What to Do Instead of Tying Commons Together

If you’re a field service technician, you might find yourself often in the position of having to engineer a solution on the fly. If you find yourself needing to add a subsystem or secondary control cabinet, perhaps for a peripheral piece of equipment such as a printer or bar code scanner, you may not receive a set of electrical prints. In a perfect world, every electrical design modification should be reviewed by an engineer. But in the real world, that doesn’t always happen. Sometimes, you’re on your own to figure it out.

Multiple Grounds

When you’re interfacing two pieces of equipment that communicate via electrical signals and that are powered by separate power supplies, each will have its own ground. While most electrical cabinets are tied to Earth ground, there is also sometimes a chassis ground or a “common” bus that is tied to terminal blocks. For example, if you have a machine that tapes boxes shut and you need to install a label applicator right next to it, the label applicator might need to receive a signal from the PLC inside the taping machine to let it know when the tape has been applied and the label can go on. Suppose both control systems run on 24 volts, and each box has its own common

One really bad practice that is sometimes used in low-voltage applications is tying commons together. In this instance, for example, you might decide to tie the taping machine’s common to the label applicator’s common so that their 24 volts will be based off of the same reference voltage. This might appear to work fine, but it’s an invitation for trouble down the road. When you daisy chain two power supplies together, you effectively ensure that if one power supply fails, they both fail. If any noise is introduced into either system, that noise will be carried into both systems because they are electrically tied together.

Optocouplers

An optocoupler, also called an optoisolator, is a component designed to solve this problem. It’s basically an LED next to a photo sensor. Optocouplers are made for situations where digital signals need to be passed between two different electrical systems on separate power supplies. The signal is passed by way of a beam of light that activates the sensor. That way, the signal can be “coupled” between the two systems without needing to connect them together electrically. This removes any risk of needlessly damaging or interfering with the operation of two power supplies at the same time.

Relays

Relays can also be used to couple a signal between two different systems. One system can be used to activate the relay coil, which closes the switch on the other side. Again, no direct electrical connection is made between the two systems. Relays are sometimes less ideal than optocouplers due to characteristics like noise, switch bounce, and frequency limitations.

Tying commons together might seem to be a quick and easy solution, but it’s the kind of thing that can lead to a phone call in the middle of the night when a system suddenly fails. It’s better to err on the safe side.

Tags: electrical, problem solving, solutions

This entry was posted on September 5th, 2013 and is filed under Electrical, Technology. Both comments and pings are currently closed.