Specifications

110
SRS/SRSZ
XX VDC
S
L
Model of relay
Nominal Coil
Voltage
Structure
Coil sensitivity
SRS
03
Sealed type
L: 0.36W
Table 34 Key Characteristics SRS - 03VDC - SL Relay
For our system, we have decided to not connect the main microcontroller directly.
Instead, we have chosen to implement a Darlington array driver. The Darlington
array driver is basically multiple transistor circuits built into one chip. The reason
for the implementation of the Darlington array driver is because when considering
the logic for our system there is potential for multiple outputs to be running at the
same time. For example, there will be times when multiple compressors and
multiple air handlers will need to be running simultaneously. Powering all of
these relays at the same time straight from the I/O pins of the microcontroller
would put unnecessary stress on the microcontroller. The Darlington array driver
also has built in “clamp diodes” which helps to suppress the inductive kick of the
relays. The array driver we have chosen is a chip that contains eight Darlington
transistors with common emitters and integral suppression diodes for inductive
loads. In specific we have chosen the ULN2803A. This specific array has a
2.7kΩ input resistor. According to the datasheet, in order to turn on each
transistor an input current of between 0.93 and 1.35mA is required. The
following equations describe how the output from the main microcontroller will
meet these requirements.
Minimum output of I/O pin: 3V, Input resistor: 2.7kΩ
    
 


 


 mA
Maximum output of I/O pin: 3.5V, Input resistor: 2.7kΩ
    
 


 


 mA
These equations show that with the main microcontroller providing between 3
and 3.5V output from the I/O pins, there will be between 1.11mA and 1.29mA