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Understanding How Solid State Relays Work

Understanding what type of solid state relay to use within a given application allows for a level of precision and reliability not even remotely possible with equivalent rated electromechanical relays. The trick is ensuring that the correct relay is selected based upon the characteristics of the load.


1. Resistive Loads (heating elements) and loads with some capacitance – pf >0.75


Zero-crossing relays are suited for almost all loads with a power factor of greater than 0.7. It is the reason why they are the most common type of relay found today in resistive heating applications.


Phase-angle and burst-fire relays are suited for resistive loads where precise temperature control is required. Burst-fire relays is suggested only if there is capacitance in the load


2. Inductive Loads (motors, contactor coils, transformers) – pf <0.75


Most Crydom zero-crossing SSRs today work well with motor loads. Nevertheless, random turnon relays are recommended most of the time. This is because the phase shift between voltage and current may lead to problems with some zero-crossing relays.


Phase-angle and burst-fire relays are not recommended here.


3. Lighting Applications


As for lighting applications, while zero-crossing, random turn-on, and phase angle SSRs are all suitable, it is important to note that zero-crossing relays are commonly used should a simple on/off function be needed. As for random turn-on and phase angle relays, they are mostly adopted for dimming purposes. In most instances, burst-fire relays are not suitable for lightings. This is because the on / off period will result in unwanted flickers.


Read more about the various SSR switching types from the fact sheet below.

Or, visit our website to browse the full range of Crydom products represented by RM3C Industries.

Source: Crydom, Solid Statements

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