This article discusses the key specifications and troubleshooting techniques for Zener diode, including breakdown voltage, tolerance ratings, current ratings, power dissipation, and impedance.
Zener Diode Specifications
When selecting a Zener diode for a circuit, key electrical specifications must be considered to ensure optimal performance. These specifications define the diode’s operating limits, stability, and efficiency in voltage regulation applications. Before beginning to explain the various specifications, it is important to study the I-V characteristics of the Zener Diode. Figure 1 shows the characteristics for an 1N1775 Diode.
Figure 1. $I_{Z}-V_{Z}$ Characteristics of an 1N1775 Zener Diode.
Zener Breakdown Voltage
Zener diodes are available in a wide range of breakdown voltages. The Zener breakdown voltage ($V_{BR}$), or simply Zener voltage, is usually identified on datasheets by the letters $V_{Z}$. Typical $V_{Z}$ values range from 1.4 to 200 V. Some low-voltage values for Zener diodes 1N746A through 1N759A are 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, and 12V. A number of other $V_{Z}$ values are also available. These values vary, to some extent, among different manufacturers.
Zener Diode Tolerance Ratings
The accuracy of the Zener voltage of a diode is an important selection consideration and is largely responsible for the cost of a Zener diode. The accuracy or tolerance rating of a Zener diode is a range of values within which the actual $V_{Z}$ will appear. If the $V_{Z}$ region of Figure 1 is examined carefully, it shows that Zener breakdown does not occur at a precise location. The curve is actually rounded in this area. This is called the $V_{Z}$ or the knee of the curve. Ideally, the knee should have a sharp edge. When it is rounded, $V_{Z}$ occurs more gradually. Zener diodes with a low tolerance rating generally have a sharp $V_{Z}$ knee.
Some of the more popular tolerance ratings are 20%, 10%, 5%, and 1%. Low-cost Zener diodes have the highest tolerance percentage, while 1% or less tolerance devices are generally the most expensive. A Zener diode rated at 9.1 V with a ±10% tolerance will have a $V_{Z}$ value that is ±0.91 V of 9.1V. Therefore, its actual Zener breakdown value could be somewhere between 8.19 and 10.01 V. As a rule, diodes with a 10% tolerance rating will work for most circuit applications. Zener diodes with a 1% tolerance are used only in special circuit applications that require critical voltage values.
Example 1
Calculate the Zener breakdown ($V_{Z}$) range for the 1N754A Zener diode, which has $V_{Z}$ of 6.8V and Tolerance Rating of 5%.
Solution
The $V_{Z}$ of a 1N754A Zener diode is 6.8 V.
The tolerance rating is 5%.
$V_{Z}$ range = $6.8 \times 0.05=0.34$
Therefore, $V_{Z}$ Range = 6.46 to 7.14.
Zener voltages are generally identified at a minimum Zener current value. A certain amount of $I_{Z}$ is needed to enter the knee of the IZ curve. In this case, approximately 3mA of $I_{Z}$ is needed to produce conduction. After this value has been reached, $V_{Z}$ remains fairly constant over a wide range of $I_{Z}$. The normal operating range of $I_{Z}$ in this region is determined by the power dissipation rating.
Zener Diode Current Ratings
Once Zener breakdown voltage is reached, a very large change in Zener current is accompanied by a small change in Zener voltage, as shown in Figure 1. Zener current, however, has a safe operating limit or maximum value that must be observed. This value is determined by the wattage, or power dissipation, rating of the diode. This section takes a closer look at the safe operating range of a Zener diode as it covers power dissipation and Zener impedance.
Power Dissipation Rating
Manufacturers rate Zener diodes according to their Zener voltage ($V_{Z}$) value and maximum power dissipation ($P_{D}$) at $25^{o}C$. Power dissipation refers to the ability of the Zener diode’s junction to give off heat. A principal cause of solid-state device destruction is heat. When power is applied to a device, the device heats up. If too much power is applied, the device will overheat and be destroyed. The power dissipation rating of a Zener diode is an indication of how much heat the device can give off or dissipate. It is rated in watts at a certain temperature.
