Current limiters to protect electrical and electronic networks from the destructive effects of voltage

Current limiter (OT) - a device that is used in electrical or electronic circuits to reduce the upper limit of direct (DC) or alternating (AC) current supplied to a load. This ensures timely reliable protection of generation circuits or electronic systems from harmful impacts due to a short circuit in the network or other negative processes leading to a sharp increase AC / DC.

Limiting techniques are used to control the amount of current flowing in a DC or AC circuit. The device guarantees that in case of exceeding its limit size, the protection will reliably and timely work. Current limiting devices can be used in various modifications depending on sensitivity, rated current load, response time and possible causes of a short short circuits in the network.

Excessive AC / DC can occur in the internal circuit due to short-circuited components such as diodes, transistors, capacitors or transformers, as well as external problems during overload of network objects, in the closing circuit or overvoltage at the input terminals nutrition.

Types of limiting devices

The choice of protective devices depends on several factors. Devices are passive and active, they can be used individually or in combination. Usually the arrester is connected in series with the load.

Types of limiting devices:

1. Fuses and resistors. They are used for simple current limiting. The fuse will usually blow if the AC / DC exceeds the nominal size. Resistors are integrated into the circuit design. The correct resistance value can also be calculated using Ohm's law I = V / R (where I is current, V is voltage and R is resistance). There are many different fuses on the electrical market that can meet any power dissipation requirement.
2. Automatic switches. They are used to disconnect power, just like a fuse, but their response is slower and may not trip on particularly sensitive circuits in expensive equipment.
3. Thermistors. Negative Temperature Coefficient (NTC) thermistors are used to limit the initial impulse currents that flow when the device is connected to the mains. Thermistors have significant cold resistance and low resistance at significant temperatures. The NTC limits the starting current instantly.
4. Transistors and diodes. Regulated power supplies use limiting circuits such as integrated circuits, transistors, and diodes. Active circuits are suitable for sensitive networks and operate by reducing the load or turning off the power, on a faulty short-circuit or on the entire network.
5. Current limiting diodes are used to limit or regulate over a wide voltage range. The two-contact OT device consists of a shutter shorted to the source. It maintains DC regardless of voltage changes.

Load current limiter in power grids

Power distribution systems have circuit breakers to turn off the power in the event of a malfunction. They have certain disadvantages in ensuring the necessary reliability, since they cannot always disconnect the minimum necessary emergency section of the network for repair. The problem arises when reconstructing the power supply by adding new capacity or cross connections, which must have their busbars and breakers retrofitted for higher short-circuit current limits (TKZ).

Improving the quality of electricity in networks directly depends on the reliability of the network equipment operating mode. Among the various types of interference that affect the quality of the mains voltage (surges, harmonic distortion, etc.) ), the most serious obstacle is voltage drops, since the associated phase angle jumps can lead to breakdown equipment, to a complete stop of production, housing and communal services facilities, which with the speed of a chain reaction will create a threat to life support population.

A common cause of voltage drop is short circuit current. In the event of a malfunction in the distribution network, voltage drops sharply on all damaged tires. The level depends on the connection point and the electrical distance of the bus to the accident site.

To reduce negative processes and disconnection of faulty sections of the network, the following limiters are applied:

• Distribution static compensator;
• dynamic voltage recuperator;
• thyristor controlled capacitor;
• semiconductor static transfer switch;
• solid state fault current limiter.

Such protective devices are not always perfect. Some of them have a disadvantage due to their high cost, while others can limit the fault current to less than 5 times the normal current, which is not enough for overloads.

Points of application of current limiters in power equipment:

• To the point of operation of the main switch on the emergency feeder of consumer loads, with the inadmissibility of interruptions in the power supply;
• on equipment, the performance of which ceases to correspond to the limiting short-circuit current, increased due to an emergency in the power supply systems.

