Even a novice radio amateur is able to assemble a solid state relay. This device is based on semiconductor radio components. Power switches are assembled on thyristors, transistors or triacs. To make a solid-state relay circuit with your own hands, it is worth finding out the principle of operation and the features of connecting the device. As a result, it can be used to improve the reliability and safety of the electrical circuit.
Content
- Advantages and disadvantages
- Types of devices
-
Manufacturing recommendations
- Electronic components and functional check
- Body design
- Filling with compound
Advantages and disadvantages
Unlike other types of relays, solid-state relays are devoid of moving contacts. The switching of electrical circuits in this device is carried out according to the principle of an electronic key made on semiconductors. So that when creating a solid-state relay there are no problems, it is necessary to understand the principle of operation of the device and its design.
However, it is worth starting by describing its main advantages:
- The ability to switch powerful loads.
- Switching takes place at high speed.
- High quality galvanic isolation.
- Capable of withstanding serious overloads in a short time period.
No mechanical relay has the same parameters. The area of application of the solid state relay (SSR) is practically unlimited. The absence of moving elements in the structure significantly increases the service life of the device. However, it should be remembered that the device has not only advantages. Some of the properties of the SSR are disadvantages. For example, during the operation of powerful devices, it becomes necessary to use an additional element for removing thermal energy.
Often, the dimensions of the radiator significantly exceed the dimensions of the relay itself. In such a situation, the installation of the device is somewhat difficult. When the device is closed, there is a leakage current, which leads to the appearance of a nonlinear current-voltage characteristic. Thus, when using SSR, attention should be paid to the characteristics of the switched voltages. Some types of devices are only capable of operating on DC networks. When connecting a solid-state relay to a circuit, it is necessary to provide means of protection against false alarms.
Types of devices
Solid state relays can be divided into several groups according to certain parameters. Most often, the category of the connected load is used to classify these devices, as well as the method of monitoring and switching the voltage. Thus, there are 3 types of relays:
- Devices operating in DC circuits.
- Switches for AC circuits.
- Universal relays.
The first group includes SSRs with switching voltages of 3-32 V. They have small dimensions, are equipped with LED indication and can work effectively in the temperature range from -35 to 75 degrees. The representatives of the second category are switches designed to operate in AC circuits at a voltage of 24-220 V. Universal devices have the ability to manually adjust for use in specific conditions.
If we classify devices by the nature of the connected load, then we can distinguish 2 types of devices operating in alternating current networks - one- and three-phase. With their help, you can control a fairly high load at a current strength of 10-75 A. it is also worth paying attention to the peak electric current indicators, which are capable of reaching 500 A.
Solid state switches can be used in various types of circuits, such as capacitive or resistive. Their design allows you to get rid of noise during operation, as well as achieve smooth control of drives, for example, electric motors or lamps. SSRs are highly reliable, but in many respects the service life of the devices depends on the manufacturer.
Manufacturing recommendations
In accordance with the design features, the device circuit should be assembled not on a textolite, but using a hinged mounting. There are quite a few circuit solutions., and the desired one should be selected depending on various parameters, for example, the switching power.
Electronic components and functional check
As an example, consider a simple circuit.
The use of the MOS3083 optical pair makes it possible to form a control signal, the input voltage of which is in the range of 5-24 V. To extend the life of the AL307A LED, a circuit is introduced into the circuit, consisting of a resistance and a zener diode. Finding all the electronic items will not be difficult. The assembled circuit must be checked for operability.
To do this, you can not connect a voltage of 220 V to the circuit, but limit yourself to connecting the tester in parallel to the control line of the triac. On the measuring device, you must first select the "mOhm" mode and apply power at 5-24 V to the control voltage generation section. If the circuit has been assembled correctly, the tester will show the difference in resistance in the mΩ-kΩ range.
Body design
The base of a homemade solid state relay will be an aluminum plate with a thickness of 3 to 5 mm. The dimensions of the plate are not of fundamental importance, and when choosing a material, it is necessary to take into account only the conditions for high-quality heat removal from the triac. It should also be remembered that the surface of the base must be flat and must first be cleaned with fine sandpaper on both sides.
The next step is to install a border made of plastic or thick cardboard around the perimeter of the plate. The result should be a box, which is then filled with epoxy. A relay circuit assembled by means of surface mounting is installed inside the case. In this case, only a triac should be located on the aluminum plate.
To improve the heat dissipation process, you should use thermal grease, placing it over the entire contact area of the aluminum base and the semiconductor element. It should also be remembered that for some triacs the anode is not insulated, and they are installed only through a mica substrate.
Filling with compound
To make the mixture, you will need alabaster and epoxy without hardener. The use of alabaster allows you to solve two problems at once - a mixture of ideal consistency is created and a sufficient amount of solution is obtained with a minimum consumption of epoxy resin. During preparation, the compound is thoroughly mixed, after which the hardener can be added and mixed again.
After that, the created circuit is carefully filled with compound to the upper level, leaving only a part of the control LED head on the surface. Any solution suitable for casting can be used in the construction of the solid state switch housing. The only criterion when choosing ingredients is the lack of the ability to conduct electric current.
A homemade SSR is a good choice for connecting to low power low voltage circuits. Collecting more powerful devices designed for high voltages is impractical. Such circuits are highly complex and it is better to buy a ready-made device.
