Transistor wiring on a PLC output.

Thread Starter


Joined Jul 30, 2008
Hello - this is my first post here so please don't bite!

I've started a job recently as a PLC/SCADA system programmer and have been left to work out quite a lot of things for myself. I've come across a piece of (hopefully) basic electronics I don't understand and was hoping someone could explain it or point me to a useful resource.

I am working with a PLC that comes equipped with 2 transistor switched outputs and a number of relay switched outputs. I understand the relay switches - two wires needed to the load, one "in" and one "out" of the PLC to form a circuit.

I am struggling to understand the transistor switch output - it has three terminals for its output. My understanding is that I should only see two - the collector and emitter, which has a power supply in line with the load (like the relay switch outputs). The terminals are labeled V0+, V0- and 0.

If I wire this up according to the PLC instructions the LED lights up brightly as expected. If I bypass the seemingly superfluous V0- connection the LED still lights up but is dimmer. I am struggling to understand the need for this extra connection and what it does, as it seems to go a step beyond a simple transistor-LED circuit. My wiring methods are shown in bmp attachments LED1 and LED2.

Also, a wiring schematic for the transistor output from the PLC's online manual is attached.

If anyone can point me to a site that explains this circuit type or point out why the system is designed and works properly with the third terminal it would be of great help. :p




Joined Feb 4, 2008
Your wiring with the bright led is the correct.

V0+ is connected to the positive line of the power supply

V0- is connected to the negative line (zero volts if the supply is unipolar) of the power supply

0 is the output of the PLC

You connect the load between O and the negative line (ground) of the power supply. When the transistor is activated you get a positive voltage at 0 with respect to ground.

Thread Starter


Joined Jul 30, 2008
Thanks for pointing that out. Why do I still get any light out of the LED if I don't connect V0- to anything at all? Should the LED not be totally dark if it means there's no circuit?