If it blocks DC current, then you wouldn’t find one in a DC circuit, would you? Or, would one use it when an intermittent DC current is desired? (I assume it would be intermittent as the capacitor builds to full charge and then discharges and recharges again)?
There are very, very few pure DC circuits. A flashlight is just about the most sophisticated DC circuit you are going to find. Most circuits that run on DC
a) use AC as a supply and convert it to DC (requiring capacitors as filters).
and
b) use oscillators internally to create AC waveforms.
Capacitors are commonly used to filter noise on the power lines of DC supplies (“bypass caps”), and to act as part of the timing circuits for oscillators. The charge / discharge cycle you mention is called a “relaxation oscillator” and is very simple to create. These days one would use a 555-type timer IC to create such a circuit - it works as a relaxation oscillator, but it has programmable setpoints so it is much more flexible than ye olde capacitor-neon light-resistor circuit.
Capacitors make a nice loud bang, as I discovered in a high school electronics shop class.
The first time I exploded a capacitor in class it was an accident.
And an inductor is, approximately, the opposite: open to DC, closed to AC (of appropriate freq).
Many capacitors are symmetric, but not the “eloctrolytic” ones. I remember several untested circuit boards being plugged into a million-dollar computer. (Don’t blame me for the lack of testing; I was just the underpaid freelancer called out of bed.) The capacitors were installed backwards; every minute or two we heard a disconcerting popcorn-popping sound from inside the million-dollar mainframe. :smack:
It’s been speculated (though with little serious support) that the Ark of the Covenant of Yahweh was a capacitor; this might explain its ability to blind and kill. (If you have a large charged capacitor, definitely exercise caution !)
Without shorting a capacitor’s terminals, how long can one remain charged?
Thanks,
Rob
How long they stay charged depends on a lot of factors, including humidity, which affects how conductive air is.
However, they can hold a charge for a surprisingly long time. One should never assume a capacitor has discharged naturally. You hear stories of old TV sets, unplugged for years, knocking some tinkerer across the room.
besides the conductivity of the air the dielectric of the capacitor affects how long it can hold a charge.
It’s reasonable to say that they get “full”. With a constant voltage applied, they’ll only pass a certain amount of current, until the voltage reaches the applied voltage. That’s more of a time domain description, versus a frequency domain description as a complex impedance. But both descriptions are valid.
Indeed. They warned at army tech school (well, the Canadian Forces School of Communications and Electronics, where I took my LCIS course) to approach the big caps with caution and use a special shorting stick:
- Make sure you’re holding the wooden nonconducting part.
- Attach flexible lead to a good ground.
- Touch fixed lead to cap electrode.
- NEVER use the stick to short the electrodes to each other in an attempt to neutralize the cap faster. At an extreme case, the current could actually permanently weld the stick in place.
Plus there was that horror story of a hydro worker who didn’t wait until a flywheel capacitor wound down (something taking at least a full day) and was disintegrated, leaving only part of his leg.
Correct me if I’m wrong, but isn’t the use of capacitors what makes it possible for me to both unplug my computer and take out its battery, but still have the correct date, time, etc. when I but them back in?
capacitors can be used for memory backup in electronic devices. they can be used to give a minute or two as you describe or in some designs eliminate the battery and hold the memory for a long time.
you can also get flying bits of molten metal from the capacitor terminals and wire as they melt and are propelled by the heated air when the capacitor is shorted.
I’ve heard that some dielectrics are hallucinogenic when vaporized, too. One of my students once told me about a time he burned out a cap, and lizards started crawling out of his electrical sockets.
plastic vapors might do that
Capacitors fail. If an electronic device stops working a dead capacitor is the most likely cause, YMMV.
A capacitor lets A.C. current through but stops D.C. current. Connected between a D.C circuit and ground it can drain away radio noise. This is useful in low level D.C. circuits such as a wheastone bridge in a transducer.
Well, not quite. A capacitor is not a “wire” when it comes to AC.
