Do-it-yourself solid-state relay: assembly instructions and connection tips
Solid-state relay (TTR) is a device from a series of electronic components of non-mechanical action. The lack of mechanics opens up more opportunities for electronics lovers to make a solid-state relay do-it-yourself for personal use.
Consider this possibility in more detail.
The design and principle of operation TTR
If most of this electronics traditionally contains moving parts of contact groups, the solid-state relay has no such parts at all. Circuit switching circuit device is carried out on the principle of electronic key. And the role of electronic switches is usually performed by semiconductors built into the body of the relay - power transistors, triacs, and thyristors.
Before trying to make a solid-state relay on its own, it is logical to become familiar with the basic design of such devices, to understand the principle of their operation.
Within the framework of a thorough study of the device, it is immediately necessary to single out the advantageous sides of TTR:
- switching of powerful load;
- high switching speed;
- ideal galvanic isolation;
- the ability to briefly hold high overload.
Among the mechanical structures to find a relay with similar parameters is not really possible. In general, the advantages regarding mechanical counterparts in solid-state relays are expressed in an impressive list.
The operating conditions for TTR practically do not limit the use of these devices. In addition, the absence of moving mechanical parts favorably affects the service life of devices. So there is every reason to take a solid-state relay - to assemble the device with your own hands.
However, in fairness, along with the positive aspects, the properties of the relay, characterized as disadvantages, should be noted. So, for the operation of high-power devices, as a rule, an additional component of the structure is required, which is designed to remove heat.
Radiators for cooling solid-state relays have overall dimensions several times greater than those of TTR, which reduces the convenience and efficiency of installation.
During operation (in the closed state), the TTR devices give a reverse leakage current and show a nonlinear current-voltage characteristic. Not all solid-state relays are permissible to use without any limitations in the characteristics of switched voltages.
Some types of devices are designed to switch only DC.The introduction of solid-state relays into the circuit usually requires an appeal to additional measures aimed at blocking false alarms.
How does a solid state relay work
The control signal (usually a low level voltage, coming, for example, from the control controller) is fed to the LED of an optoelectronic pair present in the TTP circuit. The LED begins to emit light towards the photodiode, which in turn opens and begins to pass current.
The current passing through the photodiode comes to the control electrode of the key transistor or thyristor. The key opens, closes the load circuit.
This is how the switching function of the device works. All electronics are traditionally enclosed in a monolithic case. Actually, therefore, the device was called a solid-state relay.
Types of Solid State Switches
The entire existing range of devices can be divided into groups, based on the category of plug-in load, features of control and switching voltages. Thus, in total there will be three groups:
- Devices operating in DC circuits.
- Devices operating in AC circuits.
- Universal designs.
The first group is represented by instruments with the parameters of operating control voltages of 3–32 volts. This is a relatively small-sized electronics, endowed with LED display, capable of functioning without interruption at temperatures of -35 / + 75ºС.
The second group is devices designed for installation in AC networks. Here are the designs of TTR for installation in AC networks, controlled by a voltage of 24 - 250 volts. There are devices capable of switching high power loads.
The third group - universal devices. Circuitry of this type of device supports manual configuration for use in certain conditions.
If you start from the nature of the connected load, you should distinguish two types of solid-state AC relays: single-phase and three-phase.Both types are designed for switching a sufficiently powerful load at currents of 10 - 75 A. At the same time, peak short-term values of current can reach 500 A.
Capacitive, resistive, and inductive circuits can act as a load switched by solid-state relays. Switch designs allow, without excessive noise, to smoothly control, for example, heating elements, incandescent lamps, electric motors.
The reliability of work is sufficiently high. But in many respects the stability and durability of solid-state relays depends on the quality of production of products. Thus, devices manufactured under a certain brand name “Impuls” are often marked with a short service life. On the other hand, Schneider Electric products leave no reason for criticism.
How to make TTR do it yourself
Considering the design feature of the device (monolith), the circuit is not assembled on a textolite board, as is customary, but by mounting.
