Abstract: This paper compares the similarities and differences between electromagnetic relays and solid-state relays, conducts a preliminary analysis of their relevant parameters and concepts, discusses the doubts and controversies surrounding parameters such as the turn-on voltage and turn-off voltage of solid-state relays, and puts forward views and suggestions on related issues. Keywords: solid-state relay, electromagnetic relay; turn-off voltage 1 Introduction [The General Specification for Solid-State Relays GJB1515-92L] is China's first military standard for solid-state relays, laying the foundation for the research, testing, production, and use of military solid-state relays in China. As it was the first standard, imperfections were inevitable. Therefore, some doubts arose during the implementation of this standard, such as regarding the "turn-on voltage" and "turn-off voltage." Its upgraded version, GJB1515A-201L, fundamentally modified the turn-on voltage and turn-off voltage, making their parameters more scientific and reasonable. However, doubts still exist regarding these two parameters, because these two parameters of solid-state relays are similar to the "operating voltage" and "release voltage" of electromagnetic relays; these two pairs are important parameters and best reflect the similarities between the two types of relays. Because solid-state relays have a relatively short history of development, many people lack a deep understanding of their mechanisms and fail to grasp the essential differences between them and electromagnetic relays. They often treat them with the same concepts as electromagnetic relays, or even completely regard them as electromagnetic relays, leading to misunderstandings and biases. This section focuses on the "on voltage" and "off voltage" parameters of solid-state relays. 2. Solid-State Relays vs. Electromagnetic Relays Electromagnetic relays generate macroscopic mechanical motion under the action of electromagnetic force, using the engagement and disengagement of objects to achieve circuit switching. Solid-state relays, on the other hand, utilize semiconductor technology, under the control of external physical quantities, to generate microscopic electron or hole movement within the semiconductor to achieve the switching of external circuits. Therefore, the working mechanisms of these two types of relays are fundamentally different. The following comparative analysis will clarify their main similarities and differences. 2.1 GJB1515-92 and GJB1515A-2001 GJB1515-92 adopts the test methods for operating voltage and release voltage in the national military standard for electromagnetic relays for testing the turn-on and turn-off voltages, i.e., first applying rated excitation, then testing using the ramp method. GJB1515A-2001, however, directly applies the specified values ​​for compliance judgment (i.e., the step method), only examining whether it is qualified, while directly testing the measured values ​​using the ramp method. GJB1515-92 clearly defines the measured and specified values ​​of the turn-on and turn-off voltages, while GJB1515A-2001 explicitly defines the specified values ​​as the input turn-on and turn-off voltages, and defines the measured values ​​and their test methods as the guaranteed turn-on and turn-off voltages. It is evident that GJB1515A-2001, based on the actual situation of solid-state relays, adopts the step test method for the input turn-on and turn-off voltages. The ramp test method, which could potentially damage components, was abandoned. 2.2 Characteristic Curves Taking a DC electromagnetic relay (monostable, one set of normally open contacts) and a DC solid-state relay (one set of normally open outputs) as examples, the test circuit uses the same configuration. The input-output waveforms and characteristic curves of these two types of relays were tested using the ramp method. As shown in Figures 1-4: [img=438,229]http://www.chuandong.com/uploadpic/THESIS/2009/4/200904051707228430919.jpg[/img] [img=438,559]http://www.chuandong.com/uploadpic/THESIS/2009/4/20090405170801578217.jpg[/img] It should be noted that: "slope method" is an industry term for the input waveforms in Figures 1 and 3; for simplicity, the output voltage transition region of the solid-state relay is assumed to be linear in Figures 3 and 4, and the nonlinear transition regions at both ends of the slope are approximated as two intersection points; if a perpendicular line is drawn from the apex of the slope in Figures 1 and 3 respectively, and the right side of the figure is folded to the left along the perpendicular line. The results are compared with those in Figures 2 and 4. Comparing Figures 1 and 3 and Figures 2 and 4, it can be seen that: (1) In Figures 1 and 2, when the input voltage increases or decreases, the corresponding output voltage jump is the measured action or release voltage value, and the latter is less than the former. Obviously, there are two clear output jump points in a test cycle. In Figures 3 and 4, when the input voltage increases, the output voltage does not jump. Instead, there is a continuous transition