# Bipolar power supplies and the voltage vacuum

Discussion in 'General Electronics Chat' started by bachevelle, Sep 5, 2008.

1. ### bachevelle Thread Starter New Member

Sep 5, 2008
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0
Here's another ?? How can you have a power rail that is -12v (any number would work here)? I always assumed that when you got to zero volts you found ground. Then i started to study amplifier schemos and low and behold, there are two power rails, one +, and one -. Obviously this left me unsettled. How do you go past ground? Most people, me thinks, see this the same way. + or -, positive or negative, full or empty. So, in my mind, what is - voltage. A voltage vacuum of course! Like a black hole, capable of sucking stray voltages into oblivion. Of course not. I started thinking, maybe it's like biasing a bjt to amp a/c on a single rail. My understanding is to do this you let the base of the bjt think that the midpoint in your PS, say a 9v battery is ground. (or circuit common) Thereby allowing for a swing +4.5v and -4.5v. Of course, as with anything else, I have only you folks to tell me if my assuption is correct. So, i found a power transformer the other day, it was rated 25v. There was no "pin out" diagram on it but on the side of the case it said 12.5v-0v-12.5v. AHA, this is it. The explanation i've been waiting for.... this must mean that your positive rail it actually 25v-12.6v, then circuit common, then your nagative rail is 12.5v-0v. Yes, that must be it. I thought on that for a few minutes and it dawned on me how absolutely absurd that idea was. Apparently, I've discovered a way to learn in reverse. Instead of learning new things when I study, I forget other things I thought I knew. Just to check, 1Ω represents less resistance than 1MΩ. Surely, i'm losing braincells. So, then the other day i'm changing the batteries in a semi at work (im a diesel mechanic) I start thinking, if you put the test lead of a multi meter on the POS terminal of a battery and the common lead on the NEG you will get +12v or thereabouts. (on a good batt of course) Conversely, if you switch them round you see -12v. This could be useful. So thats a matter of what the meter see's as ground? Like those old mostly European cars that had the POS terminal of the battery cabled to the frame instead of the NEG. So, in this case, have I been going about this all wrong? Perhaps I should view the + and - more as phase, or polarity that as absolute positive and negative? That make a whole lot more sense to me. But is that correct? Could I also take that to mean that the ever mystifying current may in fact be moving on opposite directions on the rails? Help....help....help??? Bueller...............Bueller.............Bueller.........

2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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It's very simple really, it's all about relative voltages and usually how they relate to the point defined as ground/common/0V

If you take a 6V battery, in most circuits we would define the negative as our common or ground or 0V and we would say that we have a +6V supply with respesct to ground.

However, in some instances we might define that positive as common/ground, in which case we would say that we have a -6V supply with respect to ground.

If we have two 6V batteries and we connect the positive of one to the negative of the other and define the connection between the batteries as common/ground then we have a +6V and a -6V with respect to ground. Depending on which point we define as ground means that, with this arrangement, we could have: 0V +6V +12V or 0V -6V -12V or +6V 0V -6V

3. ### bachevelle Thread Starter New Member

Sep 5, 2008
3
0
thingmaker3 said:
Here's another way of looking at it: "Negative" and "positive" simply indicate whether current would flow to the reference or from the reference as a result of the potential.