Geçici Bastırma Cihazları / Transient Suppression Devices

We like to think that the AC or DC power supplies we use to power our circuits are both clean and well-regulated resources. However, switching AC inductive loads or switching DC relay contacts and DC motors as part of a microcontroller project all combine to produce a power supply quality that is difficult to maintain.

These inductive switching transitions occur when some kind of inductive or reactive load, such as an engine, solenoid coil or relay coil, is suddenly turned off. The rapid collapse of the magnetic field induces a temporary voltage that is placed on the stable state source. These inductive switching voltage transitions can reach 1000 volts.

Transient events are very steep voltage steps that occur in electrical circuits due to the sudden release of previously stored inductive or capacitive energy. This causes a high temporary voltage or fluctuation. Due to some switching movements, this sudden energy release creates a temporary voltage rise in the form of a vertical energy pulse, which could theoretically be of any infinite value.

Temporary Suppression

This high DV/DT temporary switching increment can either exist for a very short period of time (milliseconds or microseconds), or it can occur randomly, for example, two or three times a day, over short periods of time. .

We must also understand that voltage transitions do not always start at zero voltage or at the beginning of a cycle, but can be overtaded by another voltage level. In both cases, temporary events are bad because they can damage electronic equipment and therefore need to be suppressed and controlled.

Temporary suppression devices can be in many forms, from arc contacts to filters to solid-state semiconductor devices. Discrete semiconductor transient suppression devices such as metal-oxide Varistors or MOV are by far the most common, as they are available in various energy absorption and voltage ratings, which enable strict control over unwanted and potentially destructive transient events. voltage spikes.

Transient current suppression devices can be installed to reduce or decrease the energy value of a transient current that prevents it from spreading throughout a circuit, or it can be used in parallel with the load to direct the transient current to the ground. Therefore, limit or stabilize the voltage now.

The weakening of the temporary voltage is usually carried out using low-pass filters that are serially connected to the load circuit. When a voltage transition occurs, it is a high-frequency ascent, which usually moves fast. Therefore, the filter reduces this high frequency transition or allows low frequency power or signal component to continue intact while blocking. A good example of temporary debilitators are extension cables with mains filters.

The routing of a transient current is usually carried out using a voltage clamping type device or using what is commonly called a crowbar type device. These parallel connected devices exhibit a nonlinear impedance feature, since the current flowing through them is not linear to the voltage throughout their terminals, as given by the Ohm Act.

Temporary Suppression

A voltage clamping device such as MOV has a variable impedance depending on the current passing through the device or the voltage in its terminal. Under normal stable condition operating conditions, the device offers a high impedance and therefore has no effect on the connected circuit.

However, when a voltage transition occurs, the impedance of the device changes by increasing the current drawn from the device as the voltage on it increases. The result is a pronounced compression of the temporary voltage. The volt-amp feature of a docking device usually depends on time, since the large increase in current causes the device to expend a lot of energy.

Crowbar devices are another type of temporary suppression device that removes excessive voltage spikes from a circuit as a result of a switching-type opening action. Crowbar devices are similar to a zener diode in terms of operation, as they have no effect on the circuit under normal stable conditions. When a transient event is detected, they quickly switch to an "ON" state, providing a very low impedance path that removes the transient event from the parallel attached load.

Discrete temporary suppression devices can then be divided into three basic categories, depending on the connection and operating types.

  • Low Pass Filters connected to serial (blocking).
  • Voltage Clamps and Voltage Trimmers connected to parallel (shunt).
  • Crowbar devices connected in parallel (shunt).

and this can be shown as follows:

Temporary Suppression Devices

Temporary Suppression
Temporary Suppression Devices

Serial Temporary Suppression Filters

Serial Temporary Suppression Filters Transient events in an AC power line can vary from a few volts to several kilo-volts of the normal mains voltage. Suppression devices that reduce or block these transient currents use filter circuits to effectively eliminate these transient currents from the network by installing a 100 Hz filter in series on the connected load.

The frequency component of a fast switching voltage can be much higher than the slow-moving base frequency of the AC source. Therefore, an obvious choice to reduce and control these unwanted transitions is to use a low-pass filter section between source and load.

Low-pass filters, such as LC filters, can be used to reduce any high frequency transition and allow low frequency power or signal to pass intact. The simplest form of the transient event suppression filter is a resistance-capacitor RC filter placed directly opposite the power line to reduce any high frequency transient event.

Filters designed for AC power applications usually consist of inductenses and capacitors to create multi-stage LC filters, the degree of weight loss depends on the number of LC stages in the filter. A typical series-connected AC mains temporary suppression filter is shown below.

