AC confuses me. - Page 1
Forum section: electronics / beginners

author: Hextejas
author_rank: Frequent Contributor
timestamp: January 11, 2018, 09:33:06 pm
content: Please excuse a complete noob question.I am doing an exercise on a breadboard that has Vin as 9v AC.My wall wart takes the 120 down to 9v and I have a plug that lets me plug it into the breadboard.The breadboard of course has the + and - rails running down the side.So, since this is AC, does it have a + and - or does it make any difference ?Thanks and please excuse if a stupid question.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2018, 09:42:12 pm
content: The + and - are simply labels for your convenience, when working with AC they are irrelevant. There is no difference internally in the breadboard, it's just like the color of wire insulation.

author: paulca
author_rank: Super Contributor
timestamp: January 11, 2018, 09:53:43 pm
content: Yes, AC is confusing. It all gets into trigonometry to explain simple things like current, voltage, power.If you have two wires coming from the power supply I expect one of them is a "reference" voltage and the other will alternate around it. My maths brain doesn't work these days, I expect it doesn't really matter which is which. They will go +9V potential between them at some point and -9V potential at another. So if you reverse them there is no difference. They will alternate between each other at the frequency, probably 50 or 60Hz in your case.I hate AC.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2018, 10:01:27 pm
content: I don't understand what is confusing about AC, you don't need any fancy math to understand it on a basic fundamental level. Just look at how a simple AC generator works, a magnet rotating on an axis with the poles passing by a coil, the voltage will ramp up in one direction, ramp down to zero and then ramp up in the other direction as the other pole passes the coil then repeat.

author: retrolefty
author_rank: Super Contributor
timestamp: January 12, 2018, 02:26:11 am
content: I can get why AC voltage understanding can be confusing for beginners to get their head around for some. Even just stating 9 vac, one has to understand the different ways and AC source voltage can be stated, RMS value, peak value, peak to peak value, average value.When stating a source is 9 vac one has to understand that it's assume to be the AC RMS value unless specified specifically. And after all that one learns that these value relationships depend on the AC wave shape and if other then a pure sine wave source the voltage values relationships differ.There is a reason that electronic fundamentals courses start off with DC fundamentals before AC fundamentals.

author: TimNJ
author_rank: Super Contributor
timestamp: January 12, 2018, 03:29:34 am
content: Here's one way to look at it.AC is alternatingcurrent. And while you need a voltage to drive that current, let's first look at it as its name implies. In AC, the direction of current changes with respect to time. That means electrons might flow from left to right in a wire for a second, and then from right to left for a second, and then left to right, and on and on. How quickly the current changes direction is called frequency.Lets say we attach a load (a resistor for instance) up to an AC source. (Look at my attached drawings.) If the direction of current is downwards through the resistor, then the voltage on thewhitewire must be positive with respect to theblackwire. This is because current flows from high potential (voltage) to low potential. Similarly, if the direction of current is upwards through the resistor, then the voltage on theblackwire must be positive with respect to thewhitewire. This is exactly the same as saying the voltage of the white wire isnegativewith respect to the black wire.That kind of introduces the ideas of a reference node, or "ground", or "0V" or "-". If we make all of our measurements with respect to the black wire, then the voltage on the white wire will alternate between positive and negative values. This is because we lock down the black wire as the 0V reference point of the circuit. All other measurements in the circuit must be referenced to it (i.e. put the black lead of your multimeter on the reference node and make all voltage measurements with respect to it.)Maybe that's all very confusing, still..So to answer your question, no, AC does not have a + or -. You can get really philosophical when you start talking about the existence of "ground" or "-" (or whatever). But for your purposes, if you hooked up an AC voltage to the + and - rails on a breadboard, then 50% of the time the markings on the breadboard are going to be correct (when current flows out of the + terminal and back into the - terminal). But the other 50% of the time, current will flow from - to +, which is the opposite of what the markings suggest.

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2018, 12:15:20 pm
content: Quote from: Hextejas on January 11, 2018, 09:33:06 pmWith single phase AC (two wires) it doesn't matter which way it's connected. To make it easier to distinguish between the two wires, call one side 0V or neutral and the other side phase.If this were the mains or a control panel, one side will be connected to earth (neutral) and the other side phase, but the output from your transformer will be floating, so it's arbitrary.

author: Hextejas
author_rank: Frequent Contributor
timestamp: January 13, 2018, 12:39:05 am
content: Quote from: TimNJ on January 12, 2018, 03:29:34 am

author: TimNJ
author_rank: Super Contributor
timestamp: January 13, 2018, 01:14:39 am
content: Quote from: Hextejas on January 13, 2018, 12:39:05 amI know no one likes getting this answer, but I suggest taking some time to digest the fundamentals of DC (first) and then (I think) AC will make sense based off your understanding of DC.Here's a video I made a few years ago:To help understand your question, can you explainwhyyou think you might need a current source on "both ends"? What do you mean by "both ends"? Remember, a circuit is a complete loop/path (just like a auto race, bicycle race, etc.) An infinitely longrealwire (if one existed) would have an infinite resistance so it wouldn't do you very good. An infinitely longidealwire has no resistance, and therefore a current source or voltage source theoretically would have no problem driving some sort of load very far away.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: January 13, 2018, 10:34:16 am
content: Look, you humans think it's confusing? You should try being IN one of these wires..