| Electronics > Beginners |
| Need somebody to talk to about AC concept |
| << < (3/11) > >> |
| dmills:
The GND is a lie! Sometimes a useful lie, but a lie none the less. All voltages are measured between two points, and all currents flow in loops, everything else stems from this. "Ground" is a really crap word in electronics, especially around beginners because it has several subtly different meanings and in some cases more then one applies in a rather context dependent way. Quite often we have some node in a circuit that it is convenient to use as a reference for measuring a pile of other voltages relative to, and quite often we will refer to this as ground, this is one of of the term. Then we have the 'safety ground' use of the term, by which we really mean a conductor that does NOT normally carry current used to provide a path back to the supply neutral by some means that serves to blow fuses and open breakers in the event that a fault makes a metal case live with respect to ground. This is the second use of the term and can imply quite a different voltage to the 'ground' in the first use. Them we have the 'ground' that is in some sense a connection to the mass of the planet, usually has little difference relative to the second case (by design) but some caution with that assumption is advised there are faults that can cause these last two to develop significant voltage between them. This is the third meaning. Regards, Dan. |
| agehall:
--- Quote from: tester43 on June 26, 2018, 10:35:20 am ---AND YES: by intuition i did as you suggested: I put rectifier bridge to "produce" myself a gnd reference. So now I have just positive AC waves all the time for the price of diodes cutting voltage a bit. --- End quote --- No no no, that wasn't at all what I was suggesting. That is how circuits that care about current flow deals with AC. You don't need a rectifier to "produce a gnd reference" - you PICK one, arbitrary. Go back to basics - imagine a 1.5V battery and a lamp hooked up to it. The natural GND point for most people is right at the negative pole of the battery. However, that doesn't have to be the case. You can just as well say that the positive pole of the battery is the GND point. The result for all calculations will be the same (well, signs might change etc, but in essence everything is still the same). You are stuck at the idea that GND is always at some global 0V reference point which simply isn't true. The sooner you get rid of that idea the sooner you will get the hang of this. |
| tester43:
I think my problem in this is that in AC in general there is no single "point" or potential that would not "float" - move constantly it's value. Guys I'm doomed :( |
| whalphen:
In your circuit, ground is whatever node you want it to be! The current is continuously changing. But you can't say the voltage is changing unless you first indicate how you are measuring the voltage. Voltage measurements represent a potential difference between two points. So voltage measurements require two contact points. You can call either of those contact points ground. Or you don't need to call it anything at all. But when you communicate or record the voltage measurement, you need to also communicate which contact points were used. To simplify this, most people call one of the contact points ground and use that same contact point or node as a reference in all of their voltage measurements -- and label this as ground on the schematic. It's then implied that, unless otherwise indicated, that ground node is the measurement reference point (i.e., one of the contact points) for all the voltage measurements. It's also usually convenient to equate (attach) the ground reference point inside the circuit with conventional ground reference points used outside the circuit. And those could be the cabinet, the mains ground, and even the Earth itself. Think of an AC supply as a DC supply that is always changing. You can measure voltage across the two supply lines just as you can measure voltage across a battery. In either case you may call one of your contact points ground. It's arbitrary and entirely up to you. But, typically people call the negative terminal of the battery ground. And, often, most people will call one of the contact points on an AC supply ground. If the AC source is the mains, then it's typical to attach the mains ground to the circuit ground. And, don't confuse the mains neutral with ground. It's intended as a current carrying conductor and is also tied to the mains ground and the Earth ground at the source. The neutral does not always have zero volts potential. At distances away from the source, it can have a significant potential due to resistance when it's carrying a large current! |
| JourneymanWizard:
You'll get this. Pick a wire on the output of your transformer. Color or tape that one blue. That is now your Return/Neutral/V-. The other wire is Source/Hot/Live/V+. The blades that plug into the wall on US outlets are different sizes due to safety in household wiring. The smallest should be the LIVE/Hot connection; the larger should be NEUT/Return. The third (round) connection on a 3-prong plug is Earth. The blades are that way mostly for Edison (light bulb) sockets and the like: at your distribution panel in your house, the NEUT wire is supposed to be bonded with the Earth connection (which is connected to a grounding rod and/or your cold water pipes). This is to help with safety and give a local safety reference. The NEUT is supposed to be connected to the most touchable part of a light bulb socket, so if your fingers brush it during installation you don't get zapped (NEUT is connected to earth, you are connected to earth, the difference is small enough for safety). Don't stress overmuch** until you are above 40V ac rms (the threshold for potential human dangerous) and if there is something referenced to earth ground (e.g. household wiring). ** which is to say normal amounts of care and attention need to be paid |
| Navigation |
| Message Index |
| Next page |
| Previous page |