Questions tagged [ac]

This tag refers to alternating current mains power. It usually applies to voltages with >100V RMS, but can also be used for ex. 24V industrial AC power.

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230V AC to 5V DC converter, lossless

Is there any IC which converts 230V AC to 5V DC? As lossless as possible. I want to connect my microcontroller to an ordinary electrical socket and I don't have enough space available. Thanks.
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48 votes
13 answers
12k views

Why do fundamental circuit laws break down at high frequency AC?

We're just starting the whole RF scene having dealt with DC and low frequency AC for all our previous courses. I understand that at high frequency AC, fundamental circuit laws don't apply anymore ...
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41 votes
3 answers
78k views

Creepage distance for PCBs handling line voltage AC?

What is the required creepage (e.g. trace-to-trace) distance for PCBs handling 240VAC rms? What about 120VAC? This is for UL and CE certification. The standards for PCB Creepage (e.g. the distance ...
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40 votes
5 answers
37k views

Why use complex numbers to represent amplitude and phase of AC

Why is it that in AC circuits, sine waves are represented as a complex number in polar form? I don't logically understand from a physical perspective why there is an imaginary part at all. Is it ...
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33 votes
3 answers
16k views

How can I detect a power outage with a microcontroller?

I have the following power supply configuration: AC MAINS -> UPS -> 24V POWER SUPPLY -> 5V VOLTAGE REGULATOR -> PCB (microcontroller). What's the best solution to detect the power outage on the mains ...
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32 votes
7 answers
74k views

What exactly are harmonics and how do they "appear"?

From reading so many sources online, I still cannot grasp why a different waveforms have harmonics. For example: when designing a silly amplitude modulation (AM) circuit that puts a square wave from ...
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31 votes
6 answers
18k views

How precise is the frequency of the AC electricity network?

If I make a digital clock which first detects the AC frequency roughly (whether it is 50, 60, 100Hz, etc), then uses it as its clock pulse source, how precise would it be? Would it work world wide ...
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31 votes
10 answers
12k views

AC: Why differentiate between Ground and Neutral?

Current flows through a conductor connecting points at different potentials. Leaving aside multi-phase details, common/conventional AC systems use a 3-wire setup: Wire-1: a line/live/hot/phase wire ...
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29 votes
7 answers
8k views

Why do power grids tend to operate at low frequencies like 60 Hz and 50 Hz?

I understand that a two-pole AC generator has to rotate at 3600 rpm (60 Hz USA) in order to maintain a 60 Hz frequency and higher frequencies would require these generators to spin even faster which ...
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  • 476
28 votes
7 answers
5k views

NEMA 5-15P blade holes

Why do most NEMA 5-15P (North America 125V 15A) line and neutral blades have holes in them? I've been wondering this for quite some time and haven't yet found a satisfactory answer.
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27 votes
8 answers
40k views

Why does a capacitor block DC and not AC? [duplicate]

If someone can explain why a capacitor blocks DC, but not AC, with some math, I will understand all of that much better. I know that there are picture animations illustrating this, but I really want ...
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27 votes
5 answers
33k views

Why can I put an electrolytic capacitor on AC?

I have seen here and there schematics with electrolytic capacitors put on AC. And this sounds weird to me. Electrolytic capacitors have a polarity, right? If we invert the polarities on DC, bad ...
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26 votes
3 answers
69k views

AC to DC without transformer. How does this thing work?

I disassembled a Chinese-made flashlight and found they use just capacitors to bring down the voltage instead of a transformer. They are using it to charge small lead-acid battery. My question is how ...
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26 votes
3 answers
29k views

How do the tiny AC->USB power supplies work?

The iPhone and Kindle both come with tiny power supplies that take in 120v AC and output around 5V at 0.85 to 1 amp. They seem to be too small and light to have a transformer, and generate very little ...
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25 votes
11 answers
34k views

Why is sine wave preferred over other waveforms?

Why did scientists chose to go with sine wave to represent alternating current and not other waveforms like triangle and square? What advantage does sine offers above other waveforms in representing ...
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25 votes
5 answers
6k views

Why are relays still used in electric ovens?

I bought a new electric fan oven recently. It has a digital thermostat and control system. Yet much to my surprise, I can hear a relay clicking on and off inside it to control the power to its heating ...
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25 votes
3 answers
8k views

Why does inverter synchronization with the grid take 5 minutes?