The temperature value is an indication of device operation at ambient temperature. Ambient refers to something that goes around or surrounds an object. In this case, ambient refers to the air surrounding the operating device. A temperature of $25^{o}C$ is considered to be the temperature of an inside room. Some manufacturers indicate the power dissipation rating at temperatures such as $50^{o}C$ and $75^{o}C$. These are generally considered to be working temperatures. The actual power that can be dissipated by a Zener diode decreases as the temperature increases and increases when the temperature decreases. In other words, the power dissipation ($P_{D}$) rating of a Zener diode has a negative temperature coefficient.
The $P_{D}$ rating of a Zener diode can be altered, to some extent, by its lead length and its mounting in a circuit. The $P_{D}$ value can be derated when these factors are considered. Derating permits a device to handle more power than its normal $P_{D}$ value indicates. This is achieved by reducing lead length or mounting the device on a piece of metal, which serves as a heat sink. Manufacturers usually provide data about device derating in a chart that correlates lead length and the area of the heat sink. Many Zener diodes are housed in packages that permit them to be attached to a heat sink. In some installations, the metal chassis that houses a circuit may be used as the heat sink. Derating is an important consideration when a Zener diode operates close to its maximum power rating.
Maximum Zener Current
In the process of dissipating heat, a certain amount of power is used or consumed by the Zener diode. This characteristic gives an indication of the maximum Zener current ($I_{Z}$) that a diode can safely conduct. A 1-W, 15$V_{Z}$ Zener diode, for example, can conduct an $I_{Z}$ of 0.066 A or 66 mA. This is determined by the basic power equation:
$$P = I \times V$$
Substitute $P_{D}$ for P, $I_{Zmax}$ for I, and $V_{Z}$ for V (using the typical or nominal voltage value of the Zener from the datasheet) in the formula. The modified formula then becomes
$$I_{Zmax}=\frac{P_{D}}{V_{Z}}$$
Therefore,
$$I_{Zmax}=\frac{1}{15}=\textrm{0.066 A or 66 mA}$$
This Zener diode has a maximum Zener current rating, $I_{Zmax}$ of 66 mA. The minimum Zener current, $I_{Zmin}$, is also indicated by the curve. This value is approximately 5mA. The region between these two extreme conditions is labeled as the safe operating range. The Zener can operate anywhere between these two points without being damaged. If the current exceeds the maximum $I_{Z}$ value, additional heat will develop. This heat may become so intense that it can permanently damage the junction. As a rule, the area beyond the maximum $I_{Z}$ point should be avoided in normal diode applications.
The safe operating range of a Zener diode is from the minimum Zener current ($I_{Zmin}$) to the maximum current rating, $I_{Zmax}$.
Safe operating range = $I_{Zmax} – I_{Zmin}$
Example 2
Calculate the safe operating range for a 1N754A Zener diode. Refer to the data sheets at the end of the chapter for PD, $V_{Z}$, and IZ values.
Solution
1. Determine the maximum current rating.
$$I_{Zmax}=\frac{P_{D}}{V_{Z}}$$
$$I_{Zmax}=\frac{500mW}{6.8V}$$
$$I_{Zmax}=\frac{0.5W}{6.8V}$$
$$I_{Zmax}=73.5mA.$$
2. Then, determine the safe operating range. The safe operating range is from the minimum Zener current (IZmin), at which the typical Zener voltage is listed in the datasheet, to the maximum current rating, IZmax. safe operating range = 73.5 mA − 20 mA.
Zener Impedance
Another matter of importance in Zener diode selection is Zener impedance (ZZ). This characteristic deals with the slope of the IZ – VZ curve after conduction has been achieved. A Zener diode actually shows a slight increase in the value of $V_{Z}$ when $I_{Z}$ increases. This increase is very noticeable because it means that the reverse characteristic is not entirely vertical. Zener impedance is responsible for this characteristic and is determined by the expression:
$$Z_{Z} =\frac{\Delta V_{Z}}{\Delta I_{Z}}$$
where the Greek letter delta ($\Delta$) denotes a change in value.
