Altafhal,
The 3 sets of ratings that are of most importance are:
Contact Voltage rating
. This rating is different for ADC vs DC applications
Contact current rating
. This rating is alsoo different for AC vs DC.
Coil voltage. For solid state relays (SSR), the anlagous paramter is either actuation voltage or actuation current, depending on the type of SSR.
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Also, current ratings are different for various types of loads. Frequently, different load ratings are given for resistive and inductive loads. If only a resistive current rating is given by the manufacturer, then divide this rating by 10 to to get the inductive rating. This is a conservative rule of thumb for reliable operation with inductive loads, such as motors.
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The "bible" for relay applications is the "Engineer's Relay Handbook" published by NARM (National Association of Relay Manufacturesrs).
Regards,
Kral