Comment on What is negative voltage?
nous@programming.dev 5 days ago
I amusingly imagine a force “sucking” the current backwards. 🤭
If you think of a positive voltage as pushing then it is not anymore inaccurate to think of a negative voltage as pulling. At least if you think in terms of DC, which is what power supplies will typically give you.
Voltage is all relative. The sign just shows the direction of flow. If you plug a volt meter in backwards it will show a negative voltage. That means the current will flow from the negative to the positive terminals.
If you have three terminals, let’s say A, B, and C. If you measure 12V between A and B. Then 24V between A and C. Then you will also be able to measure 12V between B and C. Or -12v between B and A. Or -24 between C and A. The direction matters.
You could label any of these as ground it makes no real difference to the system. Only how you talk about it. Without a ground label then you would not say C is at 24V as it has nothing to compare to. If you pick A as ground then it is said to be 24V, but if you pick B as ground then it is said to be 12V. If you pick C as ground then it is 0V. So when we say a point is at a voltage then that is implied that it is that voltage relative some other point we decided was ground.
Things get a lot more complicated with AC as electrons move back and forth in the wire they don’t have the same direction as they do in DC.
akunohana@piefed.blahaj.zone 5 days ago
That’s awesome! Thanks for the explanation! So, with my (maximum) 12V computer chassi fans, what are we comparing to? What is “ground” here? I slapped the fans onto my server rack, bought a DC adapter with variable voltage and wired them up so that I can manually select the voltage and thus the speed of the fans. Is this also a situation in which the various voltages/speeds are 3V, 4.5V, 5V etc higher than something that the person who built the electric grid in my house decided to call “ground”?
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Sorry for the blurry pic.
Onomatopoeia@lemmy.cafe 5 days ago
First, the DC voltage of these fans is a separate voltage system from the AC/mains of the house.
Whether the 12v comes from a transformer or a switching power supply, the DC and AC circuits are effectively separated by that device.
The ground of the 12v system exists only within that system (and is provided by the transformer/power supply).
Consider the tranformer/power supply to be a battery with a positive and negative terminal. Inside, a chemical reaction will take place that moves electrons from one set of plates to another, leaving “holes” in one set, and extra electrons on the other.
This creates an imbalance - an electrical potential - and entropy says everything should try to balance. So those extra electrons provide the voltage, and once a circuit is made, will immediately try to balance the potential by moving through the circuit to the other terminal.
Using a transformer/power supply provides those electrons (and a sink - somewhere for those electrons to return to) from AC/mains voltage.
This is a pretty simple way to view it, there are more details that an EE would take me to task for, but it’s a good starting point.
akunohana@piefed.blahaj.zone 4 days ago
This was great! Thanks!
nous@programming.dev 5 days ago
Nothing to do with the electrical grid. It is just that the positive lead of the output of the power supply is whatever voltage you select above the negative lead on the output side of the power supply. Those are the two relative points in this situation. You can call the negative lead ground if you want. Wiring diagrams for the device you connect it to will likely call it ground. It is not the same thing as the grounding wire in a plug socket.
Note that the grounding wires in a home have nothing to do with any live circuit in your house. They are to make faults in the system safer by giving a path for electrons to flow if a conductive casing ever gets into contact with a live wire. In normal operation nothing should be flowing through it at all. With doubly insulated devices (like that power supply) the grounding wire in the plug socket is probably not connected to anything at all in the device. The plug likely does not even have a grounding wire in it.
akunohana@piefed.blahaj.zone 4 days ago
Thanks for the clarification! So, there is an overlap, then, when it comes to the term “ground”? Grounding - and its wiring - in the greater electric grid is actually there in case there is a ground fault, while ground wires in smaller DC circuits can refer to the “outgoing” lead/terminal/wire?