I like watching science videos on YouTube and I have a very basic understanding of electronics. But I never seem to get the “negative voltage” thing.
Why is it used and how does it differ from normal voltage.
In: 5
– +
A A
A A
+ –
—–[sheet of metal]—–
imagine you have two batteries touching a sheet of metal. one battery’s negative side is down, the other battery’s positive side is down.
if you take your multimeter and touch the negative probe to the sheet of metal, and the positive probe on the battery that’s got “+” up, you’ll see +1.5v.
if you then move the positive probe and touch the battery that has its “-” sticking up, you’ll see -1.5v.
You’ve now observed negative voltage — but what’s it good for?
Move your negative probe to the battery with the “-” sticking up, and touch the positive probe to the one with the “+” sticking up. Now you’ll see +3.0v.
Where did 3.0v come from?? Ground (the sheet of metal) only showed 1.5v in each direction!
Well, that’s where negative voltages come in handy — given to themselves, they’re only a smaller voltage, but combined with the positive rail, they can be useful as a higher voltage, where needed, if the circuit uses both “rails”. Consider that the “sheet of metal” – the idea of the negative voltage’s “ground” – is totally up to you… it could, for a moment, just be considered an arbitrary wire between two batteries (DC supplies).
In EV charging (where I really, deeply learned about negative voltage in this specific case), +12v/-12v is used in J1772 EVSE signaling to “communicate” (in a crude, analog manner) between the vehicle and station. To ensure the attached “thing” is really a vehicle and not a puddle of water, the EV has a diode in it – so that only the positive side of the signal ever gets resisted (to change the voltage, and “talk back” to the charging station). No diode? Both sides see a voltage decrease. Diode? Only the positive side gets decreased – and it must be a car.
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