We had some solar panels & an inverter installed recently. Every time the inverter is turned on (after maintenance or with the first light of the day) it has to synchronize its generation phase ...
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24 votes
6 answers
11k views

Why does voltage always lead current by 90 degrees in an inductor?

I have learnt that in an inductor voltage leads current by 90 degrees. However, I do not fully understand why it is 90 degrees. I have been looking everywhere for more information on why this is so. ...
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24 votes
8 answers
30k views

Why are some AC outlets and plugs polarized?

This will show my naivety when it comes to alternating current... but why are some plugs and sockets on AC outlets polarized (one prong is taller than the other)? (See this question on DIY.) (Or this ...
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24 votes
6 answers
20k views

What are the highest DC and AC voltages or currents that can be considered as safe?

By safe I mean safe for us when we touch a wire that is not insulated. I have heard that human body resistance is normally 50k Ohm when dry and 20K Ohm when wet. Therefore, these voltages or currents ...
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24 votes
3 answers
92k views

Difference between Line and Neutral in AC

I want to find out what is the difference between the AC (220 VAC) lines, (phase and Neutral). As I know that the Alternating Current (AC) has no polarity, so why do we have a "phase" line and a "...
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23 votes
4 answers
21k views

How do I select the accompanying components for an optocoupler?

I'm using an optocoupler (MOC3021) to sense the On/Off state of an electrial appliance using a microcontroller ATmega16L. How do i go about doing this? My mains supply specs are 230V, 50Hz. How do I ...
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20 votes
3 answers
30k views

Why most appliances in US don't have third prong

Most other plugs I encountered have three prongs - hot, neutral and ground I assume. For example all BS 1363 plugs have one. Also CEE 7/7 have ground connections and while CEE 7/16 exists it is not ...
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20 votes
5 answers
4k views

Why is only the fundamental frequency component said to give useful power?

In most books it's said that the harmonic content in an AC line does not transfer power (just the fundamental frequency does), but no explanation. It seems intuitive, but why is that true? Edit: In ...
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19 votes
4 answers
12k views

Can you put a 120vac positive line into a breadboard?

Is there anything I need to know about putting a 120vac positive line in my breadboard? (Like, for example, if it's a bad idea to do that to begin with.) It will just be a christmas lights strand at ...
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19 votes
1 answer
732 views

Does this device which claims to reduce reactive energy from a house work?

I've opened a device that states that it can reduce the reactive energy from a house, by plugin it directly to the power outlet/wall socket. I think it is a con, so I've opened it and I saw this black ...
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  • 471
19 votes
6 answers
54k views

Using oscilloscope safely with AC mains

I know there is quite few discussions already on this topic, but since English isn't my first language, I though it better to get some more information about this. I have used oscilloscopes to ...
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  • 191
18 votes
8 answers
5k views

Is a sine wave (or any signal) that has no zero crossing still considered AC?

This question will most likely come down to semantics. A friend and I were discussing a point that his EE instructor made, that if you consider the output from a solar panel over more than a few days, ...
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18 votes
3 answers
4k views

Does this AC to DC + breadboard setup have the ability to kill/severely hurt me?

I'm currently building a PC peripheral that uses an AC to DC converter to power a set of DC fans. I'm not a trained EE or electrician, so I want to make sure I'm not putting myself a risk with this ...
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18 votes
6 answers
27k views

Why do we use \$s=j\omega\$ in AC analysis instead of \$s=\sigma+j\omega\$?

In AC analysis, \$s=j\omega\$ when we deal with \$sL\$ or \$1/sC\$. But for a Laplace transform, \$s=\sigma+j\omega\$. Sorry for being ambiguous but I would like to connect the questions below: Why ...
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18 votes
4 answers
11k views

Why don't AC transformers burn up

I'm marginally familiar with the way that an AC transformer works. After viewing this question: Why don't all motors burn up instantly? It got me thinking about the same thing with AC ...
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  • 1,887
17 votes
3 answers
5k views

Why do trolleybuses power cables use DC and not AC?

I am interested in multiple aspects of trolleybuses as a way of transitioning from fossil-fuel powered buses to "green" public transport. I am not an electrical engineer myself but I do have ...
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17 votes
9 answers
2k views

Why harmonic distortion and not at other frequencies?

Despite searching I can't seem to find the answer to this. In understanding harmonic distortion, one piece of the puzzle that is missing for me is: - why does the distortion manifest at harmonic ...
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17 votes
4 answers
9k views

How can energy "physically" be fed back into the grid?