zone. The measured input voltage values ​​corresponding to the first and second transition points are the guaranteed turn-off voltage and turn-on voltage defined in the specification, respectively. When the input voltage decreases, the output voltage returns along the original path through the second and first transition points, respectively. For DC solid-state relays, when the output voltage is in the transition zone, the output device and the load are voltage-divided, which will greatly increase the power consumption of the output device and may cause permanent damage. In order to correctly determine these two transition points, the same criteria are given in both specifications. Because the criteria propose minimum and maximum value requirements, some people apply the requirements of measured values ​​to the specified values: taking the minimum and maximum values ​​as the upper and lower limits of the input turn-on voltage and turn-off voltage, respectively, resulting in contradictions among the parameters. (2) In Figures 1 and 2, once the electromagnetic relay is activated (or released), it remains in the activated (or released) state. When the input voltage changes, the output does not return along the original path, but takes another route back to the previous state, thus forming the rectangular window in Figure 2. This window is similar to the hysteresis loop and the hysteresis comparison curve of the Schmitt trigger. This is the "relay characteristic". Due to the relay characteristic, the contacts have instantaneous movement, that is, the contacts can quickly switch from one state to another. This is the practical value of the electromagnetic relay. If the electromagnetic relay does not have this characteristic, assuming that the displacement of the contacts is linear with the coil voltage, then under actual load, its slow-moving contacts will be damaged. The rectangular window in Figure 2 corresponds to a broken line in Figure 4, that is, the solid relay does not have the relay characteristic, and its output is approximately linear with the input. This is the essential difference between the two types of relays. However, because the input devices of DC solid-state relays have an amplification function, the input voltage range corresponding to the amplification region is actually quite narrow. If the rate of change of the input voltage can be guaranteed not to be lower than a certain limit value, the output will pass through the transition region quickly without damaging the output device. If the window area in Figure 2 and the transition region in Figure 4 are covered, the remaining parts are exactly the same. If the input voltage in Figure 4 can be guaranteed to be outside the region between the specified turn-on and turn-off voltages, its switching characteristics are the same as those of an electromagnetic relay. Since G. IBL515-92 does not explicitly define the measured and specified values ​​of the turn-on and turn-off voltages, this set of parameters may be a certain range or a specific value. It should be ensured that the turn-on and turn-off voltages are specific measured values ​​for a single product, corresponding to the two transition points in Figure 4. It should be ensured that the turn-off and turn-on voltages are the upper and lower limits of the error range of the turn-off and turn-on regions, respectively. If this limit refers to all products of the same specification, it is an uncertain value; if it is specific to a particular product, it is the measured value. Although there are also two points in Figure 4 when the input turn-on and turn-off voltages are the specified values. However, it limits the upper and lower limits of the measured values ​​of the turn-on and turn-off voltages. This ensures that the turn-on and turn-off voltages are individual attributes of each product, while the input turn-on and turn-off voltages are specified by the manufacturer. These are common attributes that products of this specification must adhere to. 2.3 Test Methods The operating and release voltage parameters of electromagnetic relays generally use the ramp test method. However, applying this method to DC solid-state relays has significant drawbacks. Under rated load, when passing through the transition zone, especially slowly or even stopping in the middle of the transition zone, the power device T operates in the amplification region, reaching its maximum power consumption, far exceeding the device's power limit, causing the junction temperature to rapidly exceed the limit and burn out instantly. Even if not in the middle position, the device has a certain self-healing ability and may not be damaged, but this near-destructive test without any protective measures repeatedly damages the device's performance, and its cumulative effect will reduce its performance. This is especially true in the breaking life test. High requirements are placed on the leading and trailing edges of the input voltage switching waveform to prevent the power device from repeatedly passing through the amplification region for too long, resulting in excessive heat