Application of high voltage relays in various occasions

1. Introduction

Buffer circuits are usually used to protect relay contacts in low-voltage circuits. It is much more difficult to protect relay contacts in high-voltage circuits.

It is quite simple to cut off a low voltage circuit. However, when the circuit voltage increases, it is not so easy to cut off the circuit. Conventional switches and relays are almost impossible to cut off a kilovolt circuit because of arcing. Because the voltage is so high, the air between the contacts is ionized and breaks through the conductor. Therefore, even if the contacts are separated, the ionized gas will make the circuit conductive.

One way to solve the arc is to remove the ionized air in the contact area. This means that high-voltage switches work best in a vacuum. For example, in a vacuum of 10-6 mm Hg, the dielectric strength of the contact air gap is as high as 2000 volts per millimeter.

Vacuum insulation provides a stable switching environment for high-voltage contacts, reduces oxidation and corrosion, and when an arc occurs in the vacuum tube, no corrosive byproducts are produced due to the separation of air or insulating gas, and a "sucking" effect occurs when the contacts are cut. Because the vacuum is impure, it still contains some impurities. In the short time when the arc is generated, some impurities are separated from the vacuum and attached to the surface of the vacuum tube, thereby improving the purity of the vacuum.

Breakdown occurs between the electrodes in an absolute vacuum. The electrons that generate the arc come from the material of the contact itself. The temperature point at which the arc is generated depends on the work function of the contact material. Considering the work function, tungsten and molybdenum are often used as contact materials.

The work function refers to the maximum insulating electrostatic field that can be sustained between a certain contact gap. Note that in the switching of the thermal environment, when the contact gap is reduced to zero, the arc will be drawn and the electrostatic field strength will increase. Therefore, when the contacts are gradually closed, the electrostatic field at some points will be very high, enough to break through the gap.

Arcs in a vacuum, unless they are extremely strong, will generally extinguish themselves. This is possible because the arc itself is a high-pressure area of ​​vaporized metal, but it is surrounded by an extremely low-pressure area. Since there is no physical boundary between the high and low pressures, the pressures tend to equalize, the arc's strength decreases, and it quickly extinguishes. Arcs can also cause corrosion of the contacts, albeit for a short time. However, this does not usually affect the contact resistance because pure metal is transferred.

2. Inert Gas Dielectric

Not all high-voltage relays are vacuum-type. Inert gas dielectrics are also used in high-voltage components and systems. The breakdown voltage in the pressurized housing can be controlled by changing the gas mixture and/or the gas pressure, which makes it flexible. Another advantage is that the gas pressurized arc extinguishing is usually completed within a few microseconds. Gas-filled relays are used for high-voltage power switches, and their function is to close normally open contacts. One reason is that the gas mixture and gas pressure can be set in advance, and the arc discharges before closing the contacts. In addition, if the circuit voltage is above 3500 volts, even if the circuit is cut off due to contact bounce, the arc is still stable enough to maintain the current. This helps to extend the service life of gas-filled relays, which can be confirmed in capacitive discharge circuits.

Ionization is harmful when breaking a circuit. In fact, it prolongs the arc and increases the corrosion of the contacts. Tests have shown that vacuum relays are more suitable for power disconnection because they suppress the arc (arc extinguishing). Arc extinguishing reduces corrosion and prolongs the life of the contacts.

The contact resistance of a conventional relay changes with the number of uses, but the contact resistance of a vacuum relay is constant and low, with a typical value of 0.015 ohms over the entire service life. This is because standard clean components are used, there is no oxidation or contamination, and pure metal is used in the contact area. Since the contacts are sealed in a vacuum tube, safe switching operations can be achieved in explosive or corrosive environments.

The contact resistance of gas-filled relays is generally low, but higher than that of vacuum relays, and less stable. The contact resistance varies with different test methods. The resistance measured in large capacity and high current test circuits is lower. Gold plating of contacts will improve the stability of gas-filled relays and reduce contact resistance.

3. RF Applications

Good insulation quality and low and stable contact resistance are two important factors in the application of high-voltage vacuum relays in RF conversion. However, any relay must pay attention to the current and voltage limitations in RF applications. Due to the influence of the "skin effect", that is, as the frequency increases, the current will move from the center of the conductor to the surface, that is, as the frequency increases, the effective thickness of the conductive conductor surface decreases, which will cause more current to pass through a smaller cross-section. As a result, the local surface of the conductor will heat up. High temperature will affect the sealing of the relay.

