Features of using solid state relays

Features of using solid state relays

When using practically any electrical circuits, there is a need to turn on or turn off electronic devices, devices, units, etc. The equipment used for this belongs to the category of switching equipment, and includes switches, knife switches, contactors, relays, etc.

One of the devices that fall under the category of switching equipment are solid state relays . What is a solid state relay? A solid state relay is a device built on semiconductor elements and power switches, such as triacs, bipolar, or MOSFETs. As well as electromagnetic relays, they are designed to control a weak signal load with a large voltage or current.

Solid State Relays are unique devices that require little maintenance once installed. For example, they do not need to clean the contact group, since the very concept of "contact group" is absent from solid-state relays at all. The advantages of solid state relays over electromagnetic ones are well known. These are small dimensions, absence of noise and vibration, long service life, absence of sparking, constant output impedance, which does not change during the service life. But solid-state relays also have their own application features that cannot be ignored during their operation. And questions to the technical support service confirm this.

The block diagram of a solid state relay with constant current control is shown in the figure:

As you can see, the relay is controlled by an LED installed inside the relay housing. The question immediately arises - what voltage should be applied to the input of the relay, that it "turned on" or "turned off"? For HHG1 series relays, the control voltage is specified as 3-32VDC. And with this, as it were, there are no questions. But it must be borne in mind that in order to “turn off” the relay, the control voltage must be turned off completely. According to the documentation for this relay, the cut-off voltage is 1VDC. That is, when the control voltage drops from 3VDC to 1VDC, the relay will remain open. And if we take into account the possible presence of electromagnetic "pickup" in the relay control circuit, then a situation may arise that the relay will remain on all the time.

Further, it is impossible to check the operability of the relay by connecting a tester to the output contacts and “ringing” it. Unlike an electromagnetic relay, in which the switching circuit has only two states - “closed” and “open” - solid-state relays have power switches built on triacs, or transistors. You can check the performance of a solid state relay by assembling the simplest device shown in the figure

In addition, solid state relays have their own characteristics for load selection. When choosing a solid state relay, pay special attention to the load current. When operating a solid-state relay, it is worth considering not only the operating current, but also the currents that occur during the start-up process, which can exceed the nominal parameter by several times. As a rule, solid state relays withstand 10 times the current overload for 10 milliseconds. In other words, with a 10-fold current overload lasting more than 10 milliseconds, the relay is practically guaranteed to fail.

The values of exceeding the maximum current in relation to the nominal are displayed in the table:

Application current margin
For heating elements 30-40% more than the nominal value
For asynchronous motors 6-10 times
For incandescent lamps 8-12 times
Electromagnetic relays 4-10 times

Therefore, for solid-state relays, it is recommended: for an active load (lamps, heating elements), the margin for rated current is 2-4 times . When starting asynchronous motors, due to the large starting current, the current margin must be increased up to 6-10 times . It is possible to somewhat reduce the current margin by providing for starting the engine at idle with a minimum speed.

An important factor in the operation of solid-state relays is the provision of thermal conditions. Solid state relays are capable of conducting the current declared by the manufacturer at temperatures not exceeding 40°C. When heated for every 10°C, the ability of the relay to pass current decreases by 20-25%. That is, at a temperature of 80°C, the relay is practically not able to pass current.

And finally - to protect the solid state relay from failure due to a short circuit, it makes sense to protect the load by installing class B circuit breakers.

It may seem that the transition from conventional relays and contactors to solid state relays is too complicated. Indeed, such a transition requires a more responsible approach. But with the right choice of a solid state relay and the provision of appropriate conditions for its performance, the benefits will manifest themselves in the first place.