accumulation and high average temperature rise, which could damage the device. However, the test requires a ramp test method with a rated load. This ramp with extremely flat leading and trailing edges is very easy to damage the device. The measured action and release voltage parameters of electromagnetic relays are very easy to test; even without a load on the contacts, the pull-in and release values ​​can be measured relatively accurately. However, this cannot be achieved with solid-state relays. In addition, the measured value is of great significance for samples or semi-finished products under development in determining circuit parameters and improving the circuit sequence; however, it has little value for obtaining measured values ​​for finished products. This is because the real concern in practical applications is whether the solid-state relay can operate reliably in the switching state under specified input conditions. Therefore, the input turn-on and turn-off voltages are the most practically valuable. It is recommended to treat the input turn-on and turn-off voltages only as routine test parameters, and to use the guaranteed turn-on and turn-off voltages as arbitration tests. This is exactly the opposite of electromagnetic relays: routine testing uses the ramp method, not the step method. In fact, this matches the output characteristics of both. In conventional testing, the ramp method corresponds to the step characteristic of electromagnetic relays, while the step method corresponds to the ramp (turn-off) characteristic of solid-state relays. For arbitration testing, the test methods are interchanged. When it is necessary to use the ramp method with a rated load to measure the guaranteed turn-on and turn-off voltages of solid-state relays, the following method is recommended to effectively prevent device damage: Adjust the input voltage so that U_o gradually increases from the rated turn-off or input turn-off voltage (i.e., 0V). The output gradually approaches the first turn-off point along the turn-off region. When the output leakage current increases, this is the "guaranteed turn-off voltage." At this point, U_o should not be increased slowly but should be increased to the rated turn-on voltage or input turn-on voltage as quickly as possible, causing the output to quickly pass through the turn-off region and enter the conduction region. Then, gradually decrease U_o from the rated or specified value, causing the output to approach the second turn-off point along the conduction region. When the turn-on voltage drop increases, this is the "guaranteed turn-on voltage." Finally, quickly return to zero, causing the output to quickly pass through the turn-off region and return to the turn-off region. The test is complete. In addition, to address the defects in the transition zone of solid-state relays and the potential for overload or short circuit during use, appropriate protection measures and other practical functions can be added to the circuit to improve the product's cost-effectiveness and market competitiveness. 2.4 Load Conditions The contact load conditions for electromagnetic relays: Regardless of whether they operate in DC or AC, and the size and nature of the load, uniform specifications are provided for DC 10mA/6V, 100mA/28V, 1000mA/24V, etc. However, the test conditions for parameters such as the on and off voltages of solid-state relays require the output to be under rated load. Since the contact load capacity of electromagnetic relays can be guaranteed by design and materials, the above-mentioned loads are used instead of the rated load in the test to avoid damaging the initial state and performance of the contacts. The output of a solid-state relay is accomplished by semiconductor devices. It is equivalent to the contacts in an electromagnetic relay, but its switching process is microscopic. The load capacity and reliability of all products of a certain specification depend on the selected power device that performs the switching function, unlike the contacts of an electromagnetic relay which have visibility, consistency, and determinism. Furthermore, early solid-state relays used bipolar transistors as output elements. Because these are current-controlled devices, the collector current and base current must satisfy a certain relationship to operate in the saturation region when the relay is turned on. This causes the relay's turn-on and turn-off voltage parameters to vary with the output current. For example, when the output current is small, a small input voltage can saturate the output device. However, when the output current increases, the original input voltage is no longer sufficient for saturation, causing the output device to exit the saturation region and enter the amplification region. This not only results in the loss of switching characteristics but also carries the risk of device damage. Therefore, it is essential to test each relay under rated load conditions. This ensures that the measured parameters cover all load conditions below the rated load and repeatedly verifies and confirms the product's load capacity. The reliability of