Specifications

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Figure 10 Typical HVAC relay setup
Researching different types of relays led us to come across the website:
http://www.deltrol-controls.com/. This website is pretty good because deltrol-
control is a leader in the design and manufacturing of electronic components.
They also have very detailed and well explained part specification sheets that list
what type of relays are used for certain applications. The relay that our group is
interested in is the 263/268 series relay. These relays can either be surface
mounted by using a soldering gun or can be directly plugged into the circuit
board. Both the 263 and 268 series relay can carry a maximum of 15 Amps as
well as have an ambient AC temperature ranging from -45 degrees to 65 degrees
Celsius. Our HVAC control system will have multiple different sets of relays; one
relay to activate 1
st
Stage Heating, one for 2
nd
Stage Heating, one for Blower /
Fan, one for Compressor, one for Reversing Valve, and one for 24V Common.
The 268 series relay is readily available with a 24V AC coil having a DPDT
configuration that meets our systems requirements. The 24 V AC coil has an
electronic tolerance of ± 10% ranging above or below 72 ohms. It also has a
nominal coil powered at 2.2 VA.
Another consideration when researching relays is how to interface them to the
microcontroller. This involves designing a circuit that uses a transistor to ensure