can capacitors be used as power supplies ??

• Feb 12, 2009

• #1

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desperado1

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capacitor powered led

capacitors have the capacity of storing charges.........then why can't big capacitors be used as voltage supplies? if it can be used please suggest me how it can be used ??

 

• Feb 12, 2009

• #2

FvM

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can capacitor be used as battery

Generally, they can be used. But the achievable energy density is insufficient for most applications. See a comparison of capacitors and rechargable batteries for explanation.

 

• Feb 13, 2009

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desperado1

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can a capacitor be used as a battery

thanks a lot....................what could be the reason for such low energy densisty ?why do we require larger energy density ??

 

• Feb 13, 2009

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LvW

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why capacitors are used instead of batteries

desperado1 said:

capacitors have the capacity of storing charges.........then why can't big capacitors be used as voltage supplies? if it can be used please suggest me how it can be used ??

Click to expand...

There is a small light at the back of my bicycle which burns for app. 5 minutes after stopping. It is fed by a charged capacitor.

 

• Feb 15, 2009

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desperado1

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capacitor used for led light

interesting Lvw........................but what could be the reason for such low energy densisty ?why do we require larger energy density ??

 

• Feb 15, 2009

• #6

FvM

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why capacitors are used?

Supercaps have applications ranging from memory backup supply (uW) up to temporary energy storage in electrical or hybrid vehicles (kW). Obviously the power and energy density requirements are very different. Simply consider your own preferred application and try to answer the question yourself.

 

• Feb 15, 2009

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mvs sarma

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supercaps used as power supplies

There are SUPER CAPS of the order of Farads. They work at low voltage though. they are being used as energy sources. You can find them in shaking type self charging LED torches.

 

• Feb 16, 2009

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analog_match

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power a led with a capacitor

A good fundamental question by desperado which got me thinking and I am outlining my thoughts on the subject.

Based on the graph posted by FvM, looks like an electrolytic capacitor has high numbers for power density, which is the "capacity" to store power per unit volume. However it has very low "ability" to convert that to useful work which is the energy density. I am assuming that might be due to loss in the dielectric etc. Unlike a cell, where a energy is being "generated" by virtue of a chemical reaction, there is no such chemical reaction in a capacitor so its charge must be constantly replenished somehow.

Mathematically speaking you want a high energy density because you want to get the maximum energy or useful work from a unit volume of the device. Why the capacitor has such low energy density must be a question for a physics person as the answer obviously lies in the physics of the thing.

 

• Feb 16, 2009

• #9

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svhb

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how can a capacitor be used as a battery

If you have a capacitor, it has a capacitance (C) and a voltage rating. TO calculate the energy that is in the capacitor at a certain voltage you do E = C*(V^2)/2 (Joule, or WattSeconds).

If you have for example a 10.000µF capacitor charged to 5V, and you can use the energy to light some LED's until the voltage is 2.5V, you use an energy of

10000e-6*(5*5/2 - 2.5*2.5/2) = 0.09J.

If your LED has 2V with a current of 20mA (and assuming a losless driver), it takes 2*0.02 = 0.04Watt.

This means that the LED will light for 0.09/0.04 = 2.25 seconds. Not very much...

Do this with a supercap of 10Farad, you will end up with more than half an hour!

Do this with 2 2600mAH AA batteries, and you have light for some days!

That's why for serious energy storage supercaps are used. It is not only used in memory backup (low power, but long period). The supercaps are also used in traction applications, for example dynamic braking of electric vehicles. It is much better for the batteries if the short (but high) energy pulse is stored in the capacitors, instead of charging the batteries.

 

• Feb 17, 2009

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cameo_2007

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capacitor power led

yes, you can make a capacitor which functions as a normal power supply.,but the size of such a thing will be huge. probably you can name it as capacitor power supply room,if you could make one.

 

• Feb 18, 2009

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lats

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using a capacitor to power led

svhb answered so nicely. Just for a thought desperado how about a lead acid battery? will llast for months on a led..........

 

• Feb 19, 2009

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desperado1

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capacitor used in power supplies

Thanks a lot 2 all of u...............this forum is awesome...........

 

• Feb 21, 2009

• #13

Sadegh.j

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capacitor powered leds

in fact, decaps are used to reduced voltage drop in VLSI circuits

 

• Feb 22, 2009

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desperado1

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can a capacitor power an led

my next fundamental question is how can we see the single shot waveforms like those obtained from a monostable mulitvibrator in a CRO ?

 

• Feb 22, 2009

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mess123

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capacitor used for power storage

U can use ramp signal at the trigger terminal of CRO. Using simulators might also help in case of absence of such facility. Alternately, u can decrease the time period to view the waveform.

 

• Feb 27, 2009

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mickey_melomane

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capacitors used as back up supplies

capacitors can store charges ... agreed... So they may be used as voltage sources... agreed again!

But they cannot produce voltages for a long time, as they cannot deliver current!!! They deliver current to the load by a decrease in the voltage across it. This is what happens when a charged capacitor is connected to a resistor, and the waveform across the resistor eventually goes down to zero due to decrease in current.

Sidharth

 

• Mar 2, 2009

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svhb

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how to power led with capacitor

as they cannot deliver current

Click to expand...

Yes they deliver current, and a huge current. Try shorint a big capacitor with a screw driver. (eg 47000uF at 100V --> take a big screw driver, or it will be soldered on the terminals).

If you connect a resistor to a battery, the voltage will also go down to zero...

The thing to look for is stored energy. The time it can deliver a certain Watts to the load.

Stefaan

 

• Mar 3, 2009

• #18

FvM

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capacitor powersupply low power

I basically agree with shvb, that there's no principal difference between ultracap and battery characteristic. Although the characteristics look rather different, they can be described by a similar parametric model, as the below diagram suggests to my opinion.