Phil Frost
  • Member for 8 years, 11 months
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4 answers
150 votes
72k views
How can I efficiently drive an LED?
Accepted answer
247 votes

An LED requires a minimum voltage before it will turn on at all. This voltage varies with the type of LED, but is typically in the neighborhood of 1.5V - 4.4V. Once this voltage is reached, current ...

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4 answers
64 votes
327k views
Does mAh measure how long a battery would last?
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80 votes

mAh (or mA·h) is not how many milliamperes a battery can deliver in an hour. That would be mA/h. Current, measured in amperes, is already a rate of stuff. Specially, one ampere is one coulomb ...

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1 answers
46 votes
55k views
How can I implement regenerative braking of a DC motor?
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74 votes

Probably you already have it, and just didn't know it. If you are driving a motor with a half-bridge or H-bridge and PWM or similar, you have regenerative braking. Let's consider a half-bridge, since ...

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14 answers
73 votes
30k views
My understanding of RC circuits is broken
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73 votes

Let's try this Wittgenstein's ladder style. First let's consider this: simulate this circuit – Schematic created using CircuitLab We can calculate the current through R1 with Ohm's law: $$ {...

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3 answers
65 votes
27k views
MOSFET: Why the drain and source are different?
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72 votes

Myth: manufactures conspire to put internal diodes in discrete components so only IC designers can do neat things with 4-terminal MOSFETs. Truth: 4-terminal MOSFETs aren't very useful. Any P-N ...

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5 answers
30 votes
35k views
How does a capacitor smooth energy?
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69 votes

Capacitors don't store charge. That's such a worthless statement because it's based on this word "charge" that has multiple meanings. Please forget you ever heard it. They also do not smooth energy. ...

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6 answers
50 votes
69k views
Does turning a lighbulb on and off repeatedly use more energy than just leaving it on for hours?
Accepted answer
59 votes

Leaving it on would use more energy, absolutely. Sometimes, people try to convince themselves that turning a light on and off uses more energy because there is some high inrush current, or some such ...

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4 answers
75 votes
92k views
What is ground and what does it do?
55 votes

Sometimes people get confused just by the many definitions of the word. ground noun the solid surface of the earth; firm or dry land: to fall to the ground Often, grounds. the foundation or basis on ...

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2 answers
43 votes
150k views
What is input and output impedance of an opamp?
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52 votes

The short answer: input impedance is "high" (ideally infinite). Output impedance is "low" (ideally zero). But what does this mean, and why is that useful? Impedance is the relationship between ...

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2 answers
38 votes
79k views
Difference between Brushless Motor and Stepper Motor
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43 votes

The two are largely the same, fundamentally. However, they differer in intended application. A stepper motor is intended to be operated in, well, steps. A BLDC motor is intended to be operated to ...

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5 answers
2 votes
6k views
Is Ohm's Law violating itself?
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39 votes

You are confused about what the concept of infinity means. Infinity isn't a number that can ever actually measure a quantity of something, like resistance, because it's not a real number. As Wikipedia ...

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8 answers
62 votes
35k views
Why do we need an isolation transformer to connect an oscilloscope?
36 votes

The alligator clip on the scope probe: (image source) is connected, through the power cord, to Earth. If you clip it to something that isn't at Earth potential, you get a large current, and things ...

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3 answers
25 votes
10k views
Why should I worry about a motor causing my supply voltage to shoot up when the back-EMF can't exceed the supply voltage?
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36 votes

A motor driven by an H-bridge is also a boost converter. Here's an H-bridge: Replace the motor with an inductor, resistance, and voltage source (back-EMF): Let's just consider that we are driving ...

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5 answers
33 votes
35k views
What makes a pull-up/down resistor strong or weak?
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34 votes

Strong means low resistance. Weak means high resistance. Of course low and high are relative terms, and so are strong and weak. The reference for this relationship must be inferred from context. A ...

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5 answers
54 votes
44k views
How can PCB trace have 50 ohm impedance regardless of length and signal frequency?
33 votes

A transmission line has distributed inductance and capacitance along its entire length. We can think of it as infinitely many little inductors and capacitors along the line: simulate this circuit &...

