# Tagged Questions

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

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 ...
38k 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 ...
31k 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?
8k 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 ...
3k 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.
598 views

### Sharp corners in PCB traces

Why does a typical PCB always has rounded tracks? What harm can a sharp edged PCB track can cause? Please explain!
6k 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 ...
29k 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 ...
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 ...
454 views

### Which way does electricity power a circuit?

Working with some circuits this summer, I ran into what everyone eventually does: Current flows from + to - despite the electrons flowing (well, bumping into eachother) from - to +. I understand the ...
694 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?
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### 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 ...
4k 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 ...
3k 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 ...
4k 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 ...
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 ...
206 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 ...
3k 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 ...
578 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 ...
1k 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 ...
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### 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 ...
371 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 ...
5k 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 ...
1k 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 ...
510 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: $\displaystyle R=\frac{V_{s}-V_{f}}{I_{f}}$ ...
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: ...
1k 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 ...
198 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?
432 views

### Transistor current mirror circuit

simulate this circuit – Schematic created using CircuitLab Can somebody please explain me how a transistor mirror circuit works and how can it be used as over current detector? I badly ...
221 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 ...
730 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 ...
4k 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, ...
512 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?
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### 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 ...
328 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 ...
1k 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 ...
431 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 ...
2k views

### Voltage drops and current for LED?

I created the following circuit on a breadboard and used the Arduino Uno 3.3V supply for the power supply: ...
566 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 ...
875 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 ...
980 views

### Constant-Current for a 20W LED

Can someone explain "constant-current" circuits? When are they necessary? What happens without one? What's the simplest version of such a circuit? I bought some 20W LEDs off ebay intending to make ...
395 views

### Have I got these simple power calculations right? (for battery consumption)

Lets say an ideal AA battery I have provides 2.0 Watt hours. It could be different, just a quick search for this example. My very simple load is the battery connected to a 5mm LED. The LED has a ...
512 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 ...
2k views

### What is a phase difference between voltage and current?

I was reading the question here with some interest, because I am in the end stages of construction on a project that involves 16 solid-state relays. I'm using non-zero crossing type, because that's ...
347 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 ...
716 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 ...
266 views

### Will the short circuit damage electronic components or just the supply?

This I have thought about. We know that shorting the supply creates zero resistance path between the vdd and gnd. We also know that current will pass through the least resistance path and since there ...
183 views

### Should I use a resistor between an input pin of MCU/CPLD and VCC/GND?

Some times, I want my MCU or CPLD to input a static logic. So, I choose to tie it to VCC or GND. The problem is that should I put a resistor in series to limit the current? I just think by myself for ...