From Wikipedia: A solid state relay (SSR) is an electronic switching device in which a small control signal controls a larger load current or voltage.



Why Use a Solid State Relay?

Solid state relays are popular because they contain no moving parts - which are found in an electromechanical relay. This means that there is no bouncing or arcing of contacts when the relay switches current to the load. This gives them a much longer lifespan. They contain two silicon-controlled rectifiers, in "inverse-parallel", which switch the load current. When the input signal from, say an Arduino, is applied, a small amount of current flows from the AC mains through the optical isolator and into the gate of the forward biased silicon-controlled rectifier. This, in turn, turns on the SCR and allows load current to flow for that half of the AC cycle. When the polarity of the AC mains reverses, the first SCR turns off and the second one conducts load current for the next half of the AC cycle. This process repeats until the input voltage is stopped.

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The heater and the AC motor need to be controlled by different methods. Since these are AC loads, we are looking for a solid state relay that uses dual power SCRs (Silicon Controlled Rectifiers) that will allow for more precise switching as the AC waveform crosses the x-axis.



Heater

The heater will be controlled by Integral Cycle Control (also known as Burst Fire Control).

Universal AC/DC Motor

The fan will be controlled by Phase Angle Control.

Other Parameters

The SSRs also need to be easily driven by the digital output pins of the Arduino. The relays described below are reated for 3-32VDC control voltage at 25A.

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Since I measured 12A draw from the heater, and the only SSR options (from Crydom) are 10A and 25A, I chose the 25A relay.

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You will need the following Solid State Relays:



D2425 (Crydom) - http://yourduino.com/sunshop2/index.php?l=product_detail&p=348

This solid state relay is a zero-crossing relay, meaning that it only turns on and off at the zero crossing. This type of relay is very common for heater loads, which is what we'll be connecting it in line with!



D2425-10 (Crydom) - http://www.crydom.com/en/products/catalog/s_1.pdf

This solid state relay, denoted with the suffix, "-10", utilizes Instant-On Switching. This means that the The SSR output is activated immediately after applying control voltage. Consequently, this relay can turn on anywhere along the AC sinusoidal voltage curve. The typical response time is thus less than 1 ms. This SSR is particularly suitable in applications where a fast response time or phase angle control is desired. This is the relay that will be in line with the Fan.



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