AC Inputs

Now that we understand how dc inputs work, let's take a close look at ac inputs. An ac voltage is non-polarized. Put simply, this means that there is no positive or negative to "worry about". However, ac voltage can be quite dangerous to work with if we are careless. (Remember when you stuck the knife in the toaster and got a shock? Be careful) Typically, ac input modules are available that will work with 24, 48, 110, and 220 volts. Be sure to purchase the one that fits your needs based upon the input devices (voltage) you will use.

OUCH... use care!!

AC input modules are less common these days than dc input modules. The reason being that today's sensors typically have transistor outputs. A transistor will not work with an ac voltage. Most commonly, the ac voltage is being switched through a limit switch or other switch type. If your application is using a sensor it probably is operating on a dc voltage.

We typically connect an ac device to our input module as shown above. Commonly the ac "hot" wire is connected to the switch while the "neutral" goes to the plc common. The ac ground (3rd wire where applicable) should be connected to the frame ground terminal of the plc.(not shown) As is true with dc, ac connections are typically color coded so that the individual wiring the device knows which wire is which. This coding varies from country to country but in the US is commonly white (neutral), black (hot) and green (3rd wire ground when applicable). Outside the US it's commonly coded as brown(hot), blue (neutral) and green with a yellow stripe (3rd wire ground where applicable).

The PLCs ac input module circuit typically looks like this:

The only things accessible to the user are the terminals labeled COMMON, INPUT 0000, INPUTxxxx... The common terminal gets connected to the neutral wire.

A common switch (i.e. limit switch, pushbutton, toggle, etc.) would be connected to the input terminals directly. One side of the switch would be connected directly to INPUT XXX. The other end goes to the ac hot wire. This assumes the common terminal is connected to neutral. Always check the manufacturers specifications before wiring, to be sure AND SAFE.

The photocouplers are used to isolate the PLCs internal circuit from the inputs. This eliminates the chance of any electrical noise entering the internal circuitry. They work by converting the electrical input signal to light and then by converting the light back to an electrical signal to be processed by the internal circuit.

One last note, typically an ac input takes longer than a dc input for the plc to see. In most cases it doesn't matter to the programmer because an ac input device is typically a mechanical switch and mechanical devices are slowwwwww. It's quite common for a plc to require that the input be on for 25 or more milliseconds before it's seen. This delay is required because of the filtering which is needed by the plc internal circuit. Remember that the plc internal circuit typically works with 5 or less volts dc.