Nematron Pointe Controller - Manual (Page 91)

Pointe Controller User Guide Chapter 4: System Design and Installation
89
Solid State Relay Outputs
Solid state relays are semiconductor switches that operate very much like
mechanical relays. They have an advantage over mechanical relays by virtue of
the fact that they are semiconductors. Solid state relays can be switched at
relatively high frequencies and they do not wear out. However they are more
expensive and there is a small voltage drop across the contact.
The figure below illustrates a typical solid state relay output:
OptiLogic Solid state relays are designed for AC load operation.
4.6.3 Analog Inputs
Analog inputs are used to monitor the value of some continuously variable
measurement. Typical analog inputs are measurements of temperature, pressure,
weight, liquid level, pH, flow rate and many other “real world” parameters.
The purpose of an analog input module is to convert the measurement into a
format that is usable by the data acquisition or control system. To be usable by an
computer-based system, the analog measurement must be converted to digital
format. Doing so accurately and, in some cases, quickly, is the goal of the analog
to digital converter module.
A good understanding of analog input modules includes an understanding of
isolation, accuracy, single and differential inputs, multiplexing, resolution and
range. The following paragraphs provide an overview of these subjects.
Isolation
In many applications there is a good deal of benefit to be derived from isolating
the analog measurement source from the RTU’s power supply. In some cases,
signal inputs may contain voltages or noise signals which could adversely affect
the main processor’s operation. Likewise, noise on the main power bus can
degrade the accuracy of the analog value measurement. Both potential problems
can be solved by isolating the analog inputs from the main power supply.
Isolation involves totally isolating the analog to digital (A/D) converter from the
main power bus. This can be accomplished in two ways. The A/D input module
can use a separate power source input, which is isolated from the power input to
the base. The A/D module can also use the main power supply and isolated power