Do-it-yourself solid-state relay: assembly instructions and connection tips
A solid-state relay (SSR) is a device from a series of non-mechanical electronic components.The absence of mechanics opens up more opportunities for electronics enthusiasts to make a solid-state relay with their own hands for personal use.
Let's consider this possibility in more detail.
The content of the article:
Design and principle of operation of TTR
While most such electronics traditionally contain moving parts of contact groups, a solid-state relay has no such parts at all. Circuit switching by the device circuit is carried out according to the principle of an electronic key. And the role of electronic keys is usually played by semiconductors built into the body of the relay - power transistors, triacs, thyristors.
Before trying to make a solid-state relay yourself, it is logical to familiarize yourself with the basic design of such devices and understand the principle of their operation.
As part of a thorough study of the device, one should immediately highlight the advantageous aspects of TTP:
- switching of powerful load;
- high switching speed;
- ideal galvanic isolation;
- ability to hold high overloads for a short time.
Among mechanical structures, it is really not possible to find a relay with similar parameters. In general, the advantages of solid-state relays relative to their mechanical counterparts are expressed in an impressive list.
The operating conditions for TSR practically do not limit the use of these devices. In addition, the absence of moving mechanical parts has a beneficial effect on the service life of the devices. So there is every reason to start working on a solid-state relay - to assemble the device yourself.
However, in fairness, along with the positive aspects, it is worth noting the properties of the relay, which are characterized as disadvantages. Thus, for the operation of powerful devices, as a rule, an additional design component is required, which is designed to remove heat.
Cooling radiators for solid-state relays have overall dimensions several times greater than the dimensions of solid-state relays, which reduces the convenience and rationality of installation.
TSR devices during operation (in the closed state) give a reverse leakage current and show a nonlinear current-voltage characteristic. Not all solid-state relays can be used without restrictions in the characteristics of the switched voltages.
Certain types of devices are designed to switch only direct current. The introduction of solid-state relays into a circuit usually requires additional measures aimed at blocking false alarms.
Solid state relays can often be found in general electrical panel of the apartment.
How does a solid state relay work?
The control signal (usually a low-level voltage coming, for example, from a control controller) is supplied to the LED of the optoelectronic pair present in the SSR 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 arrives at the control electrode of the key transistor or thyristor. The key opens and closes the load circuit.
This is how the switching function of the device works. All electronics are traditionally enclosed in a monolithic housing. Actually, that’s why the device was called a solid-state relay.
You can read about how to connect a solid-state relay in this material.
Types of Solid State Switches
The entire existing range of devices can be divided into groups based on the category of the connected load, features of voltage control and switching.
Thus, there will be a total of three groups:
- Devices operating in DC circuits.
- Devices operating in alternating current circuits.
- Universal designs.
The first group is represented by devices with operating control voltage parameters of 3 – 32 volts. These are relatively small-sized electronics, equipped with LED indication, capable of functioning without interruption at temperatures of -35 / +75 ºС.
The second group is devices intended for installation in alternating voltage networks. Here are the designs of TSRs 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 is devices for universal use. The circuitry of this type of device supports manual configuration for use in certain conditions.
Based on the nature of the connected load, two types of AC solid-state relays should be distinguished: single-phase and three-phase. Both types are designed for switching fairly powerful loads at currents of 10 - 75 A. In this case, peak short-term current values can reach 500 A.
The load switched by solid-state relays can be capacitive, resistive, or inductive circuits. The switch designs allow smooth control of, for example, heating elements, incandescent lamps, and electric motors without unnecessary noise.
Reliability of operation is quite high. But in many ways, the stability and durability of solid-state relays depends on the quality of product manufacturing. Thus, devices produced under a certain brand name “Impuls” are often noted for their short service life.
On the other hand, Schneider Electric products leave no room for criticism.
How to make a TTP with your own hands?
Considering the design feature of the device (monolith), the circuit is assembled not on a textolite board, as is customary, but by surface mounting.