Typical Temporary Suppression Filter

Temporary Suppression
Typical Temporary Suppression Filter

This basic two-stage low-pass AC filter provides a high insertion loss between the line and even the ground along the frequency range. It provides effective temporary voltage protection by stopping any high frequency transient current and noise from reaching the connected load. In addition, these mains power filters can help reduce voltage spikes and transient currents, as well as eliminate radio frequency interference or emissions supplied by the power supply.

Voltage Compression Transient Suppressors

Voltage clamps are used to limit the amplitude of a transient current during a circuit. A voltage clamping device begins to transmit when a preset threshold voltage is exceeded, then returns to a non-conductive mode when the overvoltage drops below the threshold level. Thus, overvoltage rises are reduced to a safe level by the clamping device.

Voltage clamping devices are usually placed opposite the source and parallel to the load to protect it from unwanted high dv/dt voltage transitions. A voltage clamp can be something as simple as a zener diode along a DC source, but for bidirectional AC sources, we need to use a metal oxide varistor (MOV), suppression diodes or voltage-dependent resistance (VDR) for extreme voltage protection.

Care should be taken to ensure that voltage clamping devices direct overvoltage currents, do not absorb them as in a filter, so the path used to direct the transient current does not produce or create its own problems for the circuit.

Zener Diode Transient Suppressors

Zener diodes are used for protection in DC resources (one-way), since they behave like normal diodes in their forward-pole direction, but they deteriorate and move in their inversely polarized directions. Thus, the reverse fault voltage of a zener diode can be used as the VZ reference or docking voltage level.

VZ zener diodes show high impedance to the feed and transmit very little leakage current in the opposite direction and under the zener fault voltage. However, when the voltage on the zener is greater than the zener voltage, it exhibits a very low impedance path to the overvoltage transition, gradually increasing its transmission as the voltage on it increases.

Zener Temporary Suppression

Temporary Suppression
Zener Temporary Suppression

When connected to a source or protected components, the zener diode becomes effectively "invisible" until a temporary voltage appears, since it has a high impedance under the reverse fault voltage and a low impedance above the reverse fault voltage.

When Zener is in fail-up mode, that is, suppressing a transient event, the diode instantly clamps the overvoltage to limit the spike to a safe level, and then returns to normal when the temporary voltage drops below the zener voltage, VZ. . Then the clamping voltage, VC, therefore, equals the reverse fault voltage of the zener. Due to these docking properties, zener diode is used to suppress transient events, as it traps potentially damaging currents away from the protected load.

The overvoltage current and power capacity of the Zener diode is approximately proportional to the connection area. Most zener diodes are designed to operate at low power and voltage levels. Zener diodes, designed to operate at higher voltage levels and absorb higher surge currents without damage, are known as Avalanche Diodes.

Previously, we have said that a single zener diode can only be used for temporary suppression in stable state DC resources due to its forward-looking diode properties. But by connecting two zener diodes "back to back", we can use clamping properties in a two-way AC source.

Zener Temporary Suppression

Temporary Suppression
Zener Temporary Suppression

By connecting two zener diodes one after the other, we can now maintain both the positive half-loop with one zener diode and the negative half-loop with the other.

If both zener diodes have the same reverse fault voltage, the temporary voltage of one of the poles is interlocked at the same zener voltage level, since one zener diode will effectively be in reverse bias mode and the other will be forward bias. Mode.

Two consecutive zener diodes can be used to temporarily suppress an AC source, while temporary voltage suppressor (TVS) devices are available with contrasting connections built into a single device, making them ideal for AC power applications. Two-way avalanche diodes are available in various voltage and power levels.

MOV Temporary Suppressors

Although Zener diodes and rapid recovery avalanche diodes move quickly and are effective at clamping overvolts, the most common overvoltage suppression clamping technique is the use of metal oxide variators or MOV's. Metal oxide varistors, in addition to high voltage values, are capable of processing much larger overcurrent currents, whether at a slower speed, and can be used on both DC and AC power lines to protect them from excessive voltages such as overvoltage transitions.

MOV is a variable resistance due to semiconductor voltage placed in parallel (shunt) with load or the component to be protected. MOV's have high resistance at low voltage and low resistance at high voltage, and nonlinear voltage-current properties make them useful in protecting against power line fluctuations and excessive voltage transitions.

MOV's behave similarly to successive zener diodes, since they can be used for bi-directional voltage clamping with the transmission of the transient current as the voltage on it increases. These small disc-shaped metal oxide type varistors offer high fault voltages in both directions and can absorb a higher amount of energy, often rated joule instead of watts.

