Input Devices and Sensors
In the world of HVAC control, there is basically one type of device used to complete a digital input (DI) circuit. A switch, employed in various forms, is this device.
A switch is an electrical device used to enable or disable flow of electrical current in an electrical circuit. Switches may be actuated in a variety of ways, including movement of two conducting materials into direct contact (mechanical), or changing the properties of a semi-conducting material by the application of voltage (electronic).
Switches are typically rated in terms of voltage, voltage type (AC or DC), current carrying capacity, current interrupting capacity, configuration, and load characteristic (inductive or resistive). Also specified are applicable ranges of ambient conditions over which the ratings are valid. Current carrying capacity (or current rating) is the maximum current that may continuously flow through the closed switch contacts without exceeding the maximum permissible temperature.
Process medium property sensing switches are also rated by parameters such as adjustment range, accuracy or repeatability, and deadband or differential. The range of a control switch is specified by upper and lower process values between which the switch has been designed to operate. The accuracy or repeatability of a control switch is a value typically measured in process units or percent of range that represents the expected maximum deviation from setpoint at which the switch will operate under test conditions. The switch differential or deadband is the change in process value required to cause the state of the switch to change. For example, a pressure switch that makes at 10 psig and breaks at 8 psig has a 2 psig differential.
Switch contacts are characterized in much the same way as relay contacts.
Figure 2.1 describes the most common contact configurations using industry standard terminology and symbols. Many other configurations are available.
Getting Started | Chapter 1: Special I/Os | Top | Chapter 2: Types of Switches