Are capacitors a viable option for storing charge in electronic circuits?

[cpatel23]

I just finished my last physics class (e&m) and I was pretty intrigued by capacitors and their ability to store charge. I decided to take an electronics class next quarter so I bought a few things including breadboard, capacitor, resistor, diodes, motors, and so on.
Now...I have a 50V 1000uF capacitor. I filled(?) it with 24V (the motor I have is 3-24V range) and connected it to my motor, the motor spins for like 1/10th of a second. Then the capacitor goes down to like ~1V.
I did a little research on capacitor tanks, and even then, from what people write, capacitors don't last longer then 10 seconds. What gives? Why even use capacitors to store charge. Only useful application I can see is to implement them into circuits with a transformer to make the current a little less choppy.

I had my hopes set out on harnessing solar energy using photovoltaics and storing the energy in a capacitor, but now that seems like it isn't going to work.
Are rechargeable batteries a better option to store charge?

If someone can give me a reason on why capacitors are useful for storing charge that would be appreciated, along with answering the above. Thank you for your help (sorry for the wall of text).

 

[LordVader88]

Batteries are used for long term storage of electrical energy as you know.

I'm a near noob, but caps are used on small time scales, they are always getting charged/discharged, and they can be used to do a pile of things, but they are mostly always charging/discharging when taking part in whatever

1 basic thing is, to use a water analogy, say you where using a hose but the water pressure kept changing at random, but you wanted a smooth steady flow, a capacitor acts sort of like a little tank that the random hose would fill, but at the bottom had a steady steam coming out of it's tap.

So for a voltage that jumps up and down a bit, a capacitor is used to smooth it to a steadier level

 

[cpatel23]

Batteries are used for long term storage of electrical energy as you know.

I'm a near noob, but caps are used on small time scales, they are always getting charged/discharged, and they can be used to do a pile of things, but they are mostly always charging/discharging when taking part in whatever

Right. A camera flash circuit uses a capacitor to release a high amount of energy in a short amount of time. So that means capacitors are only good for short bursts of energy?

 

[nsaspook]

Right. A camera flash circuit uses a capacitor to release a high amount of energy in a short amount of time. So that means capacitors are only good for short bursts of energy?

No, it means that capacitors obey the rules of electrical charge movement. Batteries and capacitors can complement each other in a energy storage system. Batteries usually have a limited number of charge/discharge cycles before they start to fail and most batteries are not very efficient in storing energy when almost full. By adding a large capacitor bank that's discharged first instead of the batteries you can reduce the wear on the batteries and because the storage efficiency of a capacitor is usually the same at any point it can increase system efficiency if the load consists of a many short bursts like in a EV stuck in stop and go traffic with a regeneration system that captures the braking energy.

 

[Ratch]

cpatel23,

Why even use capacitors to store charge.

Are rechargeable batteries a better option to store charge?

If someone can give me a reason on why capacitors are useful for storing charge

Rechargable batteries and capacitors do not store charge, they both store energy. Knowing that, what is your question?

Ratch

 

[cpatel23]

cpatel23,

Rechargable batteries and capacitors do not store charge, they both store energy. Knowing that, what is your question?
Ratch

In terms of storing back up energy (for blackouts/being self sufficient), would it be better to use a battery or capacitor. From what I've learned, batteries would be more efficient even though they have their limitations.
This may seem stupid, but is it possible to store energy in a generator and then withdraw it when needed? Or is that the same as a battery?

No, it means that capacitors obey the rules of electrical charge movement. Batteries and capacitors can complement each other in a energy storage system. Batteries usually have a limited number of charge/discharge cycles before they start to fail and most batteries are not very efficient in storing energy when almost full. By adding a large capacitor bank that's discharged first instead of the batteries you can reduce the wear on the batteries and because the storage efficiency of a capacitor is usually the same at any point it can increase system efficiency if the load consists of a many short bursts like in a EV stuck in stop and go traffic with a regeneration system that captures the braking energy.

So from what I understand, if I have a breadboard whose primary function is to rotate a motor, it would be more efficient to have a capacitor give the initial push, and then use the battery to keep it going. Is this true?

thanks for your responses.

 

[Ratch]

cpatel23,

In terms of storing back up energy (for blackouts/being self sufficient), would it be better to use a battery or capacitor.

Several factors need to be considered, physicsl size, voltage, cost, and energy capacity.