solid-state relays largely depends on the semiconductor device manufacturer. Newer DC solid-state relays widely use high-performance field-effect transistors (FETs) as output elements, significantly improving product reliability. Simultaneously, because this component is a voltage-controlled device, the relay's turn-on and turn-off voltage parameters are largely unaffected by the output current, allowing for complete isolation. It is recommended to follow the approach for electromagnetic relays and use a standardized sequence of small loads instead of the rated load to test solid-state relays. During the design phase, relevant tests are conducted on the relay of this specification to obtain the necessary data, providing a basis for load testing during subsequent production, testing, and use. A uniform load can be used for testing after the product is finished. This simplifies the corresponding testing instruments and allows subsequent tests to use the ramp method without damaging the components. 2.5 Parameters and Curves To ensure the reliable operation of the electromagnetic relay, the coil should be subjected to the optimal operating or release voltage, namely the rated operating voltage and zero volt of the coil. The rated operating voltage of the electromagnetic relay should be the "rated operating voltage," and zero volt should be the "rated release voltage." Similarly, the rated input voltage of the solid-state relay should be the "rated on-state voltage," and zero volt should be the "rated off-state voltage." Based on this concept, re-observing the distribution of parameters such as "measured value," "specified value," and "rated value" in Figures 1-4 will clearly show the similarities and differences between the parameter sequences of these two types of relays, and deeply understand the connotation of each parameter and their interrelationships. The curves in Figures 2 and 4 are divided into "five zones and two points." "Five zones": "Individual zone," "error zone" (two), "operating zone" (two); "Two points": reliable switching point. The "individual zone" refers to the area between two measured values, representing the individual attribute of a single product. This area is the non-operating zone of the electromagnetic relay and the prohibited zone for the solid-state relay. The "error zone" refers to the area between the measured value and the specified value, a tolerance zone that accommodates the dispersion of the measured values ​​of the product. This area is the unreliable operating zone. The "operating zone" refers to the area between the specified value and the rated value, where the product performs its function. The "two points" refer to the two endpoints of the curve, i.e., the two "rated values," which are the two reliable operating points that guarantee the switching performance and other performance of the product. The purpose of dividing the relay into "five zones and two points" is simply to gain a general understanding of the commonalities in the parameter sequences and their arrangement of these two types of relays, and also to provide a graphical summary. The division of the "operating zone" is basically reasonable for solid-state relays, as it ensures both their function and performance. Even in the "error zone," as long as they are far from the "individual zone," they can still work normally. However, this is not entirely true for electromagnetic relays. While the "operating zone" ensures their function, the definition of "internal zone" is not always consistent. However, it does not necessarily guarantee its performance, because the pressure between the contacts is proportional to the excitation value. The closer it is to the rated value, the more reliable its performance. Therefore, electromagnetic relays are most reliable when T is applied to "two points". 3 Conclusion Through a rough comparison of the two types of relays, we can roughly distinguish their respective related parameters and concepts. DC electromagnetic relays and solid-state relays are different except for the similarity in release and turn-off, and pull-in and turn-on characteristics. [b]References:[/b] [1] GJB1515—92 "General Specification for Solid-State Relays" [S], Beijing: Commission of Science, Technology and Industry for National Defense. 1992. [2] Zheng Kunhe. Concept and application of solid-state relay turn-on voltage [J]. Electromechanical Components, 1999 (4): 14-15. [3] Yang Shu, Kou Yujie. Discussion on the article "Concept and application of solid-state relay turn-on voltage" [J]. Electromechanical Components, 2000 (2): 35-37. [4] Wang Jue. Accurate understanding and application of the concepts of "on voltage" and "off voltage" of solid-state relays [J]. Electromechanical Components, 2000(2): 38-39. [5] GJBl5l5A-2001 "General Specification for Solid-State Relays" [S]. Beijing: General Armaments Department of the Chinese People's Liberation Army. 2001. [6] Li Chao, Li Du. Design of a practical DC motor control [J]. Modern Electronics Technology, 2008, 31(15): 105-106. Click to download: A brief discussion on the on and off voltages of solid-state relays