Questions tagged [current]

Flow of electric charge - typically movement of charge carriers, such as electrons. Measured in amperes (A).

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327 votes
6 answers
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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 ...
Olin Lathrop's user avatar
157 votes
4 answers
79k views

How can I efficiently drive an LED?

I understand that I can not connect an LED directly to a battery because it will draw too much current. Thus, there must be something else in the circuit to limit the current. What options are there? ...
Phil Frost's user avatar
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96 votes
10 answers
181k views

How to get more than 100mA from a USB port

I heard that the current limit for a USB port is 100mA. However, I also heard that some devices can get up to 1.8A from a port. How do you get past the 100mA limit?
200ok404notfound's user avatar
78 votes
10 answers
460k views

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, ...
user541686's user avatar
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175 votes
12 answers
19k views

How does the current know how much to flow, before having seen the resistor?

With the following circuits as examples : and How will the current I know how much to flow? Would any other wave travel first in the circuit and then come back ...
Prabhanjan Naib's user avatar
0 votes
3 answers
2k views

Op-amp: Virtual ground principle and other doubts [closed]

I can't understand some stuff about op-amps. Here is my list of doubts: If an op-amp amplify the difference between the V+ and the V-, how can it work if for the principle of virtual ground V+ = V-, ...
user11579389's user avatar
111 votes
9 answers
117k views

Why does a resistor need to be on the anode of an LED?

Please be kind, I am an electronics nub. This is in reference to getting an LED to emit photons. From what I read (Getting Started in Electronics - Forrest Mims III and Make: Electronics) electrons ...
Spechal's user avatar
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2 votes
4 answers
1k views

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 ...
Perspicacious's user avatar
6 votes
4 answers
4k views

Source-free, under-damped, parallel RLC with 2 intial conditions

I think I'm suffering from brain-drain. I'm trying to solve a 2nd order differential equation for the natural response of an under-damped parallel RLC circuit with initial conditions of \$I_L\$ and \$...
Andy aka's user avatar
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55 votes
7 answers
64k views

What is the usage of Zero Ohm & MiliOhm Resistor?

I am new to PCB design and I noticed that some schematics use 0Ω or 100mΩ resistors. What is their purpose and why do we need to use them in our PCB design? Normally if we wish to probe how much ...
Dennis's user avatar
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7 votes
1 answer
10k views

How do I tell what gauge wire I need?

I've been working on providing power for high current applications for my project. I'm working on a project with high current (or at least compared to what I'm used to...) What a lot of people point ...
Anonymous Penguin's user avatar
16 votes
6 answers
14k views

Was Benjamin Franklin wrong (about conventional current)?

I am starting to see a lot of people claiming that convential current is 'wrong' because Franklin made an error when he first started investigating electrostatics, and that later scientists didn't ...
Myforwik's user avatar
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13 votes
2 answers
3k views

How does the current limiting resistor for an LED affect current and voltage drops?

I'm having a bit of trouble understanding currrent limiting resistors in simple LED circuits. I know that I can determine the optimal resistor like so: \$\displaystyle R=\frac{V_{s}-V_{f}}{I_{f}}\$ ...
mk12's user avatar
  • 255
6 votes
3 answers
15k views

Structure of a usb 2.0 connector [duplicate]

What is the role of the outer metal connector? (the one with two square holes) Does it have a potential? Is it insulated from the 4 pins? Are the 4 pins insulated from each other?
minusatwelfth's user avatar
6 votes
4 answers
4k views

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 ...
user8718165's user avatar
19 votes
2 answers
46k views

Input impedance of Arduino Uno analog pins?

I'm using the ACS712 hall-effect based current sensing chip on some small robotic arm motors, and reading in the analog voltage with an Arduino Uno. I've had reasonably good results, but only after ...
user avatar
1 vote
2 answers
644 views

Cant understand an ab amplifier diagram

I been trying to figure out this diagram, and Can't understand what happens from the beginning of the input , how the voltage changes in a way that "shuts off" one transistor. Also cant understand the ...
user1779374's user avatar
12 votes
6 answers
38k views

Need help calculating resistance for transistor base

I have 3 12VDC/40A automotive relays (datasheet) that I want to use with my Arduino. Based on the tutorial I am following (link) I need transistor, resistor, and diode. I am not an electrical engineer ...
jM2.me's user avatar
  • 443
2 votes
2 answers
7k views

How to sense the zero crossing of current?