MOV Temporary Suppression

Temporary Suppression
MOV Temporary Suppression

Metal oxide varistors, a voltage clamping device, offer very high resistances by acting more like a voltage-dependent resistance (VDR) when the voltage in their terminals is below the predetermined fault value. When exposed to the high transient voltage of both polarities, the electrical properties of the device change and its resistance is very small by compressing the voltage to a safe level.

Then, when used as a temporary suppression device of the metal oxide varistor, its main purpose, as in most applications, is to secure the voltage generated on it to a safe level, the device is placed in parallel with the circuit or device to be protected.

Crowbar Temporary Suppressors

Another type of temporary suppression device connected to parallel (shunt) is known as crowbar protection. Electronic crowbar devices work when a preset threshold voltage is exceeded, triggering a conductive open-state that results in a voltage drop of only a few volts, so the crowbar name .

Crowbar devices and circuits effectively short-circuit when a trigger voltage is reached, and are usually found in stabilized power supplies designed to produce a constant output voltage, for example, a constant 12 volts or 5 volts, but can also be used for the following. protect a circuit or load from temporary overvoltage.

Semiconductor-based active crowbar circuits are placed in parallel with the load (shunt) and are capable of weakening very large pulse currents. Tristors are often used in crowbar circuits, as they have low "condition" voltage and can keep voltage levels well below damaging levels. Once ignited, they can direct a significant amount of temporary energy into the soil among themselves, as they act as a very low impedance type switch.

The disadvantage here is that this short circuit can cause circuit fuses or circuit breakers to work, especially in a DC system, since it is short-circuited by the power supply, if no additional commutation circuit is provided to position the crowbar clamp in the "ON" position. the crowbar device and output voltage will therefore be zero. Consider the simple crowbar tightening circuit below.

Basic Crowbar Tightening Circuit

Temporary Suppression
Basic Crowbar Tightening Circuit

Here, a thyristor or SCR is placed opposite the source, and the load is adjusted with the voltage dividing circuit established by the R1 and R2 resistors to set the thyristor door to bias at a level too low to be triggered as "ON" during normal operation. Then scr is cut and not conductive.

However, when an overvoltage pass occurs and rises above a predetermined level, the voltage drop along the reflector R2 also increases, becoming sufficient to trigger the SCR door to the transmission, which in turn traps the voltage transition that maintains the load. The problem here is that while the load is protected from overvoltage, it does not protect the power supply and the power supply is insured. Then the protection of the load from the transient current generated by the short circuit of the power supply may be greater than the event that triggered it.

In addition to the use of thyristors for overvoltage protection of AC power supplies, triage can be used as a sweater device and similarly triggered by transmission. The advantage of using a thyristor or triage to protect the sweater of AC resources is that they automatically turn off every half cycle.

Therefore, if a temporary event as short as a small fraction of a millisecond triggers the crowbar device, the manoeuvre only short-circuits the AC power line to which it is connected for at least one half cycle, which can be very fast for insurance. – connection to blow.

Zener Crowbar Temporary Suppressors

We can improve the temporary detection and performance of the above basic crowbar circuit by using a zener diode to detect the extreme voltage state. Here the recissive voltage dividing circuit has been replaced with a zener diode as shown.

Zener Crowbar Squeezing Circuit

Temporary Suppression
Zener Crowbar Squeezing Circuit

Dc supply voltage, VS, is monitored by the zener diode, which acts as a transient event detection component and determines the zener voltage, VZ rating, level of voltage at which the SCR is turned on. When the DC supply voltage is lower than the reverse pre-redefinition value of the zener diode, zener does not transmit the diode, so no voltage or current is applied to the SCR door, so the non-conductive "OFF" remains.

If the supply voltage rises above the zener voltage level, as in the case of an extreme voltage temporary state, the zener diode allows the door current to flow to the SCR, making it "ON" and short-circuiting the load supply voltage and discarding the fuse. The load is then protected from the temporary voltages above the zener voltage, as the VZ as zener diode carries the door current so that the SCR itself will carry most of the shunt current, making the SCR "ON".

Although this zener sweater circuit is an improvement in the basic voltage dividing network, zener suffers from a soft opening motion because the knee at the refractive voltage is curved rather than a sharp rise. Basic crowbar circuit that can be replaced and further improved by adding some voltage gain to the detection and triggering circuit in the form of a single amplifier circuit or op-amp circuit.

For this purpose, thyristors with a built-in overvoltage trigger are designed to crowbar one-way or duplex transitions and voltage fluctuations. Designed to protect 15 volt power supplies, the RCA SK9345 series IC crowbars, the SK9346 and SK9347, which maintain 112 volts, which maintain a supply of 115 volts.

All use an integrated circuit that includes a built-in zener diode, transistors and an SCR. The MC3423 overvoltage crowbar detection circuit is a single IC designed for use with an external crowbar SCR.