Which is better, reed relay or solid-state relay? how to choose?

Reed relay, also called reed switch and reed switch, is composed of a pair of elastic reeds made of magnetic materials. The reeds are sealed in a glass tube filled with inert gas. The reeds overlap and there is a gap between them.

The working principle of the reed relay

When there is no magnetic field, the two reeds in the glass tube filled with inert gas are separated, otherwise, under the action of the magnetic field lines, the two reeds in the glass tube are magnetized to attract and contact each other, so that the two pins are connected to the circuit. When the external magnetic force is removed, the two reeds in the glass tube are separated again, and the connected circuit is disconnected.

The characteristics of the reed relay

High reliability; high safety; high adaptability; long life; compact structure; small size; superior electrical performance; high-speed operation; wide application.

The solid-state relay has a non-contact switch with an isolation function. There is no mechanical contact during the switching process. In addition to the same function as an electromagnetic relay, it also has logic control and protection functions, resistance to mechanical shock and vibration, good moisture and corrosion resistance, and anti-explosion, anti-gas pollution, fast switching speed, and high frequency. Because solid-state relays are non-contact switches composed of microelectronic circuits and discrete electronic devices, they are often used in small-signal control and isolation circuits that drive large loads.

The characteristics of the solid-state relay

Reliable operation, long life, little interference to the outside world, compatibility with logic circuits, fast switching speed, strong anti-interference ability, and convenient and wide use. 

Solid-state relays are composed of all-solid-state electronic components, with no movable mechanical parts and no mechanical actions during the operation. The switching function of on and off is changed by the working state of the circuit, and it also realizes weak signals' control of strong electricity.

The application areas of the solid-state relay

 Industrial control; factory and production automation; data acquisition; measuring instruments and equipment; monitoring systems. It is a substitute for ordinary relays. All equipment and systems that use weak current to control strong current can use solid-state relays.

Selection of reed relays

The structure of the reed switch relay includes a glass shell packaged reed switch, plastic-packaged reed switch, metal packaged reed switch, etc.

The contact form of the reed relay has two types: normally open contact (H) and changeover contact (Z). The control group includes a single group, two groups, three groups, etc.; Show. Its working principle is very simple, using permanent magnets or using energized coils to generate a magnetic field can transform its normally open contact action into a normally closed state to turn on the control circuit.

In general, the rated capacity of the contacts of the reed relay is very small, and can only be used as a small control current for the control signal loop. When used as a control loop signal, there is no rated working voltage parameter.

Selection of solid-state relays

When choosing a solid-state relay, the power supply type of the solid-state relay and its ability to control the load should be determined by the power supply type, power supply voltage, and power supply current of the controlled circuit.

If the controlled circuit is an AC power supply, you should choose an AC solid-state relay; if the controlled circuit's power supply voltage is a DC voltage, you should choose a DC solid-state relay accordingly.

The load capacity of the solid-state relay should be determined by the voltage and current of the controlled circuit. In general, the output power of the solid-state relay should be greater than 1.5~2 times the load power of the controlled circuit. Please note the following points.

The steady-state current at the output terminal of the solid-state relay shall not exceed the rated output current. Almost all loads have inrush current when they work, such as the so-called pure resistive load electric heating element, but it has a positive temperature coefficient, and the resistance value at low temperatures is small, so the instantaneous current when it is just started is relatively large.

The input characteristics of solid-state relays are in DC 3~32V, which has a wide control range. For single-phase DC/AC solid-state relays, the input current of the control loop is generally about 10mA. The input current of the control loop of the three-phase solid-state relay is generally about 30mA.

The control operation frequency of AC solid-state relays generally should not exceed 10 Hz, and the control signal period of DC solid-state relays should be greater than 5 times the sum of the relay's on and off time.

The load capacity of the solid-state relay is directly related to the ambient temperature. When the ambient temperature rises, the load capacity of the SSR also decreases. At the same time, when selecting the rated current of the SSR, there must be sufficient leeway. When the ambient temperature is high, pay more attention to this.