Circuit solutions in this direction can be found many. The specific option depends on the required switching power and other parameters.
Electronic components for circuit assembly
The list of elements of a simple scheme for the practical development and construction of a solid-state relay do-it-yourself:
- Optopara type MOS3083.
- Triac type VT139-800.
- Transistor KT209 series.
- Resistors, Zener diode, LED.
All specified electronic components are soldered by mounting according to the following scheme:
Due to the use of the optocoupler MOS3083 in the control signal generation circuit, the input voltage can vary from 5 to 24 volts.
And due to the chain consisting of a Zener diode and a limiting resistor, the current passing through the control LED is reduced to the minimum possible. This solution ensures a long service life of the control LED.
Checking the collected scheme for performance
The assembled circuit must be tested for performance.Connect while the load voltage of 220 volts in the switching circuit through the triac is optional. It is enough to connect a measuring device - tester parallel to the switching line of the triac.
Measurement mode of the tester should be set to “MOhm” and supply power (5-24V) to the control voltage generation circuit. If everything works correctly, the tester should show the difference in resistance from “MΩ” to “KΩ”.
Device of the monolithic case
Under the base of the body of the future solid-state relay will require a plate of aluminum with a thickness of 3-5 mm. The dimensions of the plate are not critical, but must comply with the conditions for the effective removal of heat from the triac when this electronic element is heated.
The surface of the aluminum plate must be flat. Additionally, it is necessary to process both sides - clean with fine sandpaper, polish.
At the next stage, the prepared plate is equipped with “formwork” - a thick cardboard or plastic border is glued around the perimeter. It should make a kind of box, which will later be filled with epoxy resin.
An electronic circuit of a solid-state relay assembled by a “canopy” is placed inside the created box. On the surface of the aluminum plate fit only a triac.
No other parts and conductors of the circuit should touch the aluminum substrate. The triac is applied to the aluminum with that part of the case that is designed for installation on the radiator.
Heat conductive paste should be used in the contact area between the triac housing and the aluminum substrate. Some brands of triacs with uninsulated anode must be installed through a mica gasket.
The triac needs to be pressed tightly to the base with some kind of load and poured around the perimeter with epoxy glue or fixed in some way without breaking the surface of the back side of the substrate (for example, with a rivet).
Compound preparation and body pouring
Under the manufacture of a solid body of an electronic device you will need to make a compound mixture. The composition of the compound compound is made on the basis of two components:
- Epoxy without hardener.
- Alabaster powder.
Thanks to the addition of alabaster, the master solves two problems at once - he gets an exhaustive volume of the filling compound at the nominal consumption of epoxy resin and creates a fill of the optimum consistency.
The mixture must be thoroughly mixed, after which you can add a hardener and mix again thoroughly. Next, gently pour the "mounted" installation inside the cardboard box created by the compound.
Fill do to the top level, leaving on the surface only part of the head of the control LED.Initially, the surface of the compound may not look quite smooth, but after a while the picture will change. It remains only to wait for the complete solidification of the casting.
In fact, you can apply any suitable for casting solutions. The main criterion is that the casting composition should not be electrically conductive, plus a good degree of casting rigidity should be formed after solidification. The cast case of a solid-state relay is a kind of electronic circuit protection from accidental physical damage.
Conclusions and useful video on the topic
This video shows how and on the basis of any electronic components you can make a solid-state relay. The author lucidly tells about all the details of the manufacturing practice that he personally encountered in the process of manufacturing an electronic switchboard.
Video about the problem that can be encountered after the acquisition of single-phase TTP from sellers from China. Along the way, conducts a peculiar review of the device switching device.
Independent manufacture of solid-state relays is quite a possible solution, but with respect to products for low-voltage loads, which consume relatively low power.More powerful and high-voltage appliances do it yourself difficult. And this venture on finance will cost the same amount as the factory copy. So, if necessary, it is easier to buy a ready-made industrial device.