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Solid State Relays

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Watlow solid state relays (SSR) offer many of the advantages of solid state power controllers, but at a lower cost. Watlow's extensive knowledge in power controller design led to the development of a special fast cycle input card that enables a SSR to operate from a standard 4-20mA instrumentation command signal. Test results show that a zero-cross SSR in combination with the fast cycle card promotes better temperature control and longer heater life than slow cycle relays. Through a time proportional cycle rate of 1/10 of a second, heater life will be extended.

Both low and high voltage models are available from 24 to 660VAC. All ac output models include back-to-back silicon controlled rectifiers (SCRs) for a more rugged design than the traditional triac-based SSR. The internal design allows it to tolerate high currents and harsh electrical environments of heavy industry. Watlow also offers a switched VDC model for dc heating applications.

Watlow provides all of the components necessary for trouble-free operation. This includes two standard convenience items: a thermal foil to ensure proper thermal transfer from the relay to the heat sink and belleville washers to ensure that the relay is mounted with sufficient pressure to enable good heat transfer. Matched semiconductor fuses and heat sinks are available to complete the power switching package.

Features and Benefits · Heater Life · Fast Cycle Card · Thermal Transfer · Replacing Contactors or Mercury Displacement Relays (MDRs) · Wiring Diagrams

Features and Benefits

Fast cycle card
  • Increases heater life
  • Optimizes temperature control
  • Allows for higher watt density heaters
Zero cross firing
  • Results in minimal electrical noise
Back-to-back SCR design
  • Withstands harsh or hostile industrial environments
UL® recognized File #E151484 and #E73741 CSA certified up to 600VAC, File #LR700195 VDE 60950 License #40021401, File #1995500 up to 480VAC, CE
  • Meets applications requiring agency approval

Heater Life

Watlow has extensively tested electric heating elements with a variety of power switching devices. Results prove that the life of an electric element dramatically increases when the on-off cycle time that is used to time-proportion the heater is kept at less than one second. This reduces the thermal expansion and contraction of the element and improves heater life as much as 20 times. This very fast cycle time controls temperature much more accurately and allows the use of higher watt density heating elements.

Fast Cycle Card

In order to obtain the very rapid cycling time required for longer heater life, accurate temperature control and higher watt densities, Watlow has developed a loop-powered firing card for SSRs. This card operates from a standard instrumentation signal of 4 to 20mA and controls solid state relays with a time proportional cycle rate of less than one second (4VAC cycles on and 4VAC cycles off at 50 percent power).

Thermal Transfer

A thermal foil is provided with each solid state relay for mounting on the base of the relay to improve heat transfer. In addition, two belleville washers are supplied to provide the proper pressure for this transfer of heat. Use two #8-32 screws 0.625 in. (16 mm) long to secure the relay to the heat sink.

Replacing Contactors or Mercury Displacement Relays (MDRs)

Improvements in heater life and control accuracy can be achieved with SSRs operated with rapid cycle times as compared to slower operating electromechanical relays or even MDRs. When replacing these types of relays with the SSR, it is important to consider two aspects:
    1. Heat
      Solid state devices require a small voltage to turn on, which is consumed as heat (approx. 1.5 volts x amps = watts). This heat must be removed from the device and is usually accomplished by mounting the relay on heat sinks.
    2. Failure Mode
      Solid state devices should last for many years when properly protected with voltage snubbers, mounted on appropriate heat sinks and when fused with semiconductor fuses against the high currents caused by electrical shorts. Watlow's SSRs include an internal voltage snubber. However, if the unit fails, the most probable condition will be a short. Mechanical relays also have a good probability of failing short. In all cases where uncontrolled full power can cause damage, it is recommended that a high limit temperature controller and contactor be used for protection.

Wiring Diagrams

Shorted SSR Alarm

The most prevalent concern when using solid state relays is the possibility of a relay failing in a shorted condition. With this in mind, Watlow has designed a cost effective "shorted SSR alarm".

The device monitors the output (current through the heater) and activates a triac (alarm) if there is no command signal from the temperature controller. The triac can be wired to a bell, or to a normally closed latching relay to remove power to the heater.

Note: Semiconductor power switching devices are not legal for over-temperature limit or safety devices. For limit and safety devices you must have a positive mechanical break of all electrically hot legs simultaneously.


· Single-Phase Fast Cycle Input Card

· Single-Phase Shorted SSR Detector


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