From what I've learned, batteries would be more efficient even though they have their limitations.

Efficiency is not a consideration. Other factors listed above are more important,

This may seem stupid, but is it possible to store energy in a generator and then withdraw it when needed? Or is that the same as a battery

No, batteries can store energy chemically, capacitors can store energy in a electrostatic field, but a generator converts mechanical energy into electrical energy. So unless you can spin a flywheel connected to a generator, you cannot store energy in a generator.

Ratch

 

[yungman]

Storing charges is only one of the many use of capacitors. Powering a motor or any circuit is not exactly an important use of capacitor. There are so many things involve capacitance that battery cannot do.

1) Cap don't pass DC current and voltage, so it is use to block DC from reaching from one side to the other. But at the same time, it can pass AC signals, so it can couple the AC signal from one stage to another.

2) In electronics, we use capacitor and resistors to form poles and zeros to create low pass and high pass networks.

3) Read up transmission line theory, it's all about L and C.

4) filter out ripple of the un regulated voltage source.

These are just few comes to mind, there are a lot more use that basic EM...or even advance Electrodynamics don't get into. You really need to take a class in basic electronics.

 

[the_emi_guy]

Why even use capacitors to store charge. Only useful application I can see is to implement them into circuits with a transformer to make the current a little less choppy.

There are many, many other applications where capacitors are uniquely qualified. For example capacitors can be made to charge and discharge *very* rapidly, much more rapidly than any battery. Capacitors used tune radio antennas are charged and discharged *billions* of times per second continuously.

 

[cpatel23]

Several factors need to be considered, physicsl size, voltage, cost, and energy capacity.

I was thinking of a small scale setup (until I really get the hang of things). My initial idea was to by a dozen or so 1.2V/9V rechargeable batteries. Putting them in series accordingly to achieve a needed voltage, recharging them using solar panels. As for cost, I understand they could get a little pricey but in my opinion, it's worth the experience/knowledge.

When you said energy capacity, do you mean Watts or amp hour?
Since Watts have been brought up, let's say I have something that requires a minimum amount of Watts, but I only supply 70% of the minimum. Would my appliance work?

 

[cpatel23]

Storing charges is only one of the many use of capacitors. Powering a motor or any circuit is not exactly an important use of capacitor. There are so many things involve capacitance that battery cannot do.

1) Cap don't pass DC current and voltage, so it is use to block DC from reaching from one side to the other. But at the same time, it can pass AC signals, so it can couple the AC signal from one stage to another.

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

 

[yungman]

I am kind of taken back by the example and the question! I don't mean to be sarcastic...I don't mean to. But this is like saying a TV can light up a dark room. But it is not that bright and it flicker, so what is the use of a TV! I am just kind of lost of words. Using in motor is only one of the many many...many use of a cap.

Yes, capacitor is that useful and that important.

 

[yungman]

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

You put 5V on one side, and if you connect a very high value resistor from the other side to ground, you won't measure any voltage on the side with the resistor to ground. The cap block the DC voltage. But if you put a high frequency signal on one side of the cap, you can see the signal on the other side, it passes the AC signal.

 

[cpatel23]

I am kind of taken back by the example and the question! I don't mean to be sarcastic...I don't mean to. But this is like saying a TV can light up a dark room. But it is not that bright and it flicker, so what is the use of a TV! I am just kind of lost of words. Using in motor is only one of the many many...many use of a cap.

Yes, capacitor is that useful and that important.

Lol, please forgive me, only education I have on electronics is from my physics classes. Taking an introduction to electronics this upcoming quarter. Hopefully it can clear things up.
Any good electronics books in mind that I can read until then?Also, when I use a DC voltage regulator to regulate the voltage from my transformer (25V down to 15V) it gets really hot. Is this normal?

 

[yungman]

Lol, please forgive me, only education I have on electronics is from my physics classes. Taking an introduction to electronics this upcoming quarter. Hopefully it can clear things up.
Any good electronics books in mind that I can read until then?


Also, when I use a DC voltage regulator to regulate the voltage from my transformer (25V down to 15V) it gets really hot. Is this normal?

Someone need to give you a name of an introduction book, I don't have one right off my head. You are dropping to much voltage ( from 25 to 15V) across the regulator. Remember W=IV. if you drop 10V across the regulator, and if you even draw 100mA, it is 1W power dissipating with the regulator. If you don't have a heat sink, it can get hot. Some of the LM7805 get hot even when you are not drawing current.

