Positive vs. Negative Ground

[dream_noir]

I feel kind of dumb, since I've asked questions like this before, but Ive never been able to quite grasp the concept of positive vs negative ground. From what I've read, here are some assumptions, can anyone help me out if these are correct or not?

- Negative ground is the standard, where the hot wire has +9v and the ground has 0v at it.
- Positive ground is a rarer, where the hot wire has -9v and the ground has 0v at it.

While I've heard people say electrons flow backwards in a positive ground circuit, my basic education in physics tells me that isn't right. The components are just oriented the opposite way since electrons flow from negative to positve and the polarity of the hot is reversed. However whats confusing is when people say it doesn't matter what sort of power supply is powering a single pedal, I don't really get that. Nor do I really understand how a battery can power both negative and positive ground pedals. If somoene cares to explain, I'd love to hear it. Maybe some other people who are new at this might like to hear too.

[MikeH]

I had these same questions recently, here is the thread I started:

http://www.diystompboxes.com/smfforum/index.php?topic=51123.0

[puretube]

I prefer:
tip positive; negative ground!
(but that`s not the question here...)

[dream_noir]

That thread unfortunately just left me with more questions. I think I kind of know whats going on, but why are there negative and positive voltage regulators?

[R.G.]

If you have a physics background, you're going to be fine.

Ground is whatever you say it is. That is, in a circuit not actually connected to a rod driven into the dirt, ground is a reference point. This is a little bit like the theory of relativity. We don't actually care what everything else is doing, because we have decided that our "ground" is the only stable voltage in the universe, and all other voltages will be measured with respect to our personal ground. A simpler way to say that is to say that ground is where you connect the black lead of the meter to measure voltages.

So we know what ground it. How can it be positive or negative?

It can't. Let's take a 9V battery and measure its voltage with respect to ground. We hook our black lead to our own personal ground and touch the red lead to a battery terminal. What's the voltage? Zero. We touch the other terminal - the voltage is zero. That is because the battery is disconnected from our ground, and except for extraneous field effects, lightning, cosmic rays and such, there is no way for the battery to force a voltage between itself and our ground. The METER impedance connects our ground to the battery through the leads, and whatever part of the battery it touches becomes the same voltage as our "ground".

Now we take a wire and connect the positive terminal of the battery to our ground.  With our meter we measure the battery terminals again. The positive terminal measures 0V,  because it is tied to our ground by a wire, and there is no current flowing through the wire to make a voltage drop. We have forced the positive terminal to be at ground. When we measure the negative terminal - ahah! it's -9V, as it should be. The battery forces a potential difference of 9V between its terminals, no matter what they're connected to, and with the positive terminal connected to ground, it's the negative terminal that is free to have a voltage.

Because we have grounded the positive terminal, we would now call the battery a "positive-ground power supply".

If we instead connected the negative terminal of the battery to our ground reference, it would form a "negative ground power supply" instead.

This is only confusing becase the terms negative ground and positive ground make you think that something is happening to ground. It's not. What changes is the polarity of the power supply. A positive ground power supply is the same as a negative voltage power supply, just called by a different name. A negative ground power supply is the same as a positive voltage power supply, just called by a different name. The words "positive ground" or "negative ground" just tell you which side of the battery is connected to your ground reference point.

I think I kind of know whats going on, but why are there negative and positive voltage regulators?

It has to do with what happens in bipolar power supplies.

A bipolar power supply is one with a single ground (there can only be one true ground in a circuit; all others are with reference to that one) and two "batteries". One battery has its positive terminal connected to ground, the other has its negative terminal connected to ground. So the resulting power supply has three terminals, one positive with respect to ground, one negative with respect to ground, and ground itself. This is useful because certain circuits perform best when balanced around ground, not off to one side of it.

That's a bipolar power supply with batteries. What if we want to do it with an AC powered supply? We take the AC, rectify it to pulsing DC, filter it to slightly rippling DC with capacitors and/or inductors, and then we want to make it not ripple and be an exact voltage. So we make a regulator circuit. The regulator circuit takes in the rippling DC and ground. It can't change ground, but it shaves off the rippling and a little of the top of the DC to make a constant output voltage with respect to ground.

If the regulator is fed a voltage that's positive with respect to ground and then shaves off the positive side with respect to ground, that's a positive regulator.

If the regulator is fed a voltage that's negative with respect to ground and then saves off the negative side with respect to ground, that's a negative regulator.

There are tricks that can be played with regulators to make a regulator regulate the ground side, but those are complications you don't want to mess with. These circuits are still consistent with this explanation, but it takes a lot more typing to explain them, and you don't need to know that stuff yet. Maybe never.