Transistor Outputs

The next type of output we should learn about is our transistor type outputs. It is important to note that a transistor can only switch a dc current. For this reason it cannot be used with an AC voltage.

We can think of a transistor as a solid-state switch. Or more simply put, an electrical switch. A small current applied to the transistors "base" (i.e. input) lets us switch a much larger current through its output. The plc applies a small current to the transistor base and the transistor output "closes". When it's closed, the device connected to the plc output will be turned on. The above is a very simple explanation of a transistor. There are, of course, more details involved but we don't need to get too deep.

We should also keep in mind that as we saw before with the input circuits, there are generally more than one type of transistor available. Typically a plc will have either NPN or PNP type outputs. The "physical" type of transistor used also varies from manufacturer to manufacturer. Some of the common types available are BJT and MOSFET. A BJT type (Bipolar Junction Transistor) often has less switching capacity (i.e. it can switch less current) than a MOS-FET (Metal Oxide Semiconductor- Field Effect Transistor) type. The BJT also has a slightly faster switching time. Once again, please check the output specifications of the particular plc you are going to use. 

Never exceed the manufacturers maximum switching current.

Shown above is how we typically connect our output device to the transistor output. Please note that this is an NPN type transistor. If it were a PNP type, the common terminal would most likely be connected to V+ and V- would connect to one end of our load. Note that since this is a DC type output we must always observe proper polarity for the output. One end of the load is connected directly to V+ as shown above.

Let's take a moment and see what happens inside the output circuit. Shown below is a typical output circuit diagram for an NPN type output.

Notice that as we saw with the transistor type inputs, there is a photocoupler isolating the "real world" from the internal circuit. When the ladder diagram calls for it, the internal circuit turns on the photocoupler by applying a small voltage to the LED side of the photocoupler. This makes the LED emit light and the receiving part of the photocoupler will see it and allow current to flow. This small current will turn on the base of the output transistor connected to output 0500. Therefore, whatever is connected between COM and 0500 will turn on. When the ladder tells 0500 to turn off, the LED will stop emitting light and hence the output transistor connected between 0500 and COM will turn off.

One other important thing to note is that a transistor typically cannot switch as large a load as a relay. Check the manufacturers specifications to find the largest load it can safely switch. If the load current you need to switch exceeds the specification of the output, you can connect the plc output to an external relay. Then connect the relay to the large load. You may be thinking, "why not just use a relay in the first place"? The answer is because a relay is not always the correct choice for every output. A transistor gives you the opportunity to use external relays when and only when necessary.

In summary, a transistor is fast, switches a small current, has a long lifetime and works with dc only. Whereas a relay is slow, can switch a large current, has a shorter lifetime and works with ac or dc. Select the appropriate one based upon your actual application needs.