightly different with temperature changes and different working currents.

The rated current Iz is the current value passing through the Zener diode when the rated voltage is added. For better regulation performance, the current through the Zener diode needs to be higher than this value without exceeding the rated power loss.

The dynamic resistances Zzt and Zzk are the ratio of the voltage and current across the Zener diode. Before the breakdown, the reverse dynamic resistance is large, and after the breakdown the dynamic resistance is small. Typically, the smaller the value, the better the performance. For example, 1SMB5923BT3GZzt=3.5ohms, Izt=45.7mA ; Zzk@Izk, when Izk is 0.5mA, Zzk is 400ohms.

Rated power Pz, that is, when the reverse current passes through the Zener diode, the maximum power consumption value allowed by itself. Pz=3W of 1SMB5923BT3G

The temperature coefficient α refers to the relative change in the voltage across the diode caused by a 1°C change in temperature.

The reverse leakage current Ir@Vr is the leakage current generated by the Zener diode under the specified reverse voltage. For example, when Vr=6.5V of 1SMB5923BT3G, Ir=5uA. That is, the current consumed when the regulator is not in a regulated state, the smaller the value, the better.

The Way to Check if the Zener Diode is Working Properly

To test the Zener voltage regulator, you can directly use the Rx100 or Rx1k gear of the multi-meter to measure its forward resistance and reverse resistance for judgment. First of all, how to determine the positive and negative poles, when the resistance value measured with the Rx100 gear is small, the black test pen is connected to the positive pole. The greater the difference between the measured forward and reverse resistance values, the better the unidirectional conductivity of the diode. If the following conditions occur, the diode cannot be used.

- The forward and reverse resistance values are very close, indicating that the diode has lost its unidirectional conduction effect

- When the forward and reverse resistances are very small or almost 0, it means that it has been broken down

- The forward and reverse resistances are large, indicating that the internal circuit has been broken


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