Plastics catalog - Watlow - #359

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 bevel washers are supplied to provide the proper pressure for the transfer of

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heat. Use two #8-32 screws,15.8 mm (0.625 in.) long to secure the relay to the heatsink. See the derating curve, Temperature Operating Curve, for operation without heatsinks.

Note: This rating curve applies only to 10 and 25 amp SSRs. 40 through 75 amp SSRs should not exceed a seven amp load without using a heatsink.

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Improvements in heater life and control accuracy can be anticipated with solid state relays operated with rapid cycle times as compared to slower operating electromechanical relays or even mercury displacement relays in some applications. When replacing these types of relays with the SSR, it is important to consider two aspects:

Solid state devices require a small voltage to turn on, which is consumed as heat (1.5 watts/amp). This heat must be removed from the device, and is usually accomplished by mounting the relay on heatsinks.

Solid state devices should last for many years when properly protected with voltage and RC suppressors

mounted on appropriate heatsinks, and when fused against the high currents caused by electrical shorts.

However, if the unit fails, the most probable condition will be a short. Both mechanical relays mentioned above 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.

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" SDA.

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.

SDA I Shorted SSR Detector]]

4-20mA loop powered firing card for direct mounting on a dc solid state relay. Can be used in three-phase mode with two dc input solid state relays.

Requires external power switching transformer. Consult factory for part number and voltage required.

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