Have you noticed that all solid-state relays on the market are normally open contact switches if they can be classified according to the different contacts like electromagnetic switches (of course, this classification method is not scientific because solid-state relays are semiconductor switches).
You may wonder, do normally closed solid-state relays exist? If not, how can this function be achieved? In this article, we will provide you with a detailed explanation as well as a reliable solution. So let’s get started.
What is NC solid state relay?
NC solid-state relay is a semiconductor switch like ordinary SSR, but its contact is normally closed contact. Their working principle is: when no control signal is applied, the load is always on; when a control signal is applied, the load is off.
Now that a normally closed solid state relay works very similarly to an electromagnetic relay with normally closed contacts, but does it exist? Why don’t we see it openly sold in the market? In the following section, I will explain why.
Do NC solid state relays exist and why?
In reality, normally closed solid-state relays are not very common. Why is this so? We can start by studying the components of solid-state relays. As we mentioned in previous blogs, the output circuit of a solid-state relay is built around the thyristor in an AC output solid-state relay or the MOSFET in a DC output solid-state relay.
The concept of a normally closed solid-state relay (SSR) means that it has normally closed contacts similar to an electromagnetic relay. From the above, it can be concluded that the normally closed contacts are realized by its elements (thyristors or MOSFETs). This leads to the question: Do normally closed thyristors or MOSFETs exist?
Actually, there are no normally closed thyristors or MOSFETs. However, by using external circuits or components, it is still possible to build a normally closed solid-state relay. How? The next section will teach you how.
How to realize a solid state relay normally closed function?
As a professional solid-state relay solution provider, Lorentzzi® has also encountered similar customer inquiries and demands. They asked us to provide normally closed solid-state relays. What is our solution?
We have two solutions: The first option is to confirm whether the customer accepts changing the actuator from solid-state relay (SSR) to intermediate relay (for small load) or AC contactor (for large load). As we all know, intermediate relays and AC contactors have the same functions as solid-state relays, but they are more widely used in automation control. All three products use low and safe voltage to control high-voltage heavy loads. Intermediate relays or AC contactors can have normally closed contacts.
Our second solution is to use a normally closed intermediate relay to indirectly control the solid-state relay (SSR), which means that if no control voltage is applied to the intermediate relay, its load part circuit is in a closed state, that is, there is a control signal at the control end of the solid-state relay. When the control voltage is applied to the intermediate relay, the load is disconnected and can no longer provide a control signal to the solid-state relay, so the solid-state relay disconnects the load.
Conclusion
In summary, normally closed solid-state relays are rarely used in industrial automation. If they must be used, they can be implemented through external circuits, such as the indirect control of normally closed intermediate relays or the use of normally closed AC contactors as mentioned above.
As a leading solid-state relay solution provider in China, Lorentzzi not only provides high-quality solid-state relays, but also helps customers find the best solution for their applications. If you still have questions or want us to provide solutions, please send your inquiry to shonxu@lorentzzi.com, our team will reply within 24 hours.
