Hot answers tagged

51 votes

Why are electrons not pushed out from the conductor at an open circuit under the influence of a power source?

You're imagining an open circuit to look like this: A better analogy would be this: The pipes in a circuit aren't surrounded by free space for the water to flow -- they are tunnelled through a rock. ...
  • 737
40 votes

How can voltage burn out an LED?

Voltage and current are intimately related. If you attempt to increase the voltage across an LED, the current will increase. Likewise, to increase the current through an LED, you must increase the ...
28 votes

Why are electrons not pushed out from the conductor at an open circuit under the influence of a power source?

The water analogy is very limited and does not model the way electrons move in a wire. It should always be used with great care. Electrons drift very slowly (about 1m/hour) by jumping from atom to ...
  • 18.6k
28 votes
Accepted

Why are crystal oscillators used in clocks instead of RLC circuits?

Crystal oscillators are much more accurate, they are small, have low temperature coefficients and low drift at a low cost.
  • 1,835
27 votes
Accepted

Why is capacitor a linear element?

First of all, an I-V curve does not make any sense for a capacitor. This is because a capacitor follows the following equation: $$i = C \frac{dV}{dt}$$ Note that the current depends on the rate of ...
  • 5,954
25 votes
Accepted

How do you interpret/draw out the simplified circuits in Sedra's Microelectronics?

It doesn't matter where the arrows go because they are separated from the circuit of interest by current sources. For a more practical approach you may consider the arrows pointing up to a more ...
  • 163k
25 votes
Accepted

What is negative current?

understand that voltage is relative to ground, I prefer to disagree. A voltage is against a reference point. Often that reference point is ground but not always. Taken the above into account your ...
  • 14.1k
21 votes

How can voltage burn out an LED?

As you can gather from the other answers, voltage (U) and current (I) are linked. In the case of a simple resistor: U = R * I where R is the constant resistance of the resistor. A diode is only ...
  • 331
19 votes
Accepted

How does an SR Latch get started

It is true that the latch will "wake up" in an unknown state. In the real world, given a little time, the latch will have a valid state with either Q=0 or Q=1. You make the latch "get started" by ...
18 votes
Accepted

When to use a 1/4 Watt resistor vs 1/2 Watt resistor

The watt rating tells you how much current you can run through the resistor before it overheats and burns up. Power is given by: $$P = I^2R$$ So the maximum current through a 1/4 watt resistor is: $...
  • 20.7k
18 votes

Why are crystal oscillators used in clocks instead of RLC circuits?

A quartz crystal is a mechanical resonator with particularly stable properties. Quartz is a very stable material -- it doesn't 'age', or change much with temperature. It is also possible to prepare ...
  • 16.5k
16 votes

How can a circuit not have a neutral?

The two sides are out of phase with each other. When one side is high, the other is low, and vice versa. This is a 240 V outlet.
  • 7,513
16 votes

Do capacitors waste power as heat or whatever on AC?

There would be power loss because with real life capacitors, there are parasitic losses, this means that the capacitor can not act as a pure capacitive load in real life, this is mainly because of the ...
15 votes
Accepted

Understanding this circuit with op. amps

I'll let someone else do the sums formally/generally, but there is an easy approach here that will allow you to solve it by inspection. As you know, the op-amp inputs are both going to be 0V at ...
15 votes
Accepted

What role does this diode play?

This diode is to surpress any back-EMF caused when the motor is switched off. In general, when one has an inductive load such as a motor, or an electromagnet's solenoid, when you switch it on, there ...
14 votes

Rocket launcher circuit design

Having used commercial circuits and built a few myself... The standard model rocket igniter takes a fairly hefty current to work. In the olden days, an incandescent bulb was placed ACROSS the launch ...
  • 5,544
13 votes
Accepted

555 timer: how to set values to take an hour

As awesome as the 555 is, its drift may be a bit too high for periods this long. I recommend you look at using a MCU instead, such as the ATtiny4. It comes in SOT23-6 packaging and has an internal ...
13 votes

High Current or High Voltage that damages components?

Things like resistors will fail from excessive power dissipation- they get too hot and the materials they are made from suffer irreversible degradation. For example, the lacquer on the outside of a ...
13 votes
Accepted

Voltage drop over open circuit

The meter has some high resistance across its terminals (in the megaohm range for a DMM, about 10k to 50k for an analog meter type) so a slight current will flow through it, the LED and the resistor. ...
  • 42.7k
13 votes
Accepted

Can an opamp have a negative gain?

The term "negative gain" can be ambiguous. It could mean "attenuation", and it could also mean amplification but with reversal of polarity. If a gain is given the units of decibels,...
12 votes
Accepted

When does the placement of a resistor matter?

Since you say you've been a programmer for years I am going to ask you to stretch your mind a little bit answering your question in the most general way. You refer to resistors, I am referring to any ...
12 votes
Accepted

Why is it called Wired-OR when it is functioning as Wired-NOR?

This is a historic convention. The preferred active level in many 5V TTL circuits was low as the noise immunity at the input was higher (3V as opposed to 0.8V in ...
  • 21.5k
12 votes

Problem with equivalent resistance

simulate this circuit – Schematic created using CircuitLab Here's the circuit redrawn. R1 and R2 form a voltage divider, as do R3 and R4. Since the ratios are the same for each voltage divider,...
  • 8,960
11 votes

What operation could cause the inductor or the capacitor to explode?

Inductors store flux when current flows through them. When the inductor is de-energized, the flux turns back into current. When this current attempts to pass through a very high resistance it results ...
11 votes
Accepted

What does lumped element mean?

Two theories (really one!) There are basically two ways of looking at an electrical circuit: electromagnetic theory (Maxwell's equations) and the theory of lumped elements. In fact the theory of ...
11 votes

"A zener with series diode"?

The left circuit (A) is what he means. The Zener diode limits the voltage across the inductor to its nominal value. The other diode is there just to block the Zener diode in the forward direction. ...
  • 15.8k
11 votes

How do I determine whether a switch can handle the power/current in a circuit?

Current and voltage ratings do not neccessarily apply at the same time and in a single operating point. A switch is a pretty good example for this: it must not conduct a current higher than the rated ...
  • 2,524
11 votes
Accepted

My textbook claims that base kW = base kVA, but doesn't this equality break down when power factor isn't zero?

You are correct that KW only equals KVA when power factor is 1. They are not suggesting otherwise. The text is just telling you that the base for \$KW_{pu}\$ and \$KVA_{pu}\$ is the same numerical ...
  • 3,360
10 votes

555 timer: how to set values to take an hour

What you ask for is not something a R-C timer like the 555 can do. The problem is that for such long times, the current to charge or discharge the cap is so small that leakage currents dominate. ...

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