I know a very good book that is a step above AC/DC circuits. It's the Electronic Principle by Malvino. I just bought a copy for under $10USD shipped. Go on Amazon and look for a used one.

 

[jim hardy]

cpatel -Your interest and enthusiasm are refreshing. continue experimenting.


Tinkering with capacitors can give you that intuitive "feel" for the meanings of

charge, , Q, Coulombs, a coulomb being around 6E18 charge carriers(usually electrons in our business)

current, I, Amperes, amount of charge moving past a point per unit time, one ampere being one coulomb per second

voltage , Joules per coulomb, energy per unit of charge, one volt being one joule per coulomb

energy , joules or watt seconds, amount of work that can be done, one joule being one Newton-meter

capacitance, Farads,, C a measure of the ease with which a capacitor stores charge. Units of capacitance are curiously same as length because it's area (of plates) divided by distance separating them, more later

Capacitors do store charge, in the amount Q = C X V
in other words one coulomb will charge one farad to one volt

That leads to concept of DC current into a capacitor-
one ampere into one farad will raise its voltage one volt per second
but it's not practical for that to go on for very long, voltage becoes too high.


Capacitors, by storing charge, also store the energy which pushed the charge into the capacitor, in amount 1/2 C X V^2
in other words one farad charged to one volt holds a half-joule
charged to two volts it holds two joules
charged to three volts it holds four and a half joules, etc
it's analogous to compressing a spring

The capacitor stores the energy in the dielectric separating the plates. There is an attraction between the plates because of their charge, so pulling the plates apart would add energy to the capacitor and this would show up as increased voltage.
The formula for capacitabce is C= ε X A / D
A and D being area and distance between plates, and ε is the dielectric property of the material between the plates. Doubling the distance would halve the capacitance. Practical capacitors have small separation between the plates and dielectrics with high value.

Remain curious my friend, and enjoy electronics.
www dot discovercircuits dot com has a forum that is popular with hobbyists and students alike, check it out.

EDIT to your original question

Capacitors are universally used in converting wall power AC into direct current for electronics.
AC runs through a rectifier which turns it into a series of positive (or negative) pulses. The capacitor stores charge to be delivered between pulses, thereby providing a fairly constant flow of charge(current) instead of pulses.

 

[nsaspook]

So from what I understand, if I have a breadboard whose primary function is to rotate a motor, it would be more efficient to have a capacitor give the initial push, and then use the battery to keep it going. Is this true?

With proper design that's true if you plan to recapture energy with a lead-acid battery system as most of the efficiency problems I talked about are mainly due to it's poor charge efficiency above 80% SOC and limited charge/discharge cycles times.

http://web.mit.edu/first/kart/#concept
http://web.mit.edu/first/kart/circuit.jpg

 

[mheslep]

Capacitors used for bulk energy storage are in the range of thousands or tens of thousands of Farads, not microfarads.
http://www.tecategroup.com/store/index.php?main_page=product_info&products_id=1259&gclid=CKCa2sX-q7QCFYZM4Aod3CoAUA&zenid=0eltghpum8v6g1ctjuteom0ma0

 

[cpatel23]

Capacitors used for bulk energy storage are in the range of thousands or tens of thousands of Farads, not microfarads.
http://www.tecategroup.com/store/index.php?main_page=product_info&products_id=1259&gclid=CKCa2sX-q7QCFYZM4Aod3CoAUA&zenid=0eltghpum8v6g1ctjuteom0ma0

wow. Is there a reason why all the capacitor tanks have low voltage (sub 9V)?

You are dropping to much voltage ( from 25 to 15V) across the regulator. Remember W=IV. if you drop 10V across the regulator, and if you even draw 100mA, it is 1W power dissipating with the regulator. If you don't have a heat sink, it can get hot. Some of the LM7805 get hot even when you are not drawing current.

Are these any good? >>>Heat Sink It says 1W, is that how much it dissipates or how much energy it requires to operate?

AC runs through a rectifier which turns it into a series of positive (or negative) pulses. The capacitor stores charge to be delivered between pulses, thereby providing a fairly constant flow of charge(current) instead of pulses.

So a rectifier doesn't completely convert AC to DC? It just creates pulses that flow in one direction?

http://web.mit.edu/first/kart/#concept
http://web.mit.edu/first/kart/circuit.jpg

That's some awesome stuff.

