Solid state current relay: control, connection
In automation systems, control of power circuits using a low-voltage signal uses switches, called relays. They come in many forms and devices. The simplest ones contain a winding on the core and contacts. When a control voltage is applied to the winding, a magnetic field arises in the core, which attracts the contacts. They close or open the load circuit. Along with electromagnetic, a new generation of relays — solid-state, possessing several advantages — are finding increasing use.
What is solid-state current relay
A device designed for switching high-voltage circuits of direct or alternating current using low-voltage control voltage and operating on the basis of semiconductor connections in thyristors, triacs and transistors is called a solid-state relay.
It is placed in a square case, which has mounting holes and a metal platform to install it on the radiator.Connection of the solid-state relay to the control and switched lines is carried out with the help of threaded clamps.
A solid-state relay consists of several functional blocks:
- Control input. A control voltage is applied to this input. It can be variable or constant, depending on the purpose of the relay. As a rule, this voltage is low voltage. It activates the galvanic isolation unit. In the event that the control current is variable, there is a rectifying bridge with diodes and a smoothing filter between the input and the decoupling unit.
- Block galvanic isolation. It is built on an optocoupler and is used to transmit a control signal to the power switch without transferring voltage.
- The power unit is based on a thyristor, a triac or a transistor for switching high-voltage direct current voltage. The control output of the thyristor receives the voltage from the photodetector of the optocoupler. Under the action of this opens the corresponding p-n junction, and the power switch closes the load circuit.
- The power key protection circuit implemented on the varistor is designed to protect the thyristor against overvoltage.
Since the power semiconductor transmits fairly large currents, it is often installed on a cooling radiator that removes excess heat.
Types of classification of solid-state relays
1. By the type of voltage that it can commute.
Solid state relay AC in single-phase circuits:
- when you need to include loads of resistive type or inductance;
- for three-phase circuits connected by a “star” or a “delta”, in the case when each phase is connected with its own relay.
TTP, which manages a three-phase network:
- in such a circuit, you can connect and disconnect only the resistive nature of the load;
- at the same time, all types of switching of the three phases commute.
DC Solid State Relays:
- This device is good to manage such a load that operates on direct current.
2. Depending on which range of electrical voltage that is present at the power section, a switching device can operate.
TTR overlapping standard voltage variation:
- when you need to control alternating currents, reaching up to 380 V voltage.
Solid-state relay, which is capable of switching in the range of currents of constant polarity:
- for control ranging from 20 to 250 V.
Depending on which signal is controlled:
- constant current with a voltage value of 3 to 32 V;
- alternating type of current with voltage value from 90 to 250 V;
- when the load circuit resistor replaceable resistance are controlled (manual mode);
- control of a solid-state relay with an analog signal up to 10 V.
3. By the principle of switching.
TTR, where control over zero crossing is applied:
- switching of loads of resistor type is possible: incandescent lamps, elements for heating;
- capacitance switching: filter systems that perform a smoothing function, etc .;
- connect a small inductive load: solenoids, solenoid valves.
A solid-state relay of this type allows to smooth the current of the initial increase and reduce electromagnetic interference, which reduces the wear of the connected load.
TTR enable instant (random):
- suitable for loads of heating elements, tungsten lamps;
- switches inductive loads: low power electric motors, transformers.
TTR to control the phase mode
It serves not only for switching, but also for changing the voltage in the load circuit:
- allows you to adjust the power elements of the heating type;
- You can change the brightness of incandescent bulbs;
- allows you to control the engine speed.
What you need to know when choosing a relay
Since the operation of the elements of the semiconductor (power switches) occur electrical losses, this is reflected in their intense heating. When the temperature rises, the ability to pass current by the device decreases. A specific relay can provide its technical parameters when it is heated up to 40 degrees. At 65 degrees, the TTP switching capacity drops sharply, and further heating will cause it to fail. Therefore, selecting a solid-state relay for the load, it is necessary to take it with a current margin. And in powerful power circuits, TTR should be placed on radiators and forced cooling systems should be arranged.
Also, when connecting any load at the first moment of time, shock currents arise, which are several times and sometimes an order of magnitude higher than the nominal.
Characteristics of the load relays on starting currents
When connecting the solid-state relay to the load, you need to know the characteristics of the latter:
- active loads (TEN heaters) create insignificant current surges, they can be leveled using TTP, where the inclusion is in zero;
- incandescent lamps and halogen lamps, where from 7 to 12 times more current passes through them than the rated current;
- fluorescent lamps for a time up to 10 seconds give a ripple of current 5 times higher than the nominal, or even an order of magnitude;
- mercury lamps for up to five minutes can overload the circuit with currents overestimated by 3 times;
- Electromagnetic relays of alternating electricity for a time up to two periods undergo jumps of 3-10 times;
- the current in the coils of the solenoid is 1-2 orders of magnitude higher than the nominal value of a tenth of a second;
- electric motors in half a second give an increase in current up to 10 times;
- devices of highly inductance nature with saturation cores (transformers when they are idling) are switched on in the zero phase voltage, the current reaches surges up to 20 or even 40 times longer than the nominal value in time of 0.05-0.20 seconds;
- when capacitive loads in the phase are about 90 degrees, the current exceeds the nominal value by 20–40 times in a time up to 10 milliseconds.
Solid state relay do it yourself
When assembling a relay circuit at home, the main thing is to take into account the shock currents of the connected load and choose thyristors and triacs of the appropriate power margin.
For the rest of the relay circuit, you should follow the recommendations:
- for switching small alternating currents, it is better to use triacs, large ones - thyristors;
- to install a varistor or protective diode to protect the power switch parallel to it (when switching inductive load);
- put current-limiting resistors in front of the diode bridge at the input of the control circuit;
- In succession, the optocoupler must be installed with a current-limiting resistance, in parallel with a smoothing capacitor.
Advantages of Solid State Relays
TTP is not a bulky device that takes up a lot of space. It also has no mechanically moving parts, which significantly increases its reliability in comparison with electromagnetic systems. In addition, the relay provides:
- fast switching;
- insignificant noise level at the moment of connecting the load;
- low power consumption;
- the absence of discharges of arc nature inside the case;
- resistance to vibrations.