My blog this time was supposed to be about photoelectric basics, however, I recently had a discussion with an individual who asked why the market does not offer more AC sensors. In thinking about our discussion I thought this would make an interesting blog and perhaps would spark (no pun intended) some comments from our readers.
Why DC control circuits are more common than AC and what benefits do they provide?
With machines getting smaller and faster, and costs becoming more of a concern DC sensors and components solve these issues. The circuit boards are smaller which means the sensors can be smaller and lighter thus the machines can move faster due to lighter loads. DC sensors are typically less expensive and a larger selection of products exists to solve more of the demanding applications seen today.
Some regulations go into effect at the 50 – 60 volt threshold and since a vast majority of DC control circuits are 24 volt these regulations can be avoided. DC control circuits are more universally accepted than AC plus the fact DC power supplies are getting less expensive. Today’s newly designed DC circuits consume less power which means smaller power supplies can be used. Another advantage of the DC power supply is if there is a short circuit the power supply folds back and will resume full power when the short is removed.
Not only are there more sensor options available there are more and faster interface cards for the most common control device used today, the PLC. DC sensors and components do not have the current leakage that their AC counterparts have. That being said, when using an AC sensor with the higher leakage, frequently pull down resistors are required to prevent the leakage current from causing false inputs to the PLC.
In addition to the PLC interface, more and more manufacturers use DC interfaces to their electronic devices. With AC controls you have to use relays for interface which can add to cycle times. A real money saver is being able to run instrumentation and communication cables with DC controls in the same conduit or cable tray.
DC is inherently faster than AC which means faster response. With more and more cycle times being reduced to achieve faster processes milliseconds can really add up. An AC signal introduces approximately an 8 msec delay in actuation of a device, however, this delay time is very unpredictable.
Typically, AC is used on the outputs of PLC’s to turn on motor control starters, larger solenoid valves, and higher current devices. Also, AC circuits are going to be more immune to noise that would cause problems on DC circuits. In some cases it makes sense to use an AC sensor especially if there is a long run of conduit down a conveyor with one motor and one sensor. Those wires can be run together saving installation time and money.
When it comes to speed, size, and costs DC controls seem to provide more benefits than AC. What are your thoughts?