# Questions tagged [voltage]

Voltage, otherwise known as electrical potential difference (denoted ∆V and measured in volts) is the difference in electric potential between two points (adapted from Wikipedia). Voltage can be constant (DC) or varying (AC).

4,676 questions
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### What is the difference between $V_{CC}$, $V_{DD}$, $V_{EE}$, $V_{SS}$

I've seen lots of schematics use $V_{CC}$ and $V_{DD}$ interchangeably. I know $V_{CC}$ and $V_{DD}$ are for positive voltage, and $V_{SS}$ and $V_{EE}$ are for ground, but what is the ...
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### Choosing power supply, how to get the voltage and current ratings?

Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to ...
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### What is negative voltage?

Just a general electronics question: What is negative voltage, like -5 Volt? From my basic knowledge, power is generated by electrons wandering from the minus to the plus side of the power source (...
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### Can a bird, previously at earth potential, get electrocuted by landing on a powerline at high-enough voltage due to the initial “equalization charge”?

At the voltage levels of typical overhead transmission lines in the US, a bird can land on one and be just fine (as long as it doesn't do something like spread its wings and touch a tree or something ...
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### What exactly is voltage?

Bit of a strange question, but what is it? My physics teacher said it was kind of like a "push" that pushes electrons around the circuit. Can I have a more complex explanation? Any help is much ...
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### What is ground and what does it do?

I'm a bit confused about the concept of ground, and perhaps voltage as well, particularly when trying to analyze a circuit. When I learned about Ohm's law in grade school, I learned how to apply the ...
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### How much voltage/current is “dangerous”?

Related: Safe current/voltage limit for human contact? From what I've heard: 110 V (or 220 V; household voltage pretty much) is dangerous (i.e. can kill you) I think there's consensus on this, ...
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### Why do many laptops run on 19 volts?

Typically mobile devices that have a mains-powered supply will accept voltage that is multiple of some single battery voltage. For example, 4.5 volts is 1.5 volts (AA primary battery) 3 times and 36 ...
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### How safe is 48V DC?

In many Power over Ethernet (POE) setups the transmission voltage is 48V or slightly more. While higher voltage has obvious efficiency advantages, how safe it is? Is there any risk of electrocution ...
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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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### Why aren’t the main conductors in this underground power cable made from copper?

I’ve found a piece of electrical cable left by the electricity company that supplies the area. They did a repair recently underground. I’m curious why the three big triangular conductors aren’t made ...
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### Why isn't there a potential difference across a disconnected diode?

I know this question sounds silly, as if there was a potential difference a current would be created when the terminals are connected together and this would mean energy has come from somewhere. The ...
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### Is current source also a voltage source?

I'm confused between current and voltage sources; I get the text book definition but I am not able to understand real world difference. To me both current and voltage sources seem as the same. I ...
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### What does 3V3 or 1V8 mean?

While reading a datasheet for an IC I came across the pin voltages being presented as 3V3 or 1V8. What does this representation stand for?
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### I2C 3.3 to 5.0 V conversion

I'm trying to interface a 3.3V I2C device to a 5V Arduino. I don't have any problem with SCL line. I can use a simple voltage divider but I have an issue with the SDA line as this is bi-directional. ...
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### Why don't we use low voltage Power Sources for high wattage applications?

Super nooby question involving Ohm's law, but this has been on mind this morning. Say I have a 60W device, and I want to power it. Usually this calls for a 120V source or something. However, why not ...
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### Two black boxes display the same impedance at all frequencies. Which has the single resistor?

Two black boxes display the same impedance at all frequencies. The first contains a single 1 Ohm resistor. Each end is connected to a wire, so that two wires protrude from the box. The second box ...
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### Help me understand the relationship between positive, negative, neutral and ground in AC and DC

This question stems from notations on schematics and seemingly conflicting information I'm seeing. I suspect that I'm seeing different vernacular for the same concepts--but I'm in a place where no one'...
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### Easy way to figure out a LED's Vf in order to pick an appropriate resistor

I was wondering what the easiest way to figure out a LED's forward voltage is, using measurement tools. I know we can assume red LEDs to be around 1.8V - 2.2V, and that we have similar information ...
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### Can I reduce voltage from 3.3V to 1.8V using a 1.5V battery?

I am interfacing to an EEPROM that works via SPI at 1.8V; unfortunately I do not have a 1.8V power source around and being this a weekend project, I wanted to complete it without going to the shop. I ...
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### Capacitor Gaining Voltage Over Time?

I recently bought two 3300uf 100v capacitors and have connected them in parallel. I charge them up to 100v and discharge them. I then hook up a multimeter and notice the voltage going up very slowly, ...
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### Least expensive way to boost DC voltage

What would be the least expensive way to boost a DC voltage? The aim is to convert 1.2 V/1.5 V (from an AA/AAA cell) to 3.3 V to power a small 8-bit microprocessor, like Atmel ...
19k views

### Is voltage the speed of electrons?

Current is the amount of electrons passing through a wire. Can we say that voltage is the speed of those electrons?
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### Why old PMOS/NMOS logic needed multiple voltages?

Why does old PMOS/NMOS logic needed multiple voltages like +5, -5, and +12 volts? For example, old Intel 8080 processors, old DRAMs, e.t.c... I'm interested in the causes on the physical/layout ...
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### “Ground” vs. “Earth” vs. common vs. negative terminal

This may just be me not having a degree in electrical engineering or electronics, but the whole notion of "ground" and "earth", when used in electrical circuit diagrams (especially integrated circuits)...
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### Why do condors get electrocuted by power lines?

