Negative Power Supply Clarification

[PhysicsTest]

TL;DR Summary

Need to understand the circuit as shown below

I am confused with the negative voltage supply connections in electric circuit. If i have circuit as below with -5V source

it is same as below

In the 2nd circuit the 5V is directly shorted with the ground, does it mean the circuit is not correct? Am i missing something? Please advise.

 

[berkeman]

As long as the voltage source is floating (like a battery), it can be connected in either polarity.

Many/most DC lab power supplies float the +/- outputs so you can do just that (or stack multiple power supply voltages), and they usually make an Earth Ground connection available as well in case you want to use it.

https://eleshop.eu/korad-kd6002d.html

 

[Rive]

The 'ground' for such theoretical questions is just the declaration of the reference level/point.

Since it is the reference, usually it's taken as the 0V. So those drawings declaring the ground at 5V or -5V are not really the best: to have the other (not ground-connected) output of the voltage sources marked as -5V and 5V would be better.

 

[PhysicsTest]

The 'ground' for such theoretical questions is just the declaration of the reference level/point.

Since it is the reference, usually it's taken as the 0V. So those drawings declaring the ground at 5V or -5V are not really the best: to have the other (not ground-connected) output of the voltage sources marked as -5V and 5V would be better.

Thank you for reply, i really did not understand, how the circuit will look like with the above changes.

 

[Baluncore]

In spice, node 0 is always ground, and that reference is needed somewhere, or you can't measure voltages. A voltage source can be created and connected between any two nodes in the circuit.
You can change the circuit polarity by changing the sign of the voltage, or by swapping the connections or nodes.

Try this spice directive (with two text lines) to declare and connect two voltage sources, Vc and Ve.
Vc Vcc 0 +5
Ve Vee 0 -5

That will create two power supply nodes or rails, one Vcc = +5V, the other Vee = -5V.
Any pin or node labelled Vcc or Vee will then be connected to a supply rail.

Try swapping nodes and polarity to confuse readers, or yourself later.
Vc 0 Vcc -5
Ve 0 Vee +5

 

[PhysicsTest]

I am referring to the below video https://community.nxp.com/t5/Model-...T/Module-4-Space-Vector-Modulation/m-p/737654
I don't understand the Neutral point which he refers at several points starting at time 3.57. Why does he take neutral point? I observe that at the neutral point the voltage will be VDC/2 - VDC/2 = 0V. Is it a concept or need to connect the load as mentioned at the midpoint of the power source. I am sorry if i am asking the same question several times. Please advise.

Just to add i understood the concept of floating power supply from the previous posts, it is important only the difference of voltages. The negative point need not be at 0V or ground voltage.

 

[Baluncore]

The neutral point is the common terminal at the centre of the star or Y in a three-phase system.
That point could be connected to the neutral wire, which will be close to ground.

 

[Rive]

I don't understand the Neutral point which he refers at several points starting at time 3.57. Why does he take neutral point? I observe that at the neutral point the voltage will be VDC/2 - VDC/2 = 0V.

One of the reasons is exactly your observation. The usage of the 'neutral' point as a reference in such a three phase system makes things symmetrical from the point of view of the load. Since you expect some sinus-like (symmetrical) waveforms on the load, this makes some calculations a lot easier.

If you discuss that system from the POW of the battery and control electronics, you can still use the negative terminal as Earth (reference).

As far as circuit analysis goes both approach is valid, as long as you don't mix them up.

It is confusing. Always keep your actual reference point in mind.