# Questions tagged [ac]

AC stands for Alternating Current. It usually applies to voltages with >100 V RMS, but can also be used for other AC voltages, like 24 V industrial AC power.

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### AC Circuit Having Only Capacitor

In my book, it is written that at any instant, PD across the Capacitor Plates = Applied EMF ---------(1) Assuming that it's a pure Capacitor of Capacitance C, I think if Voltage across Capacitor ...
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### 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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### 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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### 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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### How can there be a current without a voltage?

Recently while trying to understand inductive AC circuits better, I got very confused with a question. Let's suppose we have a purely inductive AC circuit with ideal wires and a switch. Let's also ...
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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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### 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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### 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 ...
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### 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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### 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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### Energy stored and Lagging of Current In a Inductive Circuit

In a purely Inductive AC Circuit, the voltage/emf Leads the current in phase by 90. The graph between Voltage and Current is given below: Now can anyone here please tell me in a more intuitive way ...
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### Help understanging ac signal connected to a charged capacitor (coupling capacitor)

(Assuming ideal elements) My friend is trying to convince me that the voltage across $R_L$ varies between 4 and 6, says the average value doesn't decrease and stays fixed at $5V$ forever. I'm not ...
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### 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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### Understanding this AC coupled circuit

I'm having a little trouble understanding this circuit and could use some help. So far I understand that this is an AC coupled circuit, and the capacitor is a blocking capacitor. I also know that ...
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### 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 ...
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### Calculating the capacitor values to control ceiling fan speed

I have an old ceiling fan motor that runs with a 1.5µF run capacitor, at what I believe is, its full intended speed. With the help of some folks here at StackExchange I've wired everything up as in ...
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### Laptop charger leaks AC while giving DC voltage at the same time. How?

So, I got some laptop charger from my neighbor and I was thinking about to do something with it. Then one day I feel tiny shock from the output of the charger. I thought ac to dc filtering mechanism ...
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### Why AC power plugs have three pins

Can someone clearly explain the tasks of each pin in a plug. Is there a difference between one phase and three phase plugs? Does the electricity enters our home from line connection and goes back to ...
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### 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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### 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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### Sensing AC high voltage to microcontroller

I'd like to sense if I have AC 220V 50HZ available, and convert it to some logical level. I made such circuit, will it work? I use C1 as a resistor. Any recommendations what can be improved? ...
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### How to know if this is a band-pass filter?

I have the circuit below. How can I see that it is a band-pass filter? I tried calculating the equivalent impedance for the parallel part of the circuit: \bar{Z}_{parallel} = \frac{\bar{Z}_C\bar{Z}...
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### AC detection for microcontroller

I need to monitor the state of multiple relays carrying 110VAC and 60VAC using a microcontroller (I will probably use an Arduino) to ensure the relays are working properly. To do this I have figured ...
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### AC Optocoupler for 230V line detection

I am trying to detect line voltage (230V AC, if the supply is ON/OFF) by a microcontroller with the help of an AC optocoupler. The optocouplers I have evaluated are PC814, H11AA1 and SFH620A-3, out of ...
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### why do rectified voltage boosts after adding a capacitor?

So I found this 9V 1A AC power supply laying around and decided to make a DC source. I rectified and added a capacitor to make things a bit even. After adding a capacitor my voltage boosted from 9V ...
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### Switch current rating AC vs DC

I'm looking for a high current DC switch (able to switch something like 5V@30A) and I've encountered several DPDT that might do what I need, however they are only rated for AC applications (3A@250VAC ...
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### multi channel ac dimmer for arduino

I have been looking for a simple ac dimmer circuit to use with my arduino and/or raspberry pi. I have used the TLC 5940 PWM Driver before just to control LEDs and I wanted to adapt this to controlling ...
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### Few questions about basic concepts in electronics

So I'm rather new to electronics and I'm struggling to understand some basic concepts because I picked up little information from many different sources so it's starting to get confusing :/ Why does ...
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### Why can I put an electrolytic capacitor on AC?

I have seen here and there schematics with electrolytic capacitors put on AC. This sounds weird to me. Electrolytic capacitors have a polarity, right? If we invert the polarities on DC, bad things ...
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### 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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### How does the circuit of a basic variable speed electric drill work?

I am trying to 'automate' the control of an electric drill motor that I'm taking from a garden variety variable speed drill (from Harbor Freight.) It has a universal AC motor which I basically ...
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### What are the differences between an AC and DC coil relay?

If a DC coil was just a coil, it would saturate. If an AC coil was just a coil, the magnetic field would drop to zero at 120 Hz and the relay would chatter. Presumably the designs in some way prevent ...
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### Why doesn't an AC solenoid coil change magnetic polarity on each cycle?

I am trying to figure out how an AC solenoid coil maintains its N and S magnetic polarity. Since in a sinusoidal AC, current alternates in both directions. How does the solenoid not change polarity (N ...
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### Aren't non-polarized plugs a little dangerous? Why still use them?

Sorry if there is an answer for this kind of question somewhere. I can't find it anywhere. Take a look at obvious toaster example: If you live in Europe and have non-polarized plug and a toaster (a ...
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### 120VAC 60Hz Zero Cross Detector

I'm designing a digital phase-controlled light dimmer, and I need a zero-crossing detector circuit. From what I've read, it's a bad idea to wire the AC main directly to the microcontroller through a ...
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### Dangerous AC or DC [closed]

Is alternating current more dangerous than direct current? And give me explanation for that... Assume that 230 V, 50 Hz AC voltage and 230 V DC voltage. Which is more hazardous to ...
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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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### 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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### 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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### 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 ...
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### Why does a neon lamp indicator on a power strip switch flicker in the dark?

Neon lamp indicators embedded in power strip switches indicate whether the switch is ON or OFF. When the neon lamp is ON in a dark environment (lights of the room are off), the lamp begins flickering ...
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### AC fan speed control

I use this fan to cool my room, but sometimes it is too fast and I have to frequenctly turn it on and off to keep normal temperature. Maybe it is possible to reduce the speed by connecting a resistor ...
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### Single-phase to 3-phase conversion

I have seen some boxes (with some capacitor circuits inside) which do single-phase power supply to 3-phase power supply conversion. The problem with them is that they cause the load motor to heat up, ...
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### To what extent to I need to be concerned about PCB punch-through when dealing with AC mains?

I'm working on a project that deals with AC mains directly. I know about maintaining proper creepage distances for adjacent tracks, and that you can add isolation slots if you lack the space to ...
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### Why do electronic circuits need their power supply to be DC, or rectified AC?

I'm pretty sure the answer to this should be obvious - even to me ... It's been a while I've been reading and attempting to construct simple electronic circuits. The supply almost always is either ...
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### Why we use conjugate of current than the original phasor in the calculation of Complex Power i-e S=VI*

i have studied different books and sites for the above question but did not get the right concept that why the complex power encounter current conjugate not the original Phasor.if someone explain it ...
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### High power three phase ac motor controller

I have come across this question a lot in the design of a special 3 phase motor controller I plan to build this summer and I think I need to ask it directly to solve it. I need to be able to ...
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### Why Does DC Load Draw Twice As Much AC Current Through Transformer?

I have designed the following circuit, comprised of 4 loads, 1 of 4.2ohms and 3 of 21ohms, drawing a total of 2.4Arms (1.5A, 300mA, 300mA and 300mA respectively) of AC current through the secondary of ...
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### How do I design a current transformer?

I want to make a Current transformer using toroidal ferrites to measure 220/50-60Hz AC current(40A max). Is it possible? And how can I calculate secondary turns count? Ferrite properties: H = 2000, ...
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