The solid state relay (TCR) is a device from the non-mechanical series of electronic components. The absence of mechanics opens more opportunities for electronics enthusiasts to make solid state relays with their own hands for personal use.
Let us consider this possibility in detail.
What is the content of the article:
- The construction and operation of a TTR
- How does a solid state relay work?
- Variants of solid state switches
- How to make a TTR with your own hands?
- Electronic components for circuit assembly
- Checking the assembled circuit for proper operation
- Design of the monolithic housing
- Compound preparation and casting the housing
- Conclusions and useful video on the subject
The design and the function of the TTR
While most of these electronics traditionally contain moving parts of the contact groups, the solid state relay has none of these parts. The circuit switching is based on the principle of an electronic key. And the role of electronic keys is usually performed 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 become familiar with the basic design of such devices and understand how they function.
In industrial production, there are solid-state relays in various configurations, designed for a variety of practical application conditions. The choice of modifications is vast
In a close study of the device, the following advantages of the TTR are immediately apparent:
- Commutation of heavy loads;
- High speed switching;
- Excellent galvanic isolation;
- Ability to hold high overloads for short periods.
In mechanical designs, one cannot really find relays with similar parameters. In general, solid state relays have an impressive list of advantages over their mechanical counterparts.
Two electronic devices that functionally switch circuits: the left one is based on a solid state design, the right one on a traditional mechanical switching system
The operating conditions for TTRs are almost unlimited. In addition, the absence of moving mechanical parts has a positive effect on the service life of the devices. So there is every reason to engage in solid-state relay - to assemble the device by your own hands.
But, for the sake of fairness, along with the positive moments should be noted properties of relays, characterized as disadvantages. For example, the operation of powerful devices usually requires an additional design component that is designed to dissipate heat.
In the case of switching a powerful load, solid state relays are almost always supplemented with powerful cooling radiators. This makes the use of SSRs somewhat more difficult
Solid state relays have several times the size of the heatsinks, which reduces the ease and rationality of installation.
Solid state relays give reverse leakage current during operation (when closed) and show a non-linear volt-ampere characteristic. Not all solid state relays can be used without restrictions on the switching voltage characteristics.
Design for use only in circuits where power is supplied by direct current. Usually these devices are characterized by small size and low switching power
Some types of devices are designed to switch direct current only. The implementation of solid state relays in a circuit usually requires the use of additional measures aimed at blocking false triggers.
Solid state relays are often found in a common apartment electric panel.
How does the solid state relay work?
The control signal (usually a low level voltage, coming for example from a control controller) is applied to the LED of the optoelectronic pair present in the TTR circuit. The LED starts to emit light towards the photodiode, which in turn opens and starts to pass current.
The generalized circuit of the TTR illustrating how the electronic device works: 1 control voltage source, 2 optocoupler inside the relay housing, 3 load current source, 4 load
The current through the photodiode arrives on the control electrode of the key transistor or thyristor. The key opens, closing the load circuit.
This is how the switching function of the device works. All electronics are traditionally enclosed in a monolithic case. This is actually why the device is called a solid state relay.
And you can read about how to wire a solid state relay in this material.
Variants of solid state switches
The entire existing range of devices can be roughly divided into groups based on the category of load connected, the features of voltage control and switching.
Thereby, in total, three groups are gathered:
- D.c. current devices.
- AC current devices.
- Universal designs.
The first group is represented by devices with 3 to 32 volt control voltages. They are relatively small-sized electronics with LED display, able to operate without interruption at -35 / +75 ºC.
Widely used electronic devices for application in single phase power networks. Other constructions are also found, but much more rarely
The second group are devices intended to be installed in AC networks. Here are RTD designs for installation in AC networks, controlled by 24 to 250 volts. There are devices capable of switching high-power loads.
The third group - devices of universal purpose. The circuitry of this type of devices supports manual adjustment for use in certain conditions.
Based on the nature of the load to be connected, two types of solid state AC relays should be distinguished: single-phase and three-phase. Both types are designed to switch sufficiently heavy loads at currents of 10 - 75 A. Peak short-time current values can reach 500 A.
Widely used in three-phase applications. Often used as a linear regulator for high-power electric heaters (TENS)
Capacitive, resistive, inductive circuits can act as loads switched by solid state relays. Switch designs allow for the smooth, noise-free control of, for example, heating elements, incandescent lamps, electric motors.
The reliability is quite high. But the stability and longevity of solid-state relays depends largely on the quality of production. Thus, devices produced under a certain brand name \"Impuls\" often have a short life time.
On the other hand, products by \"Schneider Electric\" leave no room for criticism.
How to make a TTR with your own hands?
Because of the construction feature of the device (monolithic), the circuit is not assembled on a textolite board, as is usual, but by hanging up.