There are many circuit solutions in this direction that can be found. The specific option depends on the required switching power and other parameters.
Electronic components for circuit assembly
The list of elements of a simple circuit for practical development and construction of a solid-state relay with your own hands is as follows:
- Optocoupler type MOS3083.
- Triac type VT139-800.
- Transistor series KT209.
- Resistors, zener diode, LED.
All specified electronic components are soldered by surface mounting according to the following diagram:
Thanks to the use of the MOS3083 optocoupler in the control signal generation circuit, the input voltage can vary from 5 to 24 volts.
And due to a 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 assembled circuit for functionality
The assembled circuit must be checked for functionality. In this case, it is not necessary to connect a load voltage of 220 volts to the switching circuit through a triac. It is enough to connect a measuring device – tester – parallel to the triac commutation line.
The measurement mode of the tester must be set to “mOhm” and power (5-24V) must be supplied to the control voltage generation circuit. If everything is working correctly, the tester should show a difference in resistance from “mOhm” to “kOhm”.
Construction of a monolithic body
Under the base of the housing of the future solid-state relay, you will need an aluminum plate 3-5 mm thick. The dimensions of the plate are not critical, but must correspond to the conditions for effective heat removal from the triac when heating this electronic element.
The surface of the aluminum plate must be flat. Additionally, both sides need to be processed - cleaned with fine sandpaper and polished.
At the next stage, the prepared plate is equipped with “formwork” - a border made of thick cardboard or plastic is glued around the perimeter. You should get a kind of box, which will later be filled with epoxy resin.
An electronic solid-state relay circuit assembled with a “canopy” is placed inside the created box. Only the triac is placed on the surface of the aluminum plate.
No other circuit parts or conductors should touch the aluminum substrate. The triac is attached to the aluminum with that part of the housing that is designed for installation on a radiator.
Heat-conducting paste should be used on the contact area of the triac body and the aluminum substrate. Some brands of triacs with a non-insulated anode must be installed through a mica gasket.
The triac must be pressed tightly to the base with some kind of weight and filled with epoxy glue around the perimeter or secured in some way without disturbing the smooth surface of the back side of the substrate (for example, with a rivet).
Preparing the compound and pouring the body
To manufacture the solid body of an electronic device, you will need to make a compound mixture. The composition of the compound mixture is made on the basis of two components:
- Epoxy resin without hardener.
- Alabaster powder.
Thanks to the addition of alabaster, the master solves two problems at once - he obtains an exhaustive volume of the pouring compound at the nominal consumption of epoxy resin and creates a fill of optimal consistency.
The mixture must be thoroughly mixed, after which you can add the hardener and mix thoroughly again. Next, the “hinged” installation inside the cardboard box is carefully filled with the created compound.
Filling is done to the top level, leaving only part of the control LED head on the surface. Initially, the surface of the compound may not look entirely smooth, but after some time the picture will change. All that remains is to wait for the casting to completely harden.
In fact, you can use any solution suitable for casting.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 solid-state relay housing is a kind of protection for the electronic circuit from accidental physical damage.
Conclusions and useful video on the topic
This video shows how and on the basis of what electronic components a solid-state relay can be made. The author clearly talks about all the details of manufacturing practices that he personally encountered during the production of an electronic switch:
Video about a problem that you may encounter after purchasing a single-phase SSR from sellers from China. Along the way, he conducts a kind of review of the design of the switching device:
Self-manufacturing of solid-state relays is a completely possible solution, but in relation to products for low-voltage loads that consume relatively low power.
It is difficult to make more powerful and high-voltage devices with your own hands. And this financial venture will cost the same amount as the factory copy is valued at. So, if necessary, it is easier to buy a ready-made industrial device.
If you have questions about assembling a solid-state relay, please ask them in the comments section, and we will try to give them a very clear answer. There you can share your experience of making relays yourself or provide valuable information on the topic of the article.
