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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44
votes
13answers
10k 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 ...
43
votes
7answers
178k views

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.
37
votes
4answers
63k 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 ...
36
votes
5answers
27k 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 ...
31
votes
3answers
14k 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 ...
29
votes
7answers
61k 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 ...
29
votes
6answers
14k 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 ...
27
votes
8answers
37k 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 ...
26
votes
7answers
3k 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.
25
votes
5answers
5k 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 ...
24
votes
6answers
6k 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. ...
24
votes
2answers
5k 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 ...
24
votes
3answers
62k 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 ...
24
votes
3answers
26k 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 ...
22
votes
11answers
29k 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 ...
22
votes
8answers
25k 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 ...
21
votes
10answers
7k 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 ...
21
votes
4answers
19k 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 ...
20
votes
5answers
3k 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 ...
20
votes
4answers
26k 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 ...
20
votes
3answers
81k 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 "...
19
votes
3answers
14k 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 ...
19
votes
1answer
691 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 ...
18
votes
4answers
10k 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 ...
17
votes
6answers
11k 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 ...
16
votes
4answers
10k 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 ...
15
votes
6answers
37k 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 ...
14
votes
5answers
2k 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 ...
14
votes
4answers
32k 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 ...
14
votes
7answers
9k 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 ...
14
votes
4answers
8k 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 ...
14
votes
6answers
18k 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 ...
14
votes
7answers
9k 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 ...
14
votes
5answers
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 ...
13
votes
14answers
24k views

Why are the power transmission/distribution systems AC and not DC?

Is there a good reason why we are not in the process of completely converting our electrical transmission system to DC? The main reason for using AC on the grid (no offense Tesla, I love you man) was ...
13
votes
5answers
154k views

LED bulb still emits light when turned off

I have recently purchased an IKEA LED bulb with G9 fittings, to replace a halogen one. This one. The bulb is installed in a lamp with 3 other halogen G9s in a 3 + 1 configuration with a 3-state ...
13
votes
5answers
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, ...
12
votes
10answers
5k 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 ...
12
votes
9answers
7k views

Is current with a frequency less than 1 Hz still considered DC?

We had a big argument last night with vague conclusions. Is the current with a frequency less than 1 Hz considered DC? It would still resemble a wave...
12
votes
6answers
4k 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 ...
12
votes
2answers
14k views

Does a DC toy motor emit AC when you spin it?

I just watched a video on Facebook by somebody who built a project to run an LED using a 9 volt DC motor scrounged from a battery operated toy. They simply wired the LED to the motor and then used a ...
12
votes
5answers
16k views

Schuko plug wiring

Why it is not important where we connect our hot and neutral wire when connecting them to our Schuko plug? Is it because current flows in both directions? It is only important that our ground wire is ...
12
votes
2answers
4k views

Why AC points on PCB are separated by a hole

I want to find out why, in professional PCB's, they separate between the AC (220 VAC) points on the PCB with a hole in the PCB: Is it for more isolation between the AC lines? Is leaving a distance ...
11
votes
8answers
8k views

How can a phone wire have multiple frequencies at once?

How can a phone wire have multiple frequencies? In my Networks Textbook about DSL vs Dial Up it says the following: The residential telephone line carries both data and traditional telephone ...
11
votes
4answers
2k views

Using DC in transformers?

I read in many websites that transformers can only be used, to step voltage up or down, with AC current (which is why AC is preferred for power transmission since they can transfer huge power in the ...
11
votes
4answers
679 views

Burnt AC Bulb Failover

I have a 60W, 120 VAC incandecent bulb used as a heater. Today it burned out and resulted in a water system freezing. I would like to be able to hook up a spare bulb that can come on if the first bulb ...
11
votes
3answers
1k views

How do I protect myself when testing a PCB that involves an AC line?

I have to test prototype PCBs that convert household AC into several DC voltage levels. I concern about my safety when working with AC and would like to know how to properly setup a testbench that, ...
11
votes
3answers
8k views

Control 220V AC line using MCU

I want to switch ON and OFF an incandescent bulb using MCU. Please suggest an electric switch (like relay or something) for 220V 50Hz AC line which can be used here. I would prefer something very ...
11
votes
4answers
3k views

Validate Triac circuit

I'm designing a device to switch a heating device driven by mains power. I've done quite a lot of research and realize there's a lot of information out there but as i'm dealing with potentially deadly ...
11
votes
1answer
267 views

Is this LED fixture safe for use with its current fabrication?

My father is currently interested in investing into a Ukrainian LED fixture manufacturer, and after he bought home a sample, I'm scared. There are 94 LEDs paired up with diodes, all in series, ...