Talking to Electrical Engineers

Are you having trouble understanding controls engineers?  Current sinking/sourcing, PNP/PNP, true-high/true-low, and positive/negative logic. What are they talking about? Well, here some help.

The basic device of all electronics, computers & cell phones is the transistor. And whether it’s switching, amplifying, or signaling it helps to understand how transistors work. There’s a variety of transistors and for this lesson we’ll use bipolar junction transistors (BJTs) as an example.

Let’s start with simple- transistors can act like a switch and changes states, turning current flow on or off. Early transistors switched off faster than on, so designers used negative logic (output set to low voltage) to indicate a device was on. Also, turning a BJT off and sinking current to common better absorbed electrical noise in factory environments. Now modern transistors switch states extremely fast and electrical noise protection is common so using positive logic has become the norm.

Now let’s look at sensors & switches that send inputs to a computer. If we use negative logic, a device sending a signal to an input circuit will set voltage low and sink current from the input, and using positive logic, a device will apply positive voltage and source current to the input. So a sensor can give or take current to/from an computer input.

Refer to the top circuit in the above diagram, when a photosensor (NPN output) detects an object a positive voltage will be applied to the base of the BJT, turning on the switch and allowing current to flow from C (+V) to E (-V). This sets a low voltage to the inverter which then sets a high voltage to the computer input. Usually PLC inputs are optically isolated (for noise reduction) and a positive voltage is set at the base of the input transistor, causing the switch to close, and current to flow from Vcc (+) to Common (-). Thereby turning the input on.

Note that there are two BJT transistors in this circuit, one within the photosensor and one inside the computer (PLC) input card. Prior to opto-isolation, these transistors where paired with one sinking and one sourcing (or visa-versa). Nowadays, either BJT can be sinking or sourcing. Refer to the bottom circuit for a photosensor BJT sourcing current connected to a sinking BJT input.

What causes confusion is that some EEs refer to the device (sensor) when describing current flow while others refer to the input card when describing current flow. Again, to keep it simple lets agree to only refer to one side of this electrical connection. But which side: computer or device? Since you’re in manufacturing and like action/motion I suggest you name the devices causing events, whether it’s a sensor ‘seeing’ motion or an actuator ‘causing’ motion. Sure the computer is an active device but it doesn’t move. But remember that computer guys like to refer to the BJTs within their I/O card circuits and that can cause confusion.

Now let’s get to it. Note in the diagram above that a sensor with a sinking transistor (NPN) pulls current to ‘turn on’ a sourcing transistor (PNP) in the computer input card. Even though the sensor sends a signal to the input card, it is actually receiving current from +V. This electrical coupling is important to understand. One transistor sends current and one receives current.

Now we can talk terminology: current-sourcing, PNP & true-high all refer to a BJT providing current at a positive voltage. And current-sinking, NPN & true-low all refer to a BJT providing current at a negative voltage. Again, placement of the correct transistor at +V or -V determines which direction current flows. So when you’re trying to understand a controls guy, first identify which side he’s talking about (computer I/O or device). Next determine if he’s talking positive or negative logic. Then you can read voltage conditions within the circuit to better understand your machine controls. And finally you can understand what EEs are talking about!