Vacuum relays are widely used in airborne (aviation), mobile and marine communications equipment.
Typical applications include antenna coupling, tap switching on radio frequency (RF) coils, transmit/receive switching to an antenna, switching in pulse shaping networks, and heavy duty power supplies, including those for heavy duty applications. Such relays are characterized by high speed of operation and the ability to withstand high voltages, as well as carry high currents (at frequencies up to 75 MHz) while maintaining low and stable contact resistance.
Vacuum relays are commonly available in normally open (NO) and normally closed (NC) models SPST, as well as in configurations SPDT and DPDT. In addition, there are polarized relays.
The principle of operation of the relay
A vacuum relay basically consists of two main assemblies: a ceramic "switch" assembly that contains the high voltage contacts, and an actuator assembly that holds the actuation coil.
There are two main types of relays:
- Cotton-type relay, "clapperboard" (Fig. 2)
- Membrane relay (Fig. 3).
The two types differ in mechanical actuation. In a membrane relay, the actuator is located outside the vacuum envelope, while in a cotton-type relay, the actuator is inside the vacuum.
The node containing the coil is the driving part of the relay and is connected to the control circuit. When voltage is applied to the coil, a magnetic field is created and an electromagnetic force is created. This force is used to move the mechanism and hence the moving contact inside the vacuum envelope. The contact switches from NC to NO in the SPST relay and opens the high voltage circuit or reverse direction (SPDT).
Vacuum as a dielectric
Vacuum is the ideal dielectric for switching high voltage relays. It has extremely high breakdown voltage characteristics, high recovery speed (up to 10 kV per millisecond) and provides a completely inert environment for switching contacts. Also, since there is no oxygen in a vacuum, the contacts do not oxidize.
The high dielectric strength of the vacuum ensures close contact spacing, on the order of 1000 volts per mil (= 0.0394 inches). The small amount of movement needed to actuate the vacuum relay allows the use of small, light weight actuators, allowing high operating speeds. The use of refractory metal contacts ensures exceptional breaking capacity and long contact life.
When switching the load, an arc is formed. At the point where the contacts become very close and the current density gets higher and higher, breakdown occurs. This arc will have a very low voltage of 18-23 volts and will be quite stable compared to an arc in air, which is another benefit of vacuum (Figure 1). The combination of a constant arc voltage acting as a current limiter and a short arc time means vacuum relays typically wear less than other types and provide consistent performance over the life of the relay.
Compressed gas as a dielectric
High pressure clean gas allows the relay to achieve high dielectric strength and avoid oxidation. This dielectric is ideal for capacitive inrush loads and capacitive discharge loads. Typical applications include electrostatic discharge (ESD) testing equipment, cable testing equipment, and cardiac defibrillators. Gas-filled relays also provide low, stable leakage current in applications that are sensitive to current fluctuations, especially across open contact sets for extended periods of time.
However, gas relays should not be used when it is necessary to turn off the current. When the contacts open, the gas is ionized and the arc is formed and maintained much longer than in a vacuum.
The contact resistance of gas-filled relays (eg Jennings) is usually measured at 28V. It will be higher than in a vacuum relay and will not be as stable.
(The translation was made according to information from the Jennings catalog)