VSS, ground and -v all common?


#1

Hey all,

I’m sitting building up an IO board for train detection on my model railway, using this circuit. On the voltage regulator, VSS, ground and -v come together on the common pin, effectively meaning that all are a common rail. As such, I’m puzzled as to why the circuit differentiates between them in other places. Is this just a peculiarity of notation? I don’t want to destroy my ICs!

Cheers

Rich


#2

VSS, GND and -V all mean different things in general, but in a given context, all those different things can be the same :slight_smile:

VSS means the negative supply voltage to a circuit made out of field-effect transistors (as most logic ICs are). The S is for Source, VDD is the corresponding positive supply pin (D for Drain): https://en.wikipedia.org/wiki/IC_power-supply_pin

Ground, of course, is the reference voltage that all others are measured against: the 0v line.

-V or V- is the most negative supply voltage.

So for most simple circuits, we don’t have any negative voltages, so ground is the most negative voltage; and unless we have split analogue and digital supplies, we’ll also use that as VSS :slight_smile:

On the other hand, amplifier circuits often have split power rails: +V and -V on either side of a GND/0V line. In which case, either GND or -V might be used as VSS for any associated digital circuits (DAC?), as convenient…


#3

Thanks Alaric. So essentially in this case, so although the designer has used all three labels in his design, he has ultimately found that they could be common for this circuit! I’m happy enough to be putting the links to my feed rails in now, just gotta program the IC!


#4

It’s fairly common to connect floating voltage rails so you can work with the differential voltage levels (wrt to those those rails). The thing that you need to be careful of, is somehow making those rails fixed with respect to each other rather than floating.

For example connecting two isolated +12VDC power supplies in series to give -12V (the 0V of the first), 0V (the junction of the two) and +12V (the +12V of the second) will work fine (similar to multi cell batteries) - until you connect both of them to earth. When earthed, the second power supply will essentially short-out the first because both power supplies will use earth as a reference to stop the case voltage from rising to dangerous levels…