Summary： Energy devices release energy due to over-voltage surges and spikes. This energy can prove harmful and should be reduced. The devices employed to facilitate such energy spikes are known as transient suppressors, and this article is an excellent read that will help you learn more about such devices.
When working with power electronic devices, we hope that the direct and alternating current devices are powered by circuits that receive purely regulated voltages and currents. Some of these devices, such as the DC relay switching, AC inductive loads, and DC motors, generate a power supply that is too difficult to maintain. These systems require transient suppressors to aid in energy regulation. In power electric systems, inductive switching transients originate from suddenly switching off reactive or inductive loads like motors, relay coils, and solenoids. This is simply because the rapid collapse of the magnetic fields in this system induces transient voltages that affect the power steady-state supply. The transients originate from the sudden release of the energy that was stored previously. The energy can be either capacitive or inductive, which introduces voltage surge or transient.
This transient voltage needs to be suppressed. This can be achieved through transient suppression devices, which can be connected in series or parallel to the load. Series connected transient devices attenuate the energy released from the system and prevent it from propagating through the circuit. The parallel connected transient devices divert the generated transient away and clamp or limit the remaining voltage. This article will look at the various types of transient suppressors and how they suppress surges and spikes in energy devices.
Figure 1: Voltage Transients and Spikes Courtesy of Simon Mugo
The transient effects of the surge and spikes in the area of AC power vary in a range of some few volts to very high volts over the standard voltages. Suppression devices are involved in eliminating these effects. These suppressors use filter circuits to attenuate or eliminate the transients. The filter circuit is connected in series with the load and has a frequency of about 100Hz.
Low-pass filters can be involved in the attenuation transients of high frequencies and ensure that the low-frequency signals are passed undisturbed. A resistor-capacitor forms one of the simplest transient suppressors connected directly to the power line for attenuation purposes.
For AC power attenuation, the filters are built of capacitors and inductors, which form LC filtering circuits with a degree of attenuation depending on the total number of the LC stages incorporated in that filter. Figure 2 below is a good expression_ of this type of transient suppressor.
Figure 2: Typical Transient Suppression Filter Courtesy of Simon Mugo
The diagram represents a simple two-stage filter of the low-pass type, which provides a high-voltage loss insertion between the line-to-ground and line-to-line, hence offering significant transient protection and noise reduction in the power supply system.
The purpose of the voltage clampers in the circuit is to limit the transient amplitude across the connected circuit. The voltage clamping circuit operates such that when a certain threshold is exceeded, it starts contacting and goes back to non-contacting immediately after the overvoltage is eliminated.
These devices are connected across the supply and parallel to the load to protect it against high transient voltages. Voltage clampers range from simple Zener diodes connected across the DC power supply, metal oxide varistors for the AC power supply, and VDR for protection against overvoltage.
Below, we focus on voltage clamping suppressors.
Zener Diode Transient Suppression Devices
The Zener diodes are suitable devices that offer protection to DC supplies. The Zener diode acts like the standard diode when forward-biased, but they contact reversely when they reach the breakdown point. They can get in both forward and reverse-biased directions. The breakdown point of any Zener diode can be used as clamping or reference voltage. Zener diodes have high impedance when contacting in reverse voltage, and a tiny leakage current is released. If the voltage supply across the Zener diode exceeds the Zener voltage, the diode starts breaking down, hence regulating the transient voltages.
Figure 3: Typical Zener Transient Suppressors Circuits Courtesy of Simon Mugo
The Zener diode is connected across the supply or the protected components. It is deemed invisible until a transient voltage appears because of its high impedance below its reverse mode breakdown voltage and a low impedance above its reverse mode breakdown. At the breakdown mode of the Zener diode, the transient suppression occurs, clamping the circuit overvoltage instantly and reducing the spikes to a safe operating level.
Metal Oxide Varistors Transient Suppressors
This is the most common suppression device that exists. The specifications of high voltage ratings and the ability to handle much higher current surges make them suitable for this function. These devices are used in the DC and AC power supply lines to protect against overvoltage transients. MOV is a voltage-dependent variable resistor. It is positioned parallel to the connected load, ready for protection, and at low voltage, they have a very high voltage compared to the higher voltage. Their primary purpose in the circuit is to protect the load against overvoltage transients and surges on the power line.
MOVs offer high resistance anytime the voltage across their active terminals goes below the predetermined value at the breakdown, which makes them behave like voltage-dependent resistors. Anytime the system is exposed to high transient voltages, its electric characteristics are affected, making its resistance very small, and this clamps the voltage, eradicating the spikes and the overvoltage transients. See Figure 4 below.
Figure 4: MOV Transient Suppressor Circuit Courtesy of Simon Mugo
Crowbar Transient Suppression Devices
This is another type of parallel transient suppressor based on how it is connected. They work by initiating a short circuit when the preset voltage is reached. Such systems are standard in high-stability power supply, with an output expected to be fixed. It is also used in the power supply area, protecting the circuits against transient voltages.
Some crowbars are semi-conductor-based and parallel to the connected load, attenuating huge currents. They use the SCRs due to their low on-state voltages to ensure the voltage remains safe. On firing the SCR, they can divert a recognizable level of transient voltages to their ground connection.
Figure 5: Basic Crowbar Clamping Circuit Courtesy of Simon Mugo