This is how a self-made solid state relay design looks like. It is not difficult to make something like that. All you need is basic electronic and electrical skills. The material costs are small
Circuitry solutions in this direction can be found in plenty. The particular variant depends on the required switching power and other parameters.
Electronic components for circuit assembly
The simple circuit elements for the practical mastering and the construction of a solid state relay with your own hands are:
- A MOS3083 type optic.
- Simistor type BT139-800.
- Transistor type KT209.
- Resistors, Stabilistor, LED.
All these electronic components are soldered by surface mounting according to the following schematic:
The basic scheme of a low power solid state relay for the hands-on construction. The small number of parts and the simple pre-assembly make it very easy to solder the circuit
With the control signal conditioning the input voltage can be varied from 5 to 24 volt.
A circuit consisting of a stabilizer and a suppressor reduces the current flowing through the control LED to the minimum possible. This solution ensures a long life time of the check LED.
Checking the assembled circuit for proper functioning
The assembled circuit should be checked for proper functioning. It is not necessary to connect a 220 volt load voltage to the switching circuit via a triac. It is enough to connect a measuring device, a tester, in parallel to the switching line of the triac.
Testing the operation of the solid state relay with a measuring device. If the control voltage is applied to the input of the device, the triac junction must be open
The measurement mode of the tester must be set to \"mOhm\" and the power supply (5-24V) must be applied to the control voltage generation circuit. If everything works correctly the tester should show a resistance difference of \"mOhm\" to \"kOhm\".
Construction of the monolithic case
A 3-5 mm thick aluminium plate is needed under the base of the future solid state relay case. The size of the plate is not critical but must be suitable for the effective heat dissipation from the triac when heating this electronic element.
Frame for casting the case of the future device. Made of cardboard strip or other suitable materials. Mounted on the aluminium substrate with universal glue
The surface of the aluminium plate must be flat. In addition, both sides must be finished - sanded with a fine sandpaper, polished.
In the next step, the prepared plate is equipped with a \"formwork\" - a curb of thick cardboard or plastic is glued around the perimeter. A kind of box is created, which will be later filled with epoxy resin.
Inside the created box the electronic circuit of the solid state relay, assembled \"overhanging\", is placed. Only the triac is placed on the surface of the aluminum plate.
The triac is mounted on the aluminum substrate. The main condition is that this electronic component must be pressed tightly to the metal substrate. This is the only way to ensure good heat dissipation and reliable operation
No other circuit parts or conductors may touch the aluminum substrate. The triac is placed against the aluminum with the part of the case designed to be mounted on the heat sink.
Thermal paste should be used on the contact area between the triac and the aluminum substrate. Some brands of triacs with an uninsulated anode must necessarily be mounted over a mica gasket.
An option to mount the triac to the substrate with a rivet. On the back side the rivet is flush with the surface of the substrate
The SiMistors must be pressed firmly onto the substrate with some kind of load and the perimeter must be filled with epoxy glue or fixed in some way without disturbing the surface of the substrate (for example with a rivet).
Preparing the compound and pouring the enclosure
To make the solid body of the electronic device, a compound compound mixture will have to be made. The compound mix is made based on two components:
- Epoxy resin without hardener.
- Alabaster powder.
With the addition of alabaster, the mixer solves two problems at the same time - it obtains a full casting volume with a nominal amount of epoxy resin and creates a casting of an optimal consistency.
The mixture must be thoroughly mixed, after which the hardener can be added and thoroughly mixed again. Then carefully pour the \"hinged\" assembly inside the cardboard box with the created compound.
This is how the finished example of the solid state relay assembled by the hands looks like. Somewhat unusual and not very presentable, but reliable enough
The casting is done up to the top level, leaving only a part of the head of the control LED on the surface. Initially the surface of the compound may not look quite smooth, but after some time the picture will change. All that remains is to wait until the casting is fully cured.
In fact, any suitable casting mortar can be used. The main criterion is that the casting composition must not be electrically conductive, plus a good degree of casting rigidity must be formed after solidification. The casting of the solid state relay is a kind of protection for the electronic circuitry against accidental physical damage.
Summary and helpful video on the subject
This video shows how and on what electronic components one can make a solid state relay. The author lucidly explains all the details of the manufacturing practices he has personally encountered in making the electronic commutator:
Video about the problem one may encounter after purchasing a single phase TTR from the sellers in China. By the way gives a kind of review of the device of the switching device:
Manufacturing solid state relays is quite a possible solution, but in relation to products for low voltage loads that consume relatively low power.
More powerful and high-voltage devices are difficult to make their own hands. And this endeavor will cost the same amount of money as the factory unit. So, in case of need it is easier to buy a ready-made industrial device.
If you have any questions about solid-state relay assembly, please put them in the comment block, and we will try to answer them in a very clear way. There you can also share your experience in making your own relay or provide valuable information on the topic of the article.