A question about putties batteries in series with capacitor.
This would be putting batteries in series with a capacitor to light an LED:
http://i.imgur.com/7BK9C.jpg

and this would be destructive
http://i.imgur.com/S4mWt.jpg

right?

and does the order matter? for instance battery, battery capacitor would be the same in terms of output as capacitor, battery, battery?
Sorry for all the questions, had to many, what seemed to be, close calls yesterday.

 

[pantaz]

... Sorry for all the questions, had to many, what seemed to be, close calls yesterday.

Before you hurt yourself, start here = Introduction to Electronics

 

[nsaspook]

A question about putties batteries in series with capacitor.
This would be putting batteries in series with a capacitor to light an LED:

The series capacitor in the CAP CART is switched out of the circuit when discharged and reconnected during regeneration charging. Just putting a series capacitor in a led circuit won't work. I think you need to step back and study electronics first to get an understanding of how circuits actually work.

 

[cpatel23]

Last question,
I have a circuit like so
http://i.imgur.com/WxSzV.jpg
When I let the capacitor fully charge it comes out to ~7.2V (Battery is at 8.7V).
The resistors are 180ohms each

Now when I remove the resistors and leave the LED and capacitor in series, the voltage on the capacitor will read 7.2V.

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

 

[Drakkith]

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

Once the capacitor is charged you will be reading the applied voltage from your voltage source across it. Think about it. When you apply a voltage to your circuit, current flows through it. As current flows the charges build up on each plate of the capacitor, with the negative charges on the negative side and the positive on the positive side. Since current doesn't flow through the dielectric in the capacitor these charges cannot go anywhere and instead build up until they match the applied voltage. Once this happens current has ceased to flow in the circuit. Adding resistance reduces current flow and will increase the time it takes the capacitor to charge or discharge, but since the current cannot get through the capacitor you will wind up with charges built up on either side of the capacitor still. The capacitor is "acting" like an open in the circuit at this point.

As for why your measured voltage doesn't seem to match the applied voltage, I don't really know.

 

[cpatel23]

Once the capacitor is charged you will be reading the applied voltage from your voltage source across it. Think about it. When you apply a voltage to your circuit, current flows through it. As current flows the charges build up on each plate of the capacitor, with the negative charges on the negative side and the positive on the positive side. Since current doesn't flow through the dielectric in the capacitor these charges cannot go anywhere and instead build up until they match the applied voltage. Once this happens current has ceased to flow in the circuit. Adding resistance reduces current flow and will increase the time it takes the capacitor to charge or discharge, but since the current cannot get through the capacitor you will wind up with charges built up on either side of the capacitor still. The capacitor is "acting" like an open in the circuit at this point.

As for why your measured voltage doesn't seem to match the applied voltage, I don't really know.

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

 

[Drakkith]

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

Before I answer this, I want you to do something for me.

Explain to me what Voltage, Current, and Resistance are. In whatever words you want to use, it doesn't have to be exact. I just want to get a feel for what you already know.

 

[flatmaster]

Someone has recently been developing a battery/capacator hybrid called an ultracapacitor

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor
http://www.technologyreview.com/news/417053/a-battery-ultracapacitor-hybrid/

 

[cpatel23]

Before I answer this, I want you to do something for me.

Explain to me what Voltage, Current, and Resistance are. In whatever words you want to use, it doesn't have to be exact. I just want to get a feel for what you already know.

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

 

[Drakkith]

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

If you measured the voltage in your conductor right before it connects to the terminal of your battery you would read nearly 0 volts. This is because the wire itself has very little resistance and you have almost no loss in voltage (voltage drop) across it. When you measure the voltage of something in a circuit you are measuring the voltage drop across it. In a simple series circuit the voltage drop of all components must add up to the applied voltage.

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

Hmm. Ok. I could spend an hour or so typing up what those three things mean, but your best bet is to grab a basic electronics book and study up. A few things though.

Voltage is a difference in electric potential between two points. Electrical potential exists any time you have an imbalance of charges between two points, or when you have a conductor in a changing magnetic field.
Current is the flow of electrical charges past a point. It is not the flow of voltage. Voltage does not flow.