I have found some similar questions (below), but I am still having issues understanding what is "physically" happening when energy is "fed back into the grid" and the equipment ...
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17 votes
4 answers
3k views

Can we tell which way AC power is going through a cable without cutting the cable?

Assume we have a very long cable whose ends are not visible or accessible. One end we know is a generator, and the other a load. Can we determine which end of the cable has the generator attached ...
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17 votes
8 answers
13k views

How can a faulty USB charger kill you?

This question relates to this incident here, where a women in Australia died from a faulty USB charger that allowed the full 240V mains voltage through the USB cable. Now from my understanding you ...
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16 votes
9 answers
5k views

Cheap and safe way to dim 100+ 60W Incandescent bulbs

For my wedding reception, we are building ~25 lighting fixtures with 4 bulbs each (fixture per table for dinner). For aesthetic and budget reasons, these are 60w "Edison-style" bulbs which we want to ...
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16 votes
4 answers
40k views

Does AC Power have Polarity?

When I run 24VAC into a full wave bridge rectifier followed by a 220uF electrolytic capacitor to turn it into ~32VDC, the source has two wires. Does it matter in what order I connect the AC wires to ...
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16 votes
7 answers
10k views

Why are rectifiers-inverters used to drive AC motors instead of using the rectified current to drive DC motors?

I understand the advantages of using a rectifier-inverter system to drive an AC motor instead of simply plugging it into mains power, as it allows much better control of its speed and performance; but ...
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16 votes
6 answers
1k views

Can step down transformers malfunction in such a way that they become live?

I know a few things about electronics, but transformers are still an unknown for me. I know AC to DC converters use them to isolate live voltage from the DC output, or at least I think that's what ...
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15 votes
5 answers
3k views

Why don't more devices incorporate full-wave rectifiers as reverse polarity protection?

Recently, I was introduced to the idea of using a full-wave rectifier, in order to protect against reverse polarity damage in DC devices. I hadn't even considered using a rectifier in an already DC ...
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  • 1,807
15 votes
5 answers
5k views

detecting zero cross in ac?

I need to detect zero crossing for a soft starter. A long while ago, I have done it using a 1 Mega ohm resistor directly connected to micro controller at one side and live power at another side. I was ...
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  • 507
14 votes
9 answers
6k views

Is this an AC or DC signal?

My teacher says that this signal is DC, but if the polarity varies from 0V to -8V , shouldn't it be AC? He says that DC + AC = DC
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14 votes
8 answers
4k views

Should I use AC or DC between buildings?

I’m planning to install solar panels on the roof of one building (the charge controller and batteries are in this building), to power a DC circuit in another building. No AC equipment needs to be ...
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14 votes
11 answers
9k views

How to check (with DIY methods) if an Inverter returns a Square or a Sine Wave?

I do not have an Oscilloscope, but I want to check if my 12V DC to 230V AC inverter does produce a sine or a square wave. Any way to verify that? I know how a sine wave and a square wave sound, so ...
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14 votes
4 answers
10k views

Advantage of using AC generators in cars instead of DC generators

Most cars use an AC generator then convert its voltage to DC via a bridge diode rectifier to charge the 12v battery. Why wouldn't a DC dynamo be used instead? Is it because AC dynamo gives better ...
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14 votes
4 answers
2k views

Does the number of turns in Turns Ratio matter in transformers?

Suppose we have two transformers with same turns ratio, \$N_2/N_1\$. First transformer \$A\$ is of \$100:10\$ and the second transformer \$B\$ is \$1000:100\$. Both have same turns ratio so both can ...
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  • 2,709
14 votes
3 answers
79k views

Extending the backup runtime of a UPS with an external car battery

I'm looking for a simple/inexpensive way to have a longer power backup time. My fist thought was putting 2 cheap UPSes in parallel. But I quickly found it's not that simple. You would need another ...
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  • 429
14 votes
5 answers
8k views

24VAC/5VDC power supply design

I am planning to create a water valve controller using an MCU and a set of solenoid-controlled valves. The solenoids run on 24VAC (40mA inrush, 20mA holding). The MCU is on a board that draws ~100mA, ...
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  • 4,490
13 votes
6 answers
5k views

What makes mains voltage safe in homes?

When I started learning about electrical engineering, one of the most important safety guidelines that I learned was that voltages over 36V can be dangerous. As my circuits professor put it, 120VAC ...
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