Since all circuits have current adding this tag is rarely appropriate. Flow of electric charge - typically movement of charge carriers, such as electrons. Measured in amperes (A).
55
votes
9answers
2k 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 ...
39
votes
3answers
18k 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 ...
30
votes
7answers
19k 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?
22
votes
7answers
5k 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 ...
22
votes
3answers
2k views
Whats inside a resistor?
How does a resistor "resist" current/potential?
I know it's an elementary question, but I'm sure others are wondering too.
16
votes
5answers
5k 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 ...
16
votes
5answers
1k views
In theory, is it possible to make a logic gate that uses zero current?
CMOS greatly reduces the current draw of ICs because one of the complementary FETs is always in the non-conducting mode, so there is only a flow of current during the transition between states, which ...
15
votes
2answers
551 views
Why so many pins for the MOSFET's drain?
The FDC855N comes in a 6-pin package, 4 of which are connected to the drain, and only 1 to the source. Why this difference? The source sees the same current as the drain, doesn't it?
13
votes
10answers
2k views
Ways to determine polarity (dc) without a meter
I came across this question about how to figure out the polarity of a cell phone charger without a meter. This made me think of what methods you could use to figure out the polarity other than using a ...
12
votes
4answers
2k 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 ...
12
votes
4answers
2k views
High PCB copper thickness: What are the pitfalls?
We need to carry high currents on a PCB (~30Amps sustained), so we are likely to order our PCBs with high copper thickness. So far we've only used 35 microns in our designs, so 'high thickness' for us ...
11
votes
5answers
3k views
current sourcing, current sinking
I'm a student studying electronics and I'm having trouble understanding the concept behind current sourcing and current sinking. We've covered it in a lab using a 7404 and an LED and all that. Just ...
11
votes
4answers
2k 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 ...
11
votes
2answers
291 views
If supply amperage is higher than max. amperage, do I need a resistor?
This is a pretty basic question, as I'm still learning the fundamentals of electronics. I understand the analogy where amperage is compared to the quantity of water moving through a hose.
I have a ...
11
votes
4answers
179 views
Is \$I_e = I_c\$ for phototransistors?
Due to the base current, in a common emitter circuit the emitter current is a bit higher than the collector current:
\$ I_e = \dfrac{1+\beta}{\beta} I_c \$
I was wondering how this is for a ...
10
votes
6answers
470 views
Understanding voltage and current
While reading "Electronics for dummies" went through the following block and I realised that I have some uncleared concepts about electricity:
Electrostatic discharge involves very high voltages ...
10
votes
4answers
801 views
Why does electric arc in a switch prefer a curved path over a straight path?
Recently I found this video of a 500 kilovolts line being opened under load.
When the switch contacts are pulled apart an electric arc predictably starts. While the contacts are close to each other ...
10
votes
7answers
16k views
How much voltage is “dangerous”?
Related:
Safe current 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, no ...
10
votes
7answers
663 views
Which everyday components involve flows of charge that are not electrons?
I like this explanation of why there's nothing wrong with conventional current being the opposite direction from electron current. It mentions batteries and fluorescent bulbs as two cases where the ...
10
votes
2answers
3k 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 ...
9
votes
3answers
139 views
Current produced by thermocouples
I can't seem to find an answer to this on Google.
Take a K-type thermocouple for example. The voltage produced is around 41uV/K. I'm wondering how this changes when a load is put across it. What ...
8
votes
4answers
686 views
Pumping a few Amps for a 100usec
I like to pump 4-5A to a high power LED for 100usec. My system has only 3.3V battery, this 100uSec high power event takes place once every 10sec.
What is the best way of doing this without upsetting ...
8
votes
3answers
1k views
Need help operating a transistor
I have a relay operated by an NPN transistor that needs some voltage applied to the base. I'm working with an arduino and if I give it a digital port, all's well. The circuit looks like this one:
...
8
votes
1answer
463 views
Can I burn out my multimeter's µA range by using a few mA of current?
My multimeter only has one 400mA fuse protecting the µA and mA range (and a 10A fuse protecting the amp range.) Is it possible to damage the microamp range by feeding milliamps through it?
8
votes
1answer
902 views
What does this circuit symbol mean? (Circle with horizontal line through it)
I came across the following symbol in a datasheet (functional block diagram on page 8). What does it mean?
For future searchers, the symbol is a circle with a horizontal line (diameter) through ...
8
votes
3answers
368 views
Blowing a Fuse to Permanently Disable Functionality
Certain electrical designs require permanently disabling hardware functionality on the fly. Sending an overcurrent to a weak fuse may be a method for accomplishing this, breaking the circuit in a ...
8
votes
1answer
391 views
How current limiting resistor for LED affects 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}}\$
...