As others have mentioned, a capacitor stores energy - and then later releases it - instead of dissipating it into the surrounding environment in the form of heat. That’s one of the things that makes it different from a resistor; a resistor converts electrical power into low-grade heat, whereas a capacitor will store the electrically energy internally, and then late release it back into the circuit as electrical energy.
Sometimes a capacitor is used to do just that - store energy and then later release it. One example is when a capacitor is used in a “power supply bypass” circuit. It will store energy, and then quickly release the energy into the load when the load demands it. It can do this much faster and more efficiently than a battery or 120 VAC receptacle, hence the reason it is used.
An analogy to the above is a water tower - a water tower stores water in the same way that a power supply bypass capacitor stores energy. During peak water usage around 5 to 6 PM, your water comes from the tower, not the pump. In the middle of the night, the water tower is refilled by the small pump.
As a consequence of the physical laws that define what capacitance “is,” though, capacitors have other uses besides “vessels for temporary energy storage.” They can also be uses as variable resistors.
You see, when you buy a regular 'ol resistor, its resistance is a *fixed *value. It’s fixed regardless of frequency, too. One of the cool things about a capacitor is that it can be used as a programmable resistor. How do you program its resistance, you ask? Simple… by varying the signal frequency. If you lower the frequency, the capacitor’s resistance will increase. If you increase the frequency, its resistance will decrease. In essence, it is a “frequency programmable resistor.” If you make a resistor divider using a capacitor and resistor, it will attenuate certain frequencies. (Yes, I know it is more proper to use the term “impedance.” But in this paragraph I’m ignoring the concept of phase, hence using the term “resistance” makes it easier to understand.)
Since a capacitor can be used as a frequency programmable resistor, you can also use a capacitor to “couple” the output of one circuit to the input of another circuit. It usually does a better job of doing this vs. a regular 'ol resistor. This is because a capacitor has infinite resistance at DC, which means it is able to “block” the DC component of the signal and only pass the AC part. This is desirable in many circuits.
Finally, a capacitor can be used to shift the phase of a signal.
This post was number #12345678.
Ooh.
You know, I was wondering where the breeze of greatness was coming from when I posted above…
Another practical use for capacitors that I don’t think has been mentioned yet is for storing up a bunch of energy to be released in a very short time. For instance, your camera battery contains enough energy to fire the flash, but it takes it a few seconds to release that much. So the camera charges up a capacitor for a few seconds, and then discharges the capacitor quickly to set off the flash.
Capacitors don’t store charge, any more than batteries do. Capacitors store energy.
When we “charge up” either a battery or a capacitor, we push coulombs of electrons in one side, while simultaneously an equal amount of electrons pops out of the other. This causes joules of energy to build up inside the device, yet the number of electrons inside a battery or a capacitor remains constant. If a capacitor or battery has been charged with electric energy, then it can be used as a charge-pump to power an electric circuit.
In engineering, a capacitor is a circuit element where the current through the device is proportional to the rate of change of voltage across the device.
The incorrect idea “capacitors store charge” arises because we often explain capacitors or batteries while one terminal is grounded, then we mentally erase the grounded terminal. (A one-terminal component is easier to think about?) We then can dump charge into this simplified one-terminal capacitor, while pretending that the charge isn’t just flowing right back out again via the grounded terminal. These “one terminal capacitors” can sometimes greatly help simplify our thinking about certain electronic circuits. But when it comes to explaining how capacitors function in other types of circuit, it can cause serious confusion.
If electric circuits are like drive belts, then a capacitor is like a pulley which winds up a metal spring. While the belt is running, the spring is getting tighter and tighter, requiring more and more force to keep the belt going. By pushing the belt across this pulley, we can store energy in the spring. If we let the belt go, the spring drives the pulley backwards as the stored energy is released. Here’s the misconception: spring-pulleys actually store quantities of rubber belt; we can force some rubber belt into the interior of the spring-pulley, then later the belt squirts back out again. (See how such a weird idea might arise?)
http://amasci.com/graphics/wtrcap1.gif
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