The $\Delta I_{Z}$ value is derived from the safe or normal operating range of the Zener diode. Some manufacturers select the range of values from a prescribed test area near the middle of the reverse current curve. The range of $V_{Z}$ can be used to determine corresponding values of $I_{Z}$. Note the location of the $\Delta V_{Z}$ and $\Delta I_{Z}$ values on the chart. The $Z_{Z}$ for this range of operation is, therefore,
$$Z_{Z} =\frac{\Delta V_{Z}}{\Delta I_{Z}} = \frac{17V-15 V}{66mA-5mA} = \frac{2V}{61mA} = 32.79\Omega$$
The reverse knee impedance (ZZK) of a Zener diode can be determined by the same procedure as determining Zener impedance. It is represented by the following formula:
$$Z_{ZK} =\frac{\Delta V_{ZK}}{\Delta I_{ZK}}$$
Zener knee impedance shows the impedance of the device near the breakdown point and is a good indication of the slope or sharpness of the knee area of the curve. Note that the $\Delta V_{ZK}$ value is the difference between the voltage at the start of the Zener breakdown point and the beginning of the linear portion of the curve. The values of $I_{ZK}$ are the corresponding current values on the curve.
$$Z_{ZK} =\frac{\Delta V_{ZK}}{\Delta I_{ZK}}=\frac{15V-14V}{5mA – 1mA}=\frac{1V}{4mA}=\frac{1V}{0.004A}= 250 \Omega$$
In practice, a Zener diode with a sharp knee will produce a lower value of Zener impedance than a rounded knee. Ideally, the Zener knee should be very sharp. Manufacturers usually include knee impedance and Zener impedance in their specifications of a device.
Zener Diode Troubleshooting
A Zener diode cannot be tested in the same way as a P–N junction diode. The Zener diode is designed to break down and conduct in the reverse direction when the reverse voltage equals $V_{Z}$. One way to test a Zener diode is to measure the voltage across its terminals while it is functioning in a circuit. If the voltage across the Zener is within tolerance, it is good.
A Zener diode is designed to function as a voltage regulator when it is reverse biased. In this state, it will have a minimum amount of current flowing through it. To test a Zener diode, you can measure the voltage across its terminals while it is functioning. As reverse voltage applied across the Zener diode is varied, the current flowing through it should change. If the current does not change, even when the rated $V_{Z}$ voltage is applied, the Zener diode terminals may be open. On the other hand, if the Zener is shorted, no voltage will be developed across the diode, and a significant amount of current will flow, even under reverse-bias conditions.
Review Questions
- Zener diodes are rated according to their _____ and _____.
- A Zener diode rated at 1 W, 12 $V_{Z}$ has a maximum current, or $I_{Z}$, capability of _____A or _____mA.
- The space between the minimum and maximum $I_{Z}$ values of a Zener diode is called the _____ operating range.
- When the maximum $I_{Z}$ value of a Zener diode is exceeded, damage will likely occur due to excessive _____.
- Power dissipation of a Zener diode refers to the rated _____ at a particular _____.
- When the junction temperature of a Zener diode increases, the $P_{D}$ rating decreases __ ___.
- Zener _____ is a characteristic based on the value of $\frac{\Delta V_{Z}}{\Delta I_{Z}}$.
- A low value of Zener impedance is generally a good indication of the sharpness of the Zener _____.
Answers
- Zener voltage ($V_{Z}$) and power dissipation ($P_{D}$)
- 0.083A or 83mA
- linear (or regulation)
- power dissipation (or heat)
- power ($P_{D}$), temperature
- linearly (or proportionally)
- impedance ($Z_{Z}$)
- knee
Zener Diode Key Takeaways
Understanding Zener diodes—their specifications, ratings, and behavior—is essential for designing reliable and efficient electronic circuits. These diodes play a critical role in voltage regulation, overvoltage protection, and waveform clipping, making them invaluable in power supplies, communication systems, and embedded electronics.