I am trying to sense the power factor angle using microcontroller. Now the mechanism that I have thought it that Measure the zero crossing of voltage When the voltage zero is sensed, start the ...
Santosh's user avatar
  • 161
30 votes
6 answers
50k views

Parallel MOSFETs

When I went to school we had some basic circuit design and stuff like that. I learned that this was a bad idea: simulate this circuit – Schematic created using CircuitLab Since the current ...
BufferOverflow's user avatar
12 votes
3 answers
42k views

What exactly does a diode do?

Disclaimer: I'm a total beginner here. As explained in other answers here, current in a circuit doesn't flow from negative to positive and the direction doesn't matter. So what should I make of ...
Bill's user avatar
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8 votes
4 answers
22k views

Detect car +12V using Arduino

I am looking to detect 12+V from a car wire using an Arduino. I have found the following schematic: I know how crazy automotive voltage can get so I just want to make sure the schematic I found ...
StealthRT's user avatar
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7 votes
5 answers
1k views

Professor said no current flows to ground

According to the professor, adding the ground at 0V has no effect on the current flow and current flows in this circuit as if the grounding point did not exist so the answer is E. How have people died ...
Ardalan G.'s user avatar
0 votes
3 answers
6k views

Input power rating on power supplies - how does max amp draw apply to 220V

If a power supply says the input power is 100-240V, ~1.5A, I'm assuming that the "1.5A" draw would be at 100V, not at 240V, right? I.e. would the total max watts be 150W regardless of the input ...
g491's user avatar
  • 181
26 votes
5 answers
54k views

How much current can Solderless Breadboards handle?

I'm working on a project with a few stepper motors. I want to breadboard them so I know the circuit works well before creating a physical PCB. Each of the three motors is rated 2A, but I'll be ...
Anonymous Penguin's user avatar
19 votes
4 answers
11k views

Pulse-powering heavy loads with a coin cell

Lithium coin cells are rated for fairly low standard current draws, on the order of 1 to 5 mA. Also, while they allow greater pulsed current draws (i.e., periodic bursts), this appears to be ...
boardbite's user avatar
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19 votes
9 answers
156k views

How do I calculate the temperature rise in a copper conductor?

If I pass a current through a copper conductor, how can I calculate how hot the conductor will get? For example, if I have a 7.2kW load powered by 240VAC, the current will be 30A. If I transmit this ...
BG100's user avatar
  • 5,818
18 votes
7 answers
9k views

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 ...
Giancarlo's user avatar
  • 339
27 votes
4 answers
77k views

Do electrons actually flow when a voltage is applied?

It's said in books that a circuit is a closed path and thus that electrons come back to the source. If that's the case, what would happen when there were an earth fault in a circuit? How would the ...
Ranjit's user avatar
  • 271
22 votes
5 answers
44k views

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 ...
Farrukh's user avatar
  • 231
11 votes
4 answers
43k views

Does electricity go from negative to positive or vice versa?

It's common knowledge that electrons flow from negative to positive, but I have noticed than often the direction of the current is ignored. For example, the resistor is often put AFTER the LED, or the ...
user avatar
9 votes
1 answer
27k views

How Exactly Current Flow Through a Capacitor? [duplicate]

Capacitors behaving as an open circuits, How can we understand the concept of the Electrons flowing through a capacitors.
Chad's user avatar
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1 vote
2 answers
25k views

Is 20 watts of electricity dangerous?

I have a few circumstances which invlove someone being shocked with 20 watts of electricity and whether it would be deadly. So here they are: Would 1000 volts at 20 miliamperes of AC (2MHz frequency) ...
Boris Deletic's user avatar
40 votes
13 answers
11k views

Why can current only flow in loops? [duplicate]

In the following image: Why can't current flow across the following wire? It's a simple question, but I've kind off always wondered. Thanks!
Physco111's user avatar
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33 votes
7 answers
36k views

How do I calculate the power rating for zero-ohm resistors?

