RACA Journal I June 2026 www. refrigerationandaircon. co. za 19
CONTROLS
INPUTS, OUTPUTS AND THE IMPORTANCE OF SIGNAL TYPES
At the heart of every controller, whether simple or complex, lies a straightforward concept: inputs regulate outputs. Milne unpacked this principle, as misunderstandings around inputs and outputs remain one of the most common causes of commissioning and fault-finding errors.
Analogue inputs provide continuously variable information to the controller. These include temperature probes, pressure transducers, humidity sensors and gas detectors. Depending on the sensor type, the signal may be resistance-based, a current loop such as 4 – 20mA, or a voltage signal such as 0-10Vdc or 0.5- 4.5Vdc. Each has its own advantages and limitations, particularly in terms of noise immunity, cable length and accuracy.
He highlighted calibration as a critical but often neglected aspect. Small errors introduced by cable resistance or poor installation can have a disproportionate impact on system performance. In many cases, simple field calibration methods, such as ice-water testing for temperature probes, are sufficient to achieve acceptable accuracy.
Analogue outputs, on the other hand, allow controllers to modulate equipment rather than simply switch it on or off. These outputs drive variable-speed drives, inverter compressors, electronically controlled valves and fan speed controllers, enabling smoother operation and improved energy efficiency.
DIGITAL SIGNALS AND SIGNAL FLOW DIRECTION Digital inputs and outputs are binary in nature, operating in either an on or off state. Digital inputs are commonly used for functions such as unit enable, defrost initiation, standby operation or setpoint selection. Digital outputs typically drive relays, solenoids, contactors or alarms.
Many field problems arise not from faulty equipment, but from a lack of clarity about where a signal originates and where it should terminate. Milne emphasised the importance of understanding signal flow direction. Sensors generate outputs, controllers receive inputs, and outputs from controllers act on external devices. Transducers require power supplies, and relays should be used to protect controller outputs from high loads.
Using an air handling unit example, Milne illustrated how theory translates into practice. A mains supply is stepped down to a 24V control circuit, a static pressure sensor feeds an analogue input, and the controller modulates a supply fan to maintain duct pressure. Multiple temperature probes are used for both control and safety, while interposing relays ensure that expensive controller outputs are protected from electrical faults.
This example reinforced the idea that good control design is as much about sound electrical practice as it is about software. feature
RACA Journal I June 2026 www. refrigerationandaircon. co. za 19