 

[sophiecentaur]

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

Those definitions are all pretty flawed, I'm afraid.
This may sound very pernickety but it is really important to get the definitions of these terms right in your head from the start. "Near enough ain't good enough' when it comes to these things or you are starting off on the wrong foot. Don't go by what you read on a Forum, which has been written by someone non-academic who wants to make it 'friendly' and accessible. There are a mixture of definitions. even on this thread and some of them are not 'quite right' and some of them are dodgy personal interpretations. If you aren't in a position to distinguish (which you aren't, because you are asking about them) find the right way of saying and writing things by looking at a reputable site. I always recommend the Hyperphysics site, which you should be able to rely on. (Or a decent textbook, of course)

Do not take offense at this - just think of what an eejit someone (an aging adult, for instance) can sound when they try to talk about the things in young persons' lives. It's cringeworthy - and so are the things some people say about Electricity.

 

[Drakkith]

Sophie's right. It is extremely important that you understand the proper meaning of those terms. Otherwise most of electronics just doesn't make much sense if you try to think about it.

 

[sophiecentaur]

Sophie's right. It is extremely important that you understand the proper meaning of those terms. Otherwise most of electronics just doesn't make much sense if you try to think about it.

Sophie is a grumpy old git but he is right!

 

[Ratch]

cpatel23,

When you said energy capacity, do you mean Watts or amp hour?

Energy is measured in joules, power in watts, and charge in amp-hrs. Notice those units are not capitalized.

Since Watts have been brought up, let's say I have something that requires a minimum amount of Watts, but I only supply 70% of the minimum. Would my appliance work

Ask your appliance if it is happy to run at reduced power.

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

That is a misleading description. A voltage needs two points to be defined. So if the voltage increases across a capacitor, what does passing voltage mean? Current never goes through an ideal capacitor. The charge can accumulate on one plate and deplete on the opposite plate for a transient period of time, thereby allowing a current in the branch where the cap is located. Anytime the voltage changes across the capacitor, a current will exist for a transient time until the shift of charge between the plates of the cap is complete.

When I let the capacitor fully charge it comes out to ~7.2V (Battery is at 8.7V).

Remember what I said in post #5. Caps don't charge, they energize. The cap has the same net charge at 7.2 volts that it had at 0 volts. It does contains more energy, however, at 7.2 volts.

Now when I remove the resistors and leave the LED and capacitor in series, the voltage on the capacitor will read 7.2V.

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

I would like to see a schemat, not a dimly lit breadboard.

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

Charge flows, not current. Again, a schemat will work wonders.

Voltage - from what I've learned in e&m, it is the magnitude of an EMF.

What is EMF? What does it mean?

Current - the rate at which voltage is transported

Wrong.

Resistance (in terms of circuits/electricity) - something that is against the flow of current

Other things inhibit current, but they are not resistive.

sophiecentaur,

Those definitions are all pretty flawed, I'm afraid.

That is for sure.

Ratch

 

[jim hardy]

Echo previous sentiments. Even though i am guilty of making some assertions a few posts back... i hope they painted a reasonable mental picture, but I'm not a trained educator as are some others here.

You need to go into electronics with a clear mental picture of those fundamental terms
and that comes from working problems.

It's confusing at first because everything is named after dead scientists instead of a word that takes your mind straight to the concept.

And the water analogies, while they can be useful, can easily mislead you
because water flows easily through air which charge does not
and water can be pumped copiously out of the ground
which leads to a mistaken concept of "ground" in circuits.

The basic units are probably well defined in your physics book...

i was hoping to stimulate your curiosity further, as i said

Good luck in your electronics course. Become fluent in laws of Kirchoff and Ohm.

old jim

 

[jim hardy]

Here's an illustrated explanation of what Ratch was saying about capacitors, from Sophie's suggested site:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html

a picture is worth a thousand words.

 

[sophiecentaur]

cpatel23,


Remember what I said in post #5. Caps don't charge, they energize. The cap has the same net charge at 7.2 volts that it had at 0 volts. It does contains more energy, however, at 7.2 volts.



Ratch

I agree with the rest of your post. The above is a matter of your personal preference. The Charge that is 'in' a capacitor can be made to flow around a circuit - the same as the Ahr that are 'in' battery. When the term "Battery Charger is officially replaced with "Battery Energiser" on every Charger you can buy in an Autospares Shop, I shall start to think your way.

Your obsession with this issue is spoiling the credibility of your otherwise good ideas, I feel. You will not change the