Based on my earlier question, since there supposed to be zero voltage (V=IR) drop across a 0 Ω resistor, how do we select the power rating of such a component? For example, let's say I were to ...
Dennis's user avatar
  • 1,421
22 votes
14 answers
7k views

Finding a faulty chip that draws too much current

Note that this is a theoretical question - there is no schematic I can show. I will show some schematic, but it will be a very simplified version of an actual circuit, only for illustration purposes. ...
nettek's user avatar
  • 1,035
16 votes
2 answers
70k views

When does and when doesn't current flow to ground?

I was thinking about when current can and can't flow to ground. I came up with that current will only flow to ground when we have at least two grounds in the circuit, since then we have a complete ...
Quantum spaghettification's user avatar
4 votes
1 answer
3k views

Two directional FET switch. Is it possible?

Is it possible to have a high side MOSFET switch that will allow any direction of current? I was trying to design one but without the success. Or should I use a relay?
0___________'s user avatar
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4 votes
10 answers
21k views

Voltage drop across a single resistor and across two resistors

I have been having difficulty understanding voltage drops across resistors. Now, I know the theory and how to apply Ohm's law. The question is why does the voltage drop across resistors of the same ...
Cauan Kazama's user avatar
1 vote
2 answers
11k views

A confusion about the formula for transistor Vbe Ic characterisitcs

Below is NPN BJT transistor Vbe Ic characteristics and the formula: Many texts approximate this equation as: Ic = Is*e^(Vbe/Ut) and following this they say when Vbe=0, Ic becomes equal to Is. ...
GNZ's user avatar
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1 vote
3 answers
11k views

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 ...
joaocandre's user avatar
0 votes
2 answers
599 views

Ordinary Differential Relay Problem

The answer given in my workbook to the above problem is A. The things I was able to point out secondary c-phase current = 300A and other secondary phase currents = 0A since the entire current ...
Nikhil Kashyap's user avatar
33 votes
8 answers
53k views

Current flow in batteries?

I am reading a basic electronics book: "There are no Electrons: Electronics for Earthlings" and I came across a clever passage about the fact that you need a closed circuit in order for current to ...
user avatar
29 votes
7 answers
47k views

How to carry high current on PCB

I need to pass high current on some part of my circuit. I used an online PCB track width calculator to see that required track width is about 5mm and minimum clearance is 1mm, which makes it about 7mm ...
hkBattousai's user avatar
  • 14.1k
21 votes
6 answers
55k views

MOSFET: When can we not assume that the gate current is 0?

A common rule of thumb you hear when learning Electrical Engineering is that the gate current of a MOSFET is always approximately 0. When is it not safe to assume that it is 0?
Andres Riofrio's user avatar
21 votes
5 answers
60k views

Is it OK to put LEDs in parallel? [duplicate]

I wonder if it's a good idea to put LEDs in parallel, as below: I've heard that it might not be because the voltage threshold won't be exactly the same for each LED, so they'll shine all with very ...
Kar's user avatar
  • 1,557
18 votes
2 answers
43k views

Constant current, constant power and constant impedance loads

Ive been looking for information on constant power, constant current and constant impedance/resistance loads, there's some information about the constant current loads, but very little that really ...
S.s.'s user avatar
  • 2,780
11 votes
5 answers
2k views

Using a 2 A, 48 VDC rated switch in a 8 A, 12 VDC circuit

I have a push button make/break switch rated at 2 A, 48 VDC. I want to use it on my aircraft in an 8 A, 12 VDC circuit. Can I use this switch in this circuit or will the switch burn up?
litewave1 's user avatar
10 votes
3 answers
22k views

Reading and understanding electrical specs on laptop AC adapters

My Dell laptop comes with an AC adapter with the following specs: 65 W Input AC: 100 - 240 V, ~1.5 A, 50 - 60 Hz Output DC: 19.5 V, 3.34 A My HP laptop: ...
user avatar
9 votes
2 answers
6k views

Ripple current in a linear power supply transformer

I'm a bit confused regarding linear supplies and their input currents (i.e. on the input side of the voltage regulator). To start with, here's a test circuit: \$R_{bogus}\$ is just to make LTspice ...
exscape's user avatar
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