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Is a solid state relay right for you?

A solid state relay and an electromechanical relay has always been subject to debate as to which is better. But is there a clear winner? Both have advantages and disadvantages but depending on the applications one can be preferred over the other. Throughout this article, we will assess the differences between SSRs and EMRs and help inform you on when to choose an SSR.

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What is a Solid State Relay?

solid state relay (SSR) is an electrical switch that can be used to turn things on and off using another electrical input. Unlike Electromechanical Relays (EMRs), the SSR uses semiconductor switches and optoelectronics instead of magnetism and moving contacts.

It is important to realise there are two common types of solid state relay, those designed for switching AC loads and those designed for switching DC loads. A DC SSR will at best only turn off for half the time with an AC supply – this could cause serious issues related to a half-wave rectified current. If an AC relay is used with a DC supply and the current does start to flow it will probably not stop, even with the control signal removed.

Are solid state relays more reliable?

With no wearing parts, solid state relays are less sensitive to environmental factors such as mechanical shock, vibration and external magnetic fields. This provides an increased lifetime and greater dependability of the relay. Interestingly, an EMR has an average lifespan of one million cycles, whereas an SSR has a lifespan of roughly 100 times that. Finder’s solid state relays are electronically clean with low emissions, no contact bounce, and no electrical arcing. The modular design also allows for quick and tidy easy wiring.

At full rated current an SSR generates about 10X as much heat as an electromechanical of similar current rating, so SSRs need proper thermal management whereas EMRs don’t. Spacing, ventilation and PCB layout guidelines in Finder’s data sheets need to be followed to allow heat removal, to avoid overheating and any damages. For the high current 77 ‘Hockey Puck’ Series, Finder has its 077.XX DIN rail mount heat sinks available.

Finder’s thermostat (7T Series) or fan (7F Series) would complement the SSR perfectly and would help regulate optimal temperature and keep components performing at a consistently high level.

What is a solid state relay used for?

Finder’s solid state relays are used in many industrial applications. They are particularly suitable for resistive loads and for industrial applications where high switching frequencies and high inrush currents can be problematic and are ideal for controlling motors, valves and electromagnets in energy, automation, machine and rail applications.

solid state relay is ideal when fast, frequent switching is required. For example, precision temperature control of an industrial oven might require switching off the mains supply to the heating elements every two or three seconds. Finder’s 77 Series would be the best component to operate this with input AC or DC, 230 V AC or 400 V AC output and rated voltage range (19…305) V AC or (48…480) V AC and is most suited to heater controls.

Interfacing

Finder’s solid state relays are excellent in interfacing applications. Small SSR’s such as Finder’s 34, 38 and 39 Series draw only a few mA, so they sip at the limited output currents available from PLCs.

With a high 6A rating the Finder Type 34.81.7.xxx.9024 would allow a PLC to control a small DC motor or DC coil contactor perfectly. The 2A rating of the Finder Type 34.81.7.xxx.8240 would allow a PLC to control a small AC motor, AC Coil contact, AC solenoid or mains voltage heater. In both cases with a control current of only around 7mA.

Finder's range of solid state relays

77
SERIES

77 Series – Modular solid state relays (SSR) 5 -15 – 30 -50A

The 77 Series is made up of modular and “hockey puck” style solid state relays with the following features (according to Type):

  • Input AC or DC
  • 230 V AC or 400 V AC output
  • Rated voltage range (19…305) V AC or (48…480) V AC
  • Zero-crossing and random switch-on versions
  • Suitable for lamp loads
  • 17.5 and 22.5 mm housing

 

Features (“hockey puck”)

  • Input AC or DC
  • 230 V AC or 400 V AC output
  • Switching voltage range (19…305) V AC or (48…480) V AC
  • Suggested applications: heater control
  • Mounting to heatsink with screws
  • Zero-crossing version
34
SERIES

34 Series –  Slim PCB SSR 0.1-2-6A

DC Inputs (5-60V ). DC or AC output depending on model.

The 34 Series (EMR) relays include the following features (according to Type):

  • Sensitive DC coil: 170 mW
  • 5 mm wide – 6kV (1.2/50 μs) isolation, coil – contacts

The 34 Series (SSR) relays include the following features (according to Type):

  • Sensitive DC input circuits
  • 5 mm wide
  • Silent, high speed switching with long electrical life
38
SERIES

38 Series – SSR interface modules 0.1-2-3-5-6A

AC/DC Inputs ( 6-240V ) . DC or AC output depending on model.

The 38 Series is a range of EMR or SSR modular interface relays that include the following features (according to Type):

  • 6.2mm or 14mm wide
  • DC or AC/DC coil versions
  • Special coil/input leakage current suppression types
  • Timer version
  • Screw & screwless terminals
39
SERIES

39 Series – SSR MasterINTERFACE 0.1-2-6A

AC/DC Inputs ( 6-240V ) . DC or AC output depending on model

The 39 Series is a range of modular interface relays that include the following features (according to Type):

  • 6.2mm wide
  • DC, AC or AC/DC coil versions
  • Special coil/input leakage current suppression types
  • With fuse protection module
  • Timer version
  • ATEX/HazLoc compliant (EX nC) option available
  • Push-in and screw terminals
41
SERIES

41 Series – Low-profile PCB SSR 3-5A

DC Inputs (5-60V ). DC or AC output depending on model.

The 41 Series is a range of miniature low profile printed circuit board relays (height 15.7 mm). The different Types that make up this Series are:

Solid State (SSR) version

  • Low profile, 15.7 mm height
  • Sensitive DC input circuits
  • Silent, high speed switching with long electrical life
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