When a relay is used as an insulator, there will be an RF voltage across the normally open contacts of the relay and/or between the contacts and ground. In all practical applications, the relay has a high voltage capacitance in the range of 1PF to 2PF. The leakage current flowing through this capacitance causes the loss part of the insulator to heat up, thereby limiting the RF voltage applied to it.

Current and voltage limitations make it necessary to reduce the current and voltage indicators in RF applications, and the operating frequency is also limited to below 32 MHz. These limitations must be considered when selecting a dedicated relay.

4. Power switch application

The terms "power switch" and "thermal switch" refer to the use of relays to disconnect or connect power. When a relay is used as a power switch, an arc is generated when the contacts begin to close and during the subsequent contact jitter. The arc will cause contact corrosion, and if certain precautions are not taken, it may cause contact welding, or even serious contact damage. Therefore, the duration of the arc and the level of current and voltage are decisive factors in determining the life and reliability of the relay.

High voltage power switch relay contacts are usually made of tungsten or molybdenum, because these metals are hard and have high melting points, which can withstand the high temperature of arc. Some milliamp-level high voltage relays use copper contacts, which are usually used only as "relays".

The type of circuit load is an important factor when choosing a suitable relay. Circuits are usually divided into capacitive, inductive and resistive loads.

Resistive Loads - For DC resistive loads, when the switch is opened, an arc is generated at the moment the contacts separate and will continue until the contacts separate completely. At a given voltage and current, the duration of the arc depends on how quickly the contacts open, and also on how quickly the contacts cool and deionize through self-inductance and distributed capacitance. At the same voltage, AC loads are easier to disconnect than DC loads because AC disconnects itself once every half cycle, and the polarity reversal prevents the metal from always moving in the same direction; for DC loads, this will cause contact failure earlier.

Inductive Loads – DC inductive loads are more difficult to disconnect than resistive loads. This is because the energy stored in the inductor (1/2 LI2) induces an electromotive force ((-L [di/dt])) that resists the change in current until the energy stored in the inductor is exhausted. If special fast-breaking contacts or other means are not used to disconnect inductive loads, the duration of the arc will be directly dependent on the load time constant (L/R). However, AC inductive loads do not have this problem because the polarity reversal at the end of each half cycle forces the current to cross zero. At the same time, there is a phase difference between the current and the voltage, and the supply voltage is in phase with the self-induced electromotive force in the second half of the current cycle.

Capacitive Loads – Closing contacts in a DC circuit to charge or discharge a capacitor will produce large inrush currents. The effect on the contacts depends on the initial peak current magnitude and the time constant of the circuit. Similar conditions are not common in AC circuits. For best results, the relay should be placed at the ground end of the load; otherwise, high current arcs will occur between the contacts and the case, bypassing the load. The power supply is the only limit to the inrush current.

In practice, all three components are usually present, but circuits with large capacitive or inductive loads are more difficult to switch on and off due to their energy storage. Even worse than this is the presence of large inrush currents in some circuits. In the case of large inrush currents, the contacts attempt to disconnect the extremely high current during contact bounce, resulting in strong arcing that melts the contact metal and eventually leads to welding of the contacts. Sinusoidal AC makes this worse, as the voltage and current peaks are 41% higher than the equivalent DC case for the same AC load.

V. Conclusion

High voltage relays can also be used in other applications, such as aerospace power distribution systems. Studies have shown that 270V DC systems are more reliable, easier to maintain, lighter, and have a longer life than traditional 115/200V, 400 Hz AC systems.

Currently, conventional 28V or 115/200V switching devices require significant improvements to reliably switch 270V DC loads. However, these improvements would make them large and bulky. This is obviously impractical for dedicated applications. Using vacuum as the insulating medium to achieve 270V DC loads has excellent performance and improved reliability without increasing size and weight.

The selection of high-voltage relays is not as simple as that of low-voltage relays. To select the right relay, the designer should consider the circuit conditions and the electrical, mechanical and environmental factors of the relay. The designer should also be able to comprehensively consider the various working characteristics of the relay and understand that relay terminology is a professional language with special meanings.