How do inductors, resistors, and wires protect the circuit?

Inductors, resistors, and wires are not protective devices in themselves, but they can play a role in protecting the circuit with the combination of several different protective devices.

In protection devices, the gas discharge tube is characterized by a large flux, but the response time is slow high impact breakdown voltage;

TVS tube flux flow is small, has the fastest response time, and has the best voltage clamp characteristics;

The characteristics of the varistor between the two, when a protection circuit requires a large overall flux to achieve fine protection, the protection circuit often requires the cooperation of these protective devices to achieve more ideal protection characteristics.

However, these protective devices cannot be used in parallel. For example, suppose a high-flux varistor and a low-flux TVS tube are directly connected in parallel. In that case, the TVS tube will be damaged first under the action of overcurrent, and the advantage of the high-flux varistor cannot be played. Therefore, several protection devices used in conjunction with the occasion often need inductors, resistors, wires, etc., between the different protection components to cooperate. The following is an introduction to each of these components.

Inductance.

In a series DC power protection circuit, there should not be a large voltage drop on the feeder line so that the inter-pole circuit can be used with a hollow inductor.

The inductor is used to achieve two levels of protection devices.

The role played by inductance: when the protection circuit reaches the design throughput, the overcurrent on the TVS should not reach the maximum throughput of the TVS tube, so the inductor needs to provide the sufficient current limiting capability for lightning overcurrent.

In the power circuit, the inductor design should consider several issues.

1. the inductor coil should operate normally without overheating when flowing through the full operating current of the equipment.

2. Hollow inductors should be used as much as possible. Inductors with magnetic cores can become magnetically saturated under overcurrent, and the inductance in the circuit should only be calculated as the inductance without a magnetic core.

3, the coil should be wound single layer as far as possible. This can reduce the parasitic capacitance of the coil and, at the same time, can enhance the tolerance of the coil to transient overvoltage.

4, winding inductor coil wire on the insulation layer should have sufficient thickness to ensure that the transient overvoltage effect between the coil turns does not occur in the breakdown of the short circuit.

In the design of the protection circuit of the power supply port, the inductor usually takes the value of 7 to 15uH.

Resistance.

In the signal line, the line series connected components on the high-frequency signal inhibition to be as little as possible so that the inter-pole fit can be used resistance.

The use of resistors to achieve two levels of protection devices with

Resistors should play the same role as the role above inductors. In the above figure, for example, the value of the resistance is calculated as follows: measured air discharge tube impact breakdown voltage value U1, check the TVS device manual to get the maximum TVS tube 8/20us inrush current flux I1, and the highest TVS tube clamp voltage U2, the minimum value of the resistance is: R ≥ (U1-U2)/I1.

In the signal line, the use of resistors should pay attention to several issues.

1, the resistor's power should be large enough to avoid damage to the resistor under the action of overcurrent.

2, try to use linear resistors so that the impact of resistance on normal signal transmission is as small as possible.

Conductors.

Some AC / DC equipment is full with a large operating current, more than 30A. In this case, the protection circuit between the poles using inductors will be too large for the problem. The protection circuit can be divided into two parts to solving this problem. The front and rear protection are not designed on the same circuit board, while the two levels of the circuit can be used between the specified length of the feeder line.

The use of wire to achieve the two levels of protection devices cooperate.

This combination forms the protection circuit, the role of the specified length of the feeder line. The role of the inductor is the same because the inductance of the 1-meter-long wire between 1 and 1.6uH, feeder line to a certain length, can play a good role with the feeder line diameter can be flexibly selected according to the size of the full working current, overcoming the use of inductors to do interpolate with the inductor can not flow through a large working The shortcomings of using inductors for inter-pole coordination.

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