Hot answers tagged

147 votes
Accepted

Ceramic caps vs electrolytic. What are the tangible differences in use?

1. Capacitors There are a lot of misconceptions about capacitors, so I wanted to briefly clarify what capacitance is and what capacitors do. Capacitance measures how much energy will be stored in the ...
user avatar
  • 26.4k
61 votes

Ceramic caps vs electrolytic. What are the tangible differences in use?

For example, why do I see it suggested to use ceramic caps for power decoupling per microprocessor & a larger electrolytic capacitor per board? why not use electrolytic all around? The three ...
user avatar
  • 389k
47 votes
Accepted

Why do microcontrollers have so little RAM?

There are several reasons for this. First of all, memory takes up a lot of silicon area. This means that increasing the amount of RAM directly increases the silicon area of the chip and hence the ...
user avatar
32 votes
Accepted

Why do LEDs have a maximum current?

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 ...
user avatar
  • 55.6k
19 votes
Accepted

Is Gaussian noise equal to white noise?

No, they are completely orthogonal concepts. The probability distribution says nothing about the frequency content, and the power distribution across frequency says nothing about the sample ...
user avatar
  • 165k
18 votes

Ceramic caps vs electrolytic. What are the tangible differences in use?

The obvious difference is that electrolytics are much bigger than ceramics. 1mm by 0.5mm ceramics are common garden variety, your electrolytic cans are far larger. Then, as others have pointed out ...
user avatar
  • 3,428
18 votes

Will the net current drawn by a microcontroller equal the current supplied by it

Will the net current drawn by a microcontroller equal the current supplied by it No, because the microcontroller itself (its clocks, IO control drivers, its CPU) need current, too. where can I find ...
user avatar
17 votes
Accepted

Why isn't a transformer a short circuit?

As transformers are usually used with AC rather than with DC, there is what is known as inductance \$L\$, which is a property of a conductor to "resist" the changes in the current flowing in it due to ...
user avatar
  • 9,751
17 votes
Accepted

How do I really calculate these resistors?

You've done the calculations correctly, but the entire basis for the calculations is just a rough approximation of transistor behavior. As you see, \$V_{BE}\$ isn't really exactly 0.7V (most of the ...
user avatar
17 votes
Accepted

Why are circuit results different from theorised?

Your measurements are impossible if the circuit shown is correct. 12V / 20kΩ = 0.6mA. Therefore there cannot be 8.03mA going into the Base. However the slightly high Base-Emitter voltage ...
user avatar
  • 52.1k
16 votes

Why do LEDs have a maximum current?

There's a forward voltage drop of a couple volts across the LED. This Voltage drop times the Current is the Power dissipated in the device. It creates light, but also heat. It's the heat that kills ...
user avatar
  • 7,297
16 votes

Is there a formula to determine the size of decoupling capacitors?

Remember decoupling has several purposes. On a load which draws transient, spiky currents like a CPU, decoupling caps store energy locally and close to the load, so it is available quickly (ie, with ...
user avatar
  • 61.7k
15 votes

Why do microcontrollers have so little RAM?

Memory probably takes up the most silicon space, and RAM being very fast to use is volatile - and uses power constantly to keep its state. Unless you need lots of RAM, it's not useful for many other ...
user avatar
  • 6,198
15 votes
Accepted

Caculating baud rate of Ethernet per wire

A 100BASE-TX device transmits on 1 pair (2 wires) and receives on another pair (2 more wires). Because of the 4B5B encoding -- for each 4 data bits, it transmits a series of 5 symbols -- 100BASE-TX ...
user avatar
  • 17.1k
14 votes

Magnet that needs power to turn off? Inverse coil?

It's possible to "turn a magnet off" without requiring continuous power in either position- simply shunt the field lines away from the open end. Here is an example of a magnet with an on-off switch: ...
user avatar
14 votes

Why do microcontrollers have so little RAM?