A simple OT solution in power grid equipment is to add resistance to the circuit. This limits the rate at which the TCZ can increase before the breaker is opened, but also limits the ability to circuits meet rapidly changing consumer demand, so adding or removing large loads causes unstable power.

Application of current protection in electronic circuits

The inrush current occurs when the switch is energized. This is because the difference between the equivalent series resistance of the capacitor and the line resistance is only a few milloles and results in a large inrush current. Four factors that can influence this process:

1. Input AC value.
2. Minimum resistance required by an NTC thermistor (at t = 0).
3. Constant DC.
4. Ambient temperature.

A current limiter is a device or group of devices used to protect circuit elements from inrush loads. Thermistors and resistors Negative Temperature Coefficient (NTC) are 2 simple protection options. Their main disadvantages are long cooling times and high power dissipation. The current limiting diode regulates or limits the current over a wide range. They consist of a JFET with a gate shorted to source and functioning as a two-pole current limiter.

They allow the current passing through them to rise to a certain value and compare with a given value. Unlike Zener diodes, they keep constant currentrather than stress. Current limiting diodes keep the current flowing through them constant with any change in load.

Types of current limiting diodes

There are many different types current-limiting diodes classified by:

• rated current of the regulator;
• maximum limit voltage;
• working voltage;
• power consumption.

The most common maximum usable voltages are 1.7 V, 2, 8 V, 3, 1 V, 3.5 V and 3.7 V and 4.5 V. The nominal current of the regulator can range from 0.31 mA to 10 mA, with a commonly used regulator current of 10 mA.

Constant current limiting circuit

Most power supplies have separate DC and voltage control loops to regulate their outputs in either constant voltage (CV) mode or in constant current mode (CC), which are included in the control depending on how the load resistance matches the output voltage and the current settings.

Thus, protection is carried out mainly by limiting the current value. In this case, you can apply simple circuit for source limiter using two diodes and a resistor. With any power supply, there is always a risk that the output will short-circuit. Accordingly, in these conditions it is necessary to protect it from damage. There are a number of circuits that can be applied to protect the power supply.

One of the simplest circuits includes only two diodes and an additional resistor. The circuit uses a noise measurement resistor in series with the output transistor. Two diodes located between the output of the circuit and the base of the transistor provide protection. When the circuit is operating within its normal operating range, there is a small voltage across the resistor. This voltage plus the base radiative transistor is much less than the diode junction drop required to turn on the two diodes. However, as the DC increases, the voltage across the resistor increases. When it is equal to the voltage required for operation, they turn on, the voltage of the transistor drops, thereby limiting the current.

The circuit of this diode current limiter for the power supply is simple. The value of the series resistor can be calculated so that the voltage across it increased to 0.6 volts (turn-on voltage for a silicon diode) when the maximum current. However, it is always best to ensure that there is some margin of protection and it is best to limit it to the required level.

Feedback limiter

The same simple diode form of current limiting can be included in power circuits that use feedback to determine the actual output voltage and provide a more accurately adjustable output. If the measuring point of the output voltage is taken after series current resistorthen the voltage drop can be corrected at the output.

This circuit provides much better regulation than the direct emitter regulator, and can also account for the drop voltage across a resistor with a current limit if there is sufficient voltage drop across the transistor in the source circuit nutrition. The output voltage can also be adjusted to obtain the required value using a variable resistor. The diode form of current limiting can be easily integrated into the power supply circuit. Plus, it's cheap and convenient.

Applications of current-limiting diodes

Current limiting diodes provide superior performance and ease of use compared to bipolar transistors in protection systems. They are versatile and have excellent performance in terms of dynamic temperature drift. Devices using diodes:

• signal generator circuits;
• synchronization schemes;
• charging device;
• LED control;
• replacement of holding coils in telephone communication devices.

Current limiting diodes are available from many global semiconductor manufacturers such as Calogic, Central Semiconductor, Diodes Inc., O. N. Semiconductor or Zetex. The electronics market has a very wide selection of diodes, diode circuits used, or any other device that may require current limiting.