Questions tagged [current]

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

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272
votes
5answers
358k views

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 ...
84
votes
10answers
147k 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?
64
votes
9answers
360k 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, ...
160
votes
12answers
13k 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 ...
96
votes
9answers
86k 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 ...
1
vote
3answers
613 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 ...
12
votes
5answers
9k 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 ...
44
votes
7answers
53k 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 ...
12
votes
6answers
31k 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 ...
18
votes
2answers
35k 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 ...
11
votes
3answers
37k 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 ...
1
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2answers
367 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 ...
17
votes
7answers
7k 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 ...
16
votes
4answers
9k 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 ...
4
votes
3answers
5k 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?
18
votes
4answers
46k 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 ...
0
votes
2answers
12k 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) ...
9
votes
4answers
38k 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 ...
6
votes
3answers
16k 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 ...
22
votes
5answers
27k 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 ...
2
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2answers
6k 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 ...
12
votes
2answers
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}}\$ ...
4
votes
3answers
592 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 \$...
36
votes
13answers
8k 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!
22
votes
14answers
4k 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. ...
16
votes
8answers
100k 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 ...
7
votes
1answer
5k 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 ...
25
votes
7answers
21k 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 ...
0
votes
3answers
2k 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 ...
0
votes
2answers
428 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 ...
27
votes
7answers
37k 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 ...
8
votes
3answers
2k views

Do all components obey Kirchhoff's Current Law?

Kirchhoff's Current Law states that the net current through a node is always 0. AFAIK this derives from conservation of charge principle. My question is, is KCL applicable to any electrical component? ...
19
votes
4answers
33k 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 ...
18
votes
5answers
37k 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 ...
9
votes
1answer
24k 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.
4
votes
4answers
5k views

Limiting current from a 5v power supply [duplicate]

Possible Duplicate: Choosing power supply, how to get the voltage and current ratings? I have a 5V / 1A regulated supply. I'm thinking of using it to power a PCB which asks for 5V and 0.25A. In ...
-1
votes
3answers
159 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-, ...
30
votes
8answers
43k 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 ...
24
votes
4answers
36k views

Sizing a trace on a PCB to carry 2.5 amps

I need a trace on my PCB to carry up to 2.5 amps (average) current, with 5-6 amp spikes (it's going to a switch mode power supply.) How wide should the traces be? I've got a trade off between ...
17
votes
2answers
31k 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 ...
5
votes
2answers
40k 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 ...
19
votes
4answers
40k views

Why is it bad to mix new and old batteries?

I've seen warnings that it's bad to mix new and old batteries -- why? Is it a matter of a battery's age or a battery's voltage/current (remaining)?
18
votes
5answers
21k 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 ...
7
votes
5answers
71k views

How to get high current from 9 volt batteries

I am planning to make a solenoid.. Which should be so powerful as to lift up about 600N force. I have thought about it, and I have a doubt. How can I get DC currents up to 20-30 A using a 9 V battery? ...
7
votes
6answers
32k views

electric chewing gum prank circuit diagram

I'm new to electronics. I'm trying to find out how this works out of curiosity. http://www.amazon.com/gp/aw/d/B0050UNNQS Does anyone have a circuit diagram for the circuit? Is the current safe for ...
4
votes
8answers
10k views

Does voltage cause current or does current cause voltage?

So as the question asks, say you have a circuit with a battery connected to a resistor to form a complete circuit in this case does voltage cause current through the resistor? or does the voltage ...
8
votes
2answers
5k 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 ...
4
votes
4answers
29k views

Why does temperature modify the characteristics of a diode?

Everyone knows that temperature affects every semiconductor device you can think of, and diodes are no exception. From the below graph, we can see that the temperature has marked effects on pn ...
11
votes
4answers
18k views

Is it good practice to parallel relay contacts for increased current capacity?

Say I have a DPDT relay, like T92S7D12-24. The contacts of this relay are rated for 30A, but there are two sets of contacts. Can I parallel the contacts to get an effective 60A relay? Further, could I ...
6
votes
1answer
3k views

Should I calculate a resistor value for driving indicator LEDs from TTL outputs?

Traditionally I have always used a \$1\:\mathrm{k\Omega}\$ resistor as a current limiting resistor for driving an LED direct from a TTL (or 5V CMOS) output. I can't remember how I came by this value -...