Solid-State Devices Used in Industrial Logic Circuits: A COMPARISON OF SOLID-STATE LOGIC AND RELAY LOGIC: Strengths and Weaknesses of Solid-State Logic Devices

One of the reasons that the solid-state logic has such a long life expectancy is that it operates in an enclosed environment, since each device is packaged as an integrated circuit. This is extremely important when the circuit is exposed to the harsh environment of the typical industry where all kinds of dust and dirt may accumulate and cause traditional contacts to wear out or seat incorrectly. Another strength of the solid-state devices is that since they are enclosed in the IC, they don't pose a hazard in explosive atmospheres, such as in control circuits for painting systems or where vapors are used for cleaning.

The fact that solid-state logic devices are smaller means that they need less room in electrical cabinets. Today the floor space in a factory is very expensive and designers try to keep the size of electrical cabinets as small as possible. Some of the critical solid-state ICs in a circuit are now mounted in sockets or the entire circuit is modularized on a single board with an edge connector that plugs into a card cage. These features allow the faulty components or circuits to be removed and replaced more easily during troubleshooting and repair.

Some of the weaknesses of solid-state logic devices are that they are somewhat more difficult to troubleshoot because you need to have a good pin outline of the entire circuit and you need a logic probe or scope. If the solid-state IC is soldered into a printed circuit board, replacement is very difficult. Sometimes the circuitry is considered proprietary and the board must be sent back to the original equipment manufacturer. Other manufacturers may encapsulate circuit devices in plastic cases that are not made to he tested or repaired. Instead these circuits are simply swapped with a known good one to determine if they are good. If the circuit is bad, it's discarded or sent back to the factory for repair.

Another problem with solid-state logic circuits is that they tend to add (dissipate) heat into the cabinet where they are mounted, which means the cabinet may require a cooling fan or air conditioning. This is especially critical in industries like the steel industry, glass industry, and some areas of the plastic industry where the controls must be mounted near the high-temperature area of the process. Other problems arise with small-voltage levels of the signals. The small voltages used for inputs are susceptible to induced voltages from large motors or other inductive devices on the factory floor. Since the voltages are usually less than 5 volts, the logic circuitry must be mounted fairly close to the input switches. This may increase the likelihood of picking up interference from the induced voltages.