Plenty has been said about why birds are safe to land on power lines. But condors, apparently, are not. The linked article explains that condors are electrocuted often enough and endangered enough ...
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### When is it okay to exceed the absolute maximum rating on a part?

I've always thought that the absolute maximum ratings on a part are the limits that thou shalt not violate. Period. End of story. However, another engineer is making the case that it's okay to ...
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### What can I put a kilowatt into? (Or: help me save my hairdryers)

TL;DR: I need something to discharge roughly 160A at 14V into, or 2.24 kilowatts. Any comments, or answers, with either a) something I can dump a kilowatt into, b) some way I can modify a common item ...
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### Voltage regulator vs. voltage reference

I need a cheap, somewhat accurate (~0.5%) voltage reference for some DACs. At first I was going to use a LDO voltage regulator (a TC1223 specifically) for this, it seems to fit the bill looking at the ...
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### Compensating the forward voltage drop of a diode signal rectifier

I am reading Arts of Electronics and they are showing this circuit: It says that D1 compensates for D2's forward drop by providing 0.6V of bias. I don't understand this circuit at all. Is the +5V an ...
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### Why is the diode forward voltage constant?

When you have a diode with a certain barrier voltage (e.g., 0.7 V for Si) and you apply a voltage higher than this barrier potential, why does the voltage across the diode remain at 0.7V? I ...
208k views

### Reducing voltage with resistors

I have been looking around for an easy way to convert 12V to 5V. I have seen some people saying that a simple resistor is all that is needed. $$Volts = Ohms \cdot Amps$$  Amps = \frac{Volts}{...
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### Whats does “hr” mean among “V” and “Ah” in batteries?

I have a battery which indicates 12V25Ah/10hr. What does 10hr mean here? I guess "hr" stands for "hour". But 25Ah already means if battery is fully charged and if I load 12V 25A (=300W) to it, it ...
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### Purpose of 2 serial resistors on voltage divider

In the schematic/picture below, what is the purpose of 2 serial resistors for voltage divider? Temperature, thermal runaway, stocks, prices, or something else? Thank you. simulate this circuit &...
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### Why does USB have Vcc=5V and high=3.3V?

I am thinking about adding USB support to a device of mine using V-USB. From what I read there and on other sites USB seems to have only 3.3V as a high level on the data pins, whereas the voltage ...
318k views

### how to reduce DC voltage using resistors?

How would one go about using a 12 V DC power source to power something which needs 4.5 V DC using resistors? Is there a way to determine how much adding a resistor would drop the voltage?
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### When people talk about a device “drawing” current, what do they mean? Why do devices under load “draw” more current?

In my (extremely rudimentary) understanding, the amount of current flowing in a circuit is determined by a) its resistance, and b) the voltage of the power source (voltage from beginning to end), ...
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### Difference between 0-18V and -9V - +9V

What is the principle of the difference between a supply which provides 0-18V and +9V and -9V? If I am using a circuit which takes a +/-9V power supply, and instead start powering it with 2 9v ...
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### Why is home voltage value higher during the night?

I have been monitoring the voltage in my own home (Mexico 120V 60Hz). I have been graphing the values every minute and I'm starting to notice that during the morning voltage gets up to 127, and during ...
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### Why is a current signal preferred to a voltage signal for long analog transmission?

Some sensors act like current sources, and I have seen it several times, especially for very long wires even at outdoors like wind vanes. 4-20 mA current loops are used instead of 0-10 V ...
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### Why are LEDs in my home unaffected by voltage drop?

As a result of heavy snowfall yesterday night, I woke up to find out that the regular 230 volts delivered to my house had dropped to 110 volts (fixed later that day by the power company). Old ...
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### Is MOSFET gate threshold voltage a limit or minimal “Full-on” switching voltage?

I've been shopping for some mosfet transistors, for a starter kit, and noticed listings that state a mosfet is suitable for 5v logic, but the data sheets says Gate Threshold is 1-2v. 4v gated mosfets,...
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### How does a nine volt battery make a spark?

With a nine volt battery, touching the two terminals together (or using a faulty terminal) will cause a spark roughly where I would want it to be. How is this possible? Is it ionizing only a very ...
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### How much time does it take for a switch to go from on to off?

The equation for the voltage across an inductor is v = L di/dt. When a switch changes the circuit from closed to open, it changes the current very quickly. My question is how would one know exactly ...
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### Why can sticking fingers in an electric outlet kill you?

I just wanted to learn some differences between volts, amps, ohms and so forth and came up with this question. If your skin has 100k ohms resistance and the outlet is 220v wouldn't the current flowing ...
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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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### Unplugging a router from the wall socket

When the internet appears to have crashed, the service provider invariably gives this advice as a first line remedy: unplug the router from the wall socket, then wait 5 minutes, then plug it back in. ...
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### Combining and Charging multiple Lithium Ion Battery Cells (3.6 V or 3.7 Volts)

1] Voltage: 3.6V or 3.7V Are all 18650 lithium ion battery cells 3.6 or 3.7 voltsor or are there different voltage Lithium Ion cells in the market as well? 2] Possible Voltage Shortage? Do all 3.6/3....
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### Why is mains power sometimes 110V and other times 120V?

(The same question can apply to locations with 220/240V mains, if I am not mistaken.) Frequently I see mixed ratings indicating that something is suitable for 110, 115, 118 or 120V (in the US). I've ...
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### High Voltage Capacitor, in a low voltage system?

Quick question, is using a Capacitor rated for High Voltage (lets say 35v) in a System that lets say supplies 5v (like for LEDS.....or what have you) Dangerous? Since it can store UP to 35v, will it ...