How Does Zener Diode Do Overvoltage Protection in Circuit?Follow article
Over-voltage protection is necessary to prevent damage as a result of electrical transients. It is a power supply feature which shuts down the supply or clamps the output when the voltage exceeds a preset level. Most power supplies use an over-voltage protection circuit to prevent damage to the electronic components. They offer some form of overvoltage-protect (OVP) circuit to detect and then quickly pull down the overvoltage. Here introduces the Zener diode protection, which is the most common way.
1. Over Voltage Background
Every circuit design operates at various voltage levels, with 3.3V, 5V, and 12V being the most common voltage levels for a digital circuit. But every design is special, and having more than one operating voltage is also normal for a circuit. For example, a standard computer SMPS system will work at six different levels of voltage, namely ±3.3V, ±5V, and ±12V. In these cases, if a low-power device is operated by a high voltage, the component will be permanently impaired if various voltage levels are used to power different types of components. Therefore, to avoid over-voltage harm, the designer should always concentrate on implementing an over-voltage security circuit in his designs.
There will be three different voltage ratings for any part or circuit, namely the minimum operating voltage, the suggested or normal operating voltage and the maximum operating voltage. For any circuits or parts, any value over the maximum operating voltage can be fatal. Using a Zener diode over-voltage protection circuit is a very common and cost-effective solution.
2. Zener Diode Input Protection Basics
In order to protect the circuit from overvoltage conditions, Zener diodes are often the first option. A Zener diode follows the same diode theory, which blocks the current flow in the reverse direction. However, there is a drawback that the Zener diode blocks the flow of current in the reverse direction only for a restricted voltage defined by the voltage rating of the Zener diode. A 5.1V Zener diode blocks current to flow in the opposite direction up to 5.1V If the voltage is greater than 5.1V through the Zener diode, it allows the current to pass through it. This Zener diode function makes it an excellent over-voltage security component.
3. Simple Overvoltage Protection Circuit Using Zener Diode
Consider a circuit, where need microcontroller over-voltage protection. Anything that has a maximum rating of 5V across the microcontroller IO pins. So voltage more than 5V will damage the microcontroller.
Figure 1. Overvoltage Protection for Microcontroller
The diode used in the circuit above is a Zener diode of 5.1V. During an over-voltage case, it will work perfectly. It can transfer the current and regulate the voltage up to 5.1V if the voltage is more than 5.1V. In practice, however, it will behave as a regular diode and block less than 5.1V
The image below is a simulation of the spice circuit of Zener diode protection. For the full simulation description, you can make it based on your need.
Figure 2. Simulating Overvoltage Protection Circuit
There is an input voltage in the schematic above, which is a dc supply. The R1 and D1 are two components that protect the output from protection from over-voltage. The D1, 1N4099, in this case, is a Zener diode. When the V1 reaches 6.8V, the output will be protected. The output would remain at a maximum of 6.8V as a reference voltage of 1N4099.
Let's see how the above circuit works as the protection circuit of the Zener diode input and protects the output from more than 6.8V voltage.
Using PSpice cadence, the above circuit is simulated. The output remains constant at 5.999V at the 6V input voltage across the V1 (Which is 6.0V).
The input voltage in the above simulation is 6.8V. The performance, therefore, is 6.785V, which is similar to 6.8V. Let's further raise the input voltage and create a situation of overvoltage.
Now, 7.5V, which is more than 6.8V, is the input voltage. The performance is now at 6.883V. This is how a Zener diode is successful in saving the connected circuit from a situation of overvoltage, even when the voltage returns to less than 6.8V, as shown in the previous stage, the circuit will operate normally again. In other words, a Zener diode does not get fried even during an overvoltage state, unlike a fuse.
To pick different overvoltage margins in the above circuit, any other Zener diodes with different values such as 3.3V, 5.1V, 9.1V, 10.2V can be used.
4. How Do You Choose a Zener Diode to Protect a Circuit?
The next critical part is choosing the value of the Zener Diode. The points below will assist you in selecting the correct Zener Diode value and part number.
1) Choose the voltage of the Zener diode first. It is the voltage value that will serve as a close circuit for the Zener diode and protect the load from overvoltage. The Zener voltage is 6.8V in Pspice, for the above example.
There will be some cases where there is no usable targeted Zener diode voltage. In such instances, it is possible to choose a near value of the Zener diode. For example, for overvoltage security up to 7V, a near value is a 6.8V Zener diode.
2) Calculate the load current that is linked across the circuit of overvoltage safety. This is 50mA for our example discussed above. Other than the load current, biasing current is required by Zener diodes. Therefore, the total current, plus the Zener diode biasing current, should be equal to the load current. For the above-mentioned example, it can be total current=50mA+10mA=60mA.
3) There is a power ranking for Zener diodes. Therefore, for proper heat dissipation, the correct Zener diode power rating is required. Based on the measured total current in Phase - 2, which is 60mA, the power rating can be calculated. Therefore, the power rating of the Zener diode would be equal to the voltage of the Zener diode, which connects the total current flowing through the diode.
4) Calculate the value of the resistor by differentiating the voltage of the source and the general voltage. The limit which can be applied to the circuit would be the source voltage. For example, it can be 13V to maximize overvoltage that can occur or can be added as a supply voltage.
The voltage drop through the resistor will then be = 13V-6.8V = 6.2V According to the law of the ohm, the resistor value will be = 6.2V / 0.060 A = 103R It is possible to choose the standard value 100R resistor.
5) Zener diode typical values are 5.1V, 5.6V, 6.2V, 12V and 15V -most common; they also have 3V, 5V, 12V, 18V, 24V.
5. Zener Overvoltage Protection Overview
The easiest and simplest process to protect devices from overvoltage is an overvoltage-protect circuit using Zener diodes. The voltage remains regulated in this technique and the cost of this circuit is much lower compared to other techniques.
Although, surely, there are disadvantages to this sort of circuit. Power dissipation is the main downside of this type of circuit. It still dissipates heat due to the linked series resistor and results in energy wastage.