Electromechanical Relays Versus Solid

The market is full of products that claim extraordinary performances. In the electrical world there is an abundance of options for power supplies, DC-DC converters and redundant power solutions. You could easily get overwhelmed when considering all the possibilities. How do you find the best one? How do you find the best fit for your project or solution?  I’m at technician by trade; I’ve been installing, testing, breaking and fixing electrical and electronic equipment since I was 14 years old. When someone shows me a spec sheet of something exciting, I always say “Let’s test it.”

Recently, we have been developing and integrating remote management and redundant equipment for some customers’ solutions. There are several products for single phase redundant power solutions, most of them are based on electromechanical relays (EMR’s), and after testing some of them we have faced some challenges, which has made us look for other alternatives such a Solid State Relays (SSR’s).

Below is a list of highlights for SSRs vs EMRs:

•  Service Life: Because of solid state technology, the SSR definitely exhibits a longer operational life. Since there are no moving parts to jam, degrade or warp, the life span is virtually infinite.

•  Reliability: During initial operation, both types of relay will exhibit similar levels of reliability. Over time, however, the solid state relay will gain the edge for the same reasons it has a longer service life, there are no moving parts. Also, bounce free operation increases reliability and ensures consistent load control.

•  Isolation Voltage: Again, by the characteristics of construction, the solid state relay will almost always exhibit higher input to output isolation voltages than an electromechanical relay. For many telecommunication applications, a minimum of 3750VAC is desired, clearly making the SSR the optimal choice in telecom design.

•  On Resistance: Electromechanical relays have an On Resistance in the range of 100 milliohms, whereas SSRs have an On Resistance in the range of 10 Ohms. The higher On Resistance of SSRs is due to the nature of the MOSFET. The low On Resistance of the EMR allows for greater load current capability and less signal attenuation.

•  Output Capacitance: Electromechanical relays typically have an output capacitance of less than 1 picoFarad, whereas SSRs typically have a capacitance of greater than 20 picoFarads. Capacitance becomes an issue in high frequency signals, and EMRs are a better option for HF applications.

•  Package Dimensions: The internal components of the relays control the overall package dimensions. Because there are mechanical parts (coil, core, arm, contact lever arms, and spring mechanism) within the EMR, the package size is limited to the physical dimensions of functional internal components. The SSR on the other hand, is limited to only the size of the semiconductor components, and is clearly capable of being manufactured in a much smaller package.

Basically, both EMR’s and SSR’s have their applications but the overlap between them is bigger with today’s technology; such as SSR’s based on thyristors (four layered semiconductor) making them robust, safe and more efficient than many the traditional EMR’s. SSR’s provide the performance of EMR’s with a much compact package and better efficiency.

What do you think? Share your thoughts with me below.

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