8
votes
1answer
186 views
LED current while off
I have a vague recollection that while off, a LED can actually receive light and generate a small current (in uA).
Is that correct? How does that work? What is the best way to prevent this?
8
votes
2answers
235 views
Wilson current mirror
I built a super wilson current current mirror. The Iin is 4-20mA at 24Vdc. Iin to Iout accuracy was a really bad 5%. I put 100 ohm resisters on the source to ground, and that helped a little. How can ...
7
votes
5answers
2k 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 ...
7
votes
3answers
2k views
How to measure current to a DC motor?
I'm trying to measure current to a DC motor on an RC car I took apart. The car is powered by 3 AA batteries and seems to be made of three different boards. Two of the boards are for switches and LEDs, ...
7
votes
4answers
330 views
Do Linear regulators have a minimum current to maintain their output voltage?
When I was searching Digikey for a ~6V to 3.3V Linear regulators, there was a minimum current column in the listing results. This kind of threw me because I didn't know that there was such a thing, or ...
7
votes
3answers
1k views
Can I using nichrome wire to light a candle electronicaly?
I want to be able to create a set of candles that light automatically (for a presentation) however it's almost 100 candles and I need an inexpensive solution. I know that nichrome wire get's hot when ...
7
votes
1answer
614 views
will a circuit breaker designed for 230VAC work for 12VDC?
Will a standard home automatic circuit breaker, designed and sold as to work on 230V AC (alternating current), and for example 16A, or 25A etc, will it work (break the circuit if it exceeds the rated ...
7
votes
3answers
252 views
IC Max IO Pin Current
I was looking at the spec sheet for the ATTiny2313A microcontroller and it specifies an absolute maximum rating for the DC current through each IO pin at 40mA, and ...
7
votes
1answer
265 views
Excited diode allow flow opposite direction, possible?
I searched for some information regarding LED's and found out that they can only go one way because of the two metals inside of the LED have 2+/- electrons making it easy to go one way but difficult ...
7
votes
3answers
6k 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
176 views
Current flow priorities in PSU / PSU review
Being an electronics hobbyist, I'm having quite a challenge to design a PSU, which must run both on batteries (during the night time) as well as on Solar energy (daytime). The device is intend to run ...
6
votes
3answers
392 views
What would be the difference in a coil of wire and just a cylinder of solid copper?
Ok so I am learning about induction and I always see coils of wire with no insulator. So I was wondering, What is the significance of it being a coil of wire rather than just a solid cylinder since ...
6
votes
6answers
6k views
How electrons inside copper wire behave when source is ac or dc?
When a simple copper wire is connected to AC or DC, what actually happens to the electrons inside the wire?
I.e. What kind of path do they follow when they reach the end of the wire? If anyone can ...
6
votes
5answers
1k views
How can I measure the conductivity of a copper rod?
I would like to perform an experiment to measure the conductivity of a copper rod. What device can I use to perform that experiment? Is there such a thing as a conductivity meter? All I found was an ...
6
votes
4answers
326 views
Why Batteries with same voltage has different currents?
Experts say " Current depends on Voltage". So, if the voltage is high, current would be high.
Agreed; (I=V/R)
If the voltage is low current also would be low. Agreed -> I=V/R
But why then two ...
6
votes
3answers
638 views
Current and Voltage Ratings for Multi-Conductor Connectors
I am currently looking at some multi-conductor connectors that will handle reasonably high currents (approximately 30 A at 24 v). When reading datasheets, I see that the connectors have both a maximum ...
6
votes
3answers
303 views
Why can't a power supply supply low current?
I've just read this article and don't quite understand how a power supply can't supply such a low current. I was always under the impression a power supply could supply anything up to its rating, but ...
6
votes
4answers
992 views
Is it OK to attach an LED directly to a 5V Attiny?
From the datasheet, I though the AT90S1200 had current-limited pins and when running at 5V would sink the proper amount of current through a green LED attached to + (0 turns on the LED, 1 turns it ...
6
votes
2answers
270 views
Is there any way to boost DC current?
I'm designing a circuit to charge mobile battery. My source is giving a DC output of 15V, 100mA, whereas I need DC 6V, 450mA for charging. Is there any way to boost DC current?
6
votes
4answers
519 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 ...
6
votes
3answers
2k views
current limiting a shorted load to 20A
analog newbie here and first time on this forum... thank you for reading!
What i have is a control for pyrotechnics. I have all the digital control stuff figured out, but analog bits are not my ...
6
votes
2answers
817 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 ...
6
votes
1answer
732 views
Current limiting at mains voltage
I am driving a BTB16-600BW triac using a MOC3052 exactly as suggested in the MOC3052 datasheet:
The current into the power triac gate is limited using the highlighted resistor. My triac requires at ...