Besides the excellent points brought up in the other answers, another reason for limited RAM is the architecture of the microcontroller. For example, take the the Microchip PIC10LF320, which has only ...
user avatar
  • 47.1k
14 votes
Accepted

Is there any relationship between frequency of signal and distance it travels?

In general, yes, higher frequencies attenuate more the further distance they travel. There are two effects that are responsible for this. First, higher frequency radio waves tend to be absorbed more ...
user avatar
  • 3,724
12 votes

Noise and what does V/√Hz actually mean?

Is this value useful in calculating signal to noise ratio? Or what fun calculations can I do with this number? To convert the spectral density \$\tilde v\$ (in nV/√Hz) to a voltage (in VRMS), you ...
user avatar
  • 27.9k
12 votes
Accepted

Theory question about "j" imaginary unit (AC circuit analysis)

If you put a minus sign in front of the number "5" it becomes "-5". Try and look at this differently. Try thinking that it rotates the number "5" (tied to the origin by a piece of string of length 5)...
user avatar
  • 389k
12 votes

Why do LEDs have a maximum current?

TL;DR: Flowing current creates heat, and for LEDs, heat kills the part. Whenever electrons flow through a conductor, Joule heating happens. This is partly because of what heat really is, particles ...
user avatar
  • 2,823
12 votes
Accepted

How serial resistors actually reduce EMI?

Well one it slows down the rise time of your signal which reduces the high frequency content. So that could help you if you didn't need your edges to be that fast. It also lowers the current flowing ...
user avatar
12 votes

How do I really calculate these resistors?

You have done the calculations correctly. However, your biassing scheme will only work well with one set of transistor parameters, it's very sensitive to variations in beta. A better biassing scheme ...
user avatar
  • 144k
12 votes
Accepted

Estimate capacitor parameters

Capacitance is given by: This is the formula applicable for free space or vacuum. If you are using a dielectric material between the plates, you multiply the resultant capacitance with the relative ...
user avatar
  • 7,562
12 votes
Accepted

Why is power of a signal equal to square of that signal?

If the signal is represented as a voltage \$v(t)\$ or a current \$i(t)\$ and it is connected to a (1 ohm) resistor, the power dissipated in the resistor is proportional to \$v^2(t)\$ or \$i^2(t)\$. ...
user avatar
  • 3,661
12 votes
Accepted

Will the net current drawn by a microcontroller equal the current supplied by it

Here's a diagram of a microcontroller IC, with four outputs. The top two outputs OUT1 and OUT2 are sinking current from Vcc (via R1 and R2), and the bottom two outputs OUT3 and OUT4 are sourcing ...
user avatar
  • 9,064
12 votes
Accepted

About power dissipation of capacitors and inductors & I-V phase difference

If the voltage across a capacitor is described as sin(x), then the current through the capacitor (being 90 degrees out of phase) can be described as cos(x). The power is given by the product of ...
user avatar
  • 15.1k
10 votes

Theory question about "j" imaginary unit (AC circuit analysis)

In pure maths we use \$ i \$ to represent the prime square root of \$-1\$. The other square root of \$-1\$ being \$-i\$. If you imagine a number line with real numbers placed horizontally. We ...
user avatar
  • 4,710
10 votes
Accepted

Why pulsed DC passes through a capacitor?

Zero volts doesn't mean zero current. Assume your circuit looks like this: simulate this circuit – Schematic created using CircuitLab When the switch turns on (connects to 10V), current flows ...
user avatar
  • 20.7k
10 votes
Accepted

Electricity through water can injure a person?

I do remember MythBusters asking a similar question: can urinating on a live track on a railway kill a man. First question is how the water is travelling: if it is anything other than a continuous, ...
user avatar
  • 2,986
10 votes
Accepted

What's the rationale behind these warnings about extension cords?

Let's say you're extension cables are 1 mm² cross-sectional area. The loop resistance is 33.6 mΩ/m. A 25 m cable will have a loop resistance of 840 mΩ. The voltage drop at 11 A will be given by V = ...
user avatar
  • 161k

Only top scored, non community-wiki answers of a minimum length are eligible