When using solid-state relays, we may encounter 6 common problems: overheating, over-voltage, over-current, load cables firing, LED indicator not lighting up, and failure to operate the load.
What should we do when these problems occur?You are lucky to have found this article, which will help you fully understand the reasons and our best solutions for these problems. Let’s get started now!
What is a solid state relay?
There is an old Chinese saying: “Sharpening your axe will not delay your job of chopping wood. Instead, it will help you efficiently do your work.” Therefore, In order to better analyze the causes of solid-state relay problems, first we should know what a solid-state relay is.
Unlike power relays, intermediate relays or AC contactors that connect or disconnect loads through electromagnetic effects, solid-state relays are a new type of semiconductor switch without moving parts.
Its working principle can be simply understood as follows: after a small voltage (usually DC3-32V or AC70-280V) is applied to its control end, the load end is closed. Once the control signal is removed, the load part will be disconnected accordingly.
It is mainly used as an actuator controlled by a PID temperature controller in a temperature control system, and can also be used as a switch to control the start and stop of a motor. It is a good substitute for contactors or mechanical relays, with the advantages of fast switching time, no sparks, and no noise.
Tip: Want to learn more about the differences between solid-state relays (SSRs) and AC contactors or intermediate relays? See this article: SSR Vs. AC Contactor: Which Is Better In A Temperature Control System?
Solid state relay problems and reasons
Although there are many advantages to using solid-state relays instead of intermediate relays or AC contactors, there are still 6 common problems in their use:
Overheating:
Overheating means that after prolonged use, the temperature of the solid state relay will exceed 80℃, which is a serious problem because the internal semiconductors will be permanently damaged due to such high temperatures.
Over-current:
Figure 1 SSR specifications, courtesy: https://docs.rs-online.com/5800/A700000009423612.pdf
Over-current means that the current required by the load far exceeds the maximum output current of the solid-state relay. This is generally caused by improper selection. For example, if a 10A AC solid-state relay is used to control a 220 VAC 1 kw motor, when the motor starts, its instantaneous current can reach 30 A. Such a large current far exceeds the maximum withstand current of the solid-state relay thyristor (see the above figure 1: 110 amps within 20 ms), and generally the motor startup duration exceeds 20 ms(see below figure 2), which will cause the thyristor to break down and damage the solid-state relay.
Figure 2 motor starting current curve,courtesy: https://myelectrical.com/notes/entryid/77/motor-starting-introduction
Over-voltage:
Over-voltage: As can be seen from Figure 1, the Peak Repetitive Reverse Voltage(VRRM) is 600V. If the load input voltage exceeds 600V, the thyristor will reverse breakdown, resulting in uncontrolled conduction or even damage.
Load cables overheating or fire:
Figure 3 Different current wire selection, courtesy: https://support.boshart.com/what-is-wire-gauge
Improper load wire may cause the solid-state relay load wire to overheat, thus causing a fire. For example, the single phase solid state relay is 40 amps, the suitable single core wire diameter should be more than 2.6 mm. Please refer to Figure 3 above for more options.
LED indicator not lighting up:
This problem is usually caused by loose input terminal screws or broken control signal wires.
Unable to operate the load:
Another problem is that the solid state relay cannot turn the load on or off by the control signal. This may be due to its insufficient load capacity to control the load. Therefore, if the load is a resistive load, we should amplify the SSR current to twice the rated current of the resistive load. If the load is a motor, we should use the SSR current to 5-7 times the rated current of the motor.
Solutions for solid state relay problems
Solution for overheating:
Overheating is usually caused by the large power loss of the internal components of the SSR. Therefore, when designing and using solid-state relays, you should pay close attention to the heat distribution of each component and take effective heat dissipation measures, such as using a passive heat radiator and adding an active heat dissipation fan.
Solution for over-current:
Overcurrent is caused by the load current exceeding the rated current of the SSR. In order to avoid overcurrent, when selecting a solid-state relay, the rated current value should be selected according to the actual current demand of the load, or an overcurrent protection circuit such as a current limiting resistor, a fast fuse, etc. should be installed on the load part of the solid-state relay.
Solution for over-voltage:
Over-voltage is caused by the input voltage at the load end exceeding the rated 600V. Therefore, make sure the voltage is within 600V when using a solid-state relay. In addition, an over-voltage protection circuit such as a varistor can be added to the load end of the solid-state relay as shown below, its working principle is that when an over-voltage enters the circuit, the varistor becomes very low to absorb the over-voltage.
Solution for load line overheating or fire:
To choose the right cable for your controlled load, please refer to the figure below:
Solution for LED indicator not lighting up:
Check whether the control signal wires are well or not, if the wires are broken, please change to another ones, if it is caused by the control terminal screw loose, it is better to change the SSR to a new one.
Solution for unable to operate the load:
First, check whether the SSR is damaged due to overheating, over-voltage, over-current, etc. These three reasons will cause permanent damage to the internal control components of the SSR, resulting in failure to switch the load smoothly.
If not caused by SSR damage, check whether the SSR output current is enough to operate the load, as mentioned above, for resistive load, the SSR current should be twice than its rated current, for the inductive or capacitive load such as motor, the SSR output current should be 5-7 times than its rated current.
A quick overview of SSR issues and solutions
| Problem | Cause | Solution |
|---|---|---|
|
Overheat |
Heat dissipation is not enough |
Use heat radiator or cooling fans |
|
Over-voltage |
Input voltage is too high |
Add a varistor to absorb the extra voltage |
|
Over-current |
Load consumes too much current |
Use fast fuse to protecting |
|
LED indicator not lighting up |
Control signal cables are broken, SSR terminals are loose |
Change to new cables or replace a new SSR |
|
Load cables fire |
Improper selection of the cables |
Select the wire according to its maximum current |
|
Unable to run the load |
The selected SSR is too small or SSR damaged |
Choosing the SSR according to different loads |
Conclusion
After reading the above, we hope we can help you understand the problems that may occur during the use of SSR: overheating, over-voltage, over-current, load cables firing, LED indicators not lighting up, and failure to operate the load, and how to solve these problems.
Lorentzzi Electric, as a leading solid-state relay (SSR) supplier in China, has a very professional engineering team with rich experience in manufacturing SSRs and solving SSR problems. If you still have questions, please contact us immediately and we will reply to your email within 24 hours.