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7 answers
14 votes
3k views
Why do LEDs have a maximum current?
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32 votes

It's difficult to come up with an analogy because the usual analogies for electrical systems are fluid systems. A great thing about fluid systems is that the working fluid is also good at cooling ...

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2 answers
21 votes
10k views
Burned out filament but the light bulb has small resistance
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32 votes

These bulbs contain a shunt wire which is normally insulated from the bulb but closes when a high voltage is applied. From How Stuff Works: You can also see the wire in your image, below the burned ...

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7 answers
37 votes
44k views
How antenna radiates(how currents flows through wire)
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31 votes

I'm guessing you don't understand how current can flow if there is no complete circuit. Let's take a simple quarter-wave dipole as an example: simulate this circuit – Schematic created using ...

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2 answers
9 votes
39k views
Does a capacitor store charge?
Accepted answer
30 votes

If by charges you mean electric charges, then no, a capacitor does not store charges. This is a common misconception, maybe due to the multiple meanings of the word charge. When some charge goes in ...

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3 answers
8 votes
14k views
ESR and parallel capacitors
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29 votes

An equivalent circuit for what you describe is: simulate this circuit – Schematic created using CircuitLab Note that the resistors aren't in parallel, so we can't use the usual parallel ...

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1 answers
26 votes
27k views
How can I measure back-EMF to infer the speed of a DC motor?
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27 votes

One way to do this is to briefly stop driving the motor, long enough to let any residual current from the driving voltage die down, and then simply measure the voltage. The time it takes the current ...

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5 answers
9 votes
97k views
How to get high current from 9 volt batteries
27 votes

A 9V battery has approximately a stored energy of: $$ \require{cancel} \frac{560\cancel{m}A\cancel{h} \cdot 9V}{1} \frac{3600s}{\cancel{h}} \frac{1}{1000\cancel{m}} \approx 18144VAs \approx 18 kJ $$ ...

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5 answers
26 votes
32k views
Why can I put an electrolytic capacitor on AC?
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27 votes

"Can" and "should" are two things. Should you do this? No: this use is outside the specified operating parameters of ordinary electrolytic capacitors. You seem to understand this already. Can you do ...

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3 answers
36 votes
139k views
Help me understand the relationship between positive, negative, neutral and ground in AC and DC
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27 votes

Ground means whatever is attached to this symbol in the schematic: Everything that touches this symbol in the schematic is actually connected to everything else that touches the symbol. Since so many ...

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6 answers
17 votes
21k views
Use of global variables in Embedded Systems
25 votes

The reasons you would not want to use global variables in an 8-bit system are the same you would not want to use them in any other system: they make reasoning about the program's behavior difficult. ...

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6 answers
39 votes
3k views
How fast does electricity flow?
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25 votes

How fast does electricity flow? This is a good question, because it seems like a simple enough question, but usually it indicates some underlying misconceptions. The first difficulty in answering the ...

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3 answers
23 votes
14k views
What is killing my MOSFETs
Accepted answer
25 votes

From the driver to the gates, the wires are ~15cm. Does this cause rining? Almost certainly, and it's a fair bet that this is destroying your MOSFETs, by one or more of these mechanisms: exceeding \$...

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4 answers
20 votes
58k views
Do electrons actually flow when a voltage is applied?
25 votes

Thinking about current in terms of electrons moving around is the start down a path to a poor mental model of how electricity works. Here are just a few things wrong with it: Electrons are but one of ...

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3 answers
21 votes
134k views
What is "offset null" in IC 741?
Accepted answer
25 votes

The datasheet gives an example. By adjusting the pot we can null any offset error. An offset error is when the inputs are exactly equal but the output isn't exactly zero. This error is also ...

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10 answers
29 votes
14k views
Why can a voltmeter still measure potential difference if it has a (theoretically) infinite resistance?
Accepted answer
24 votes

The underlying difficulty seems to be the belief that some current must flow to measure voltage. This is false. Since you are a physics teacher, I'll explain by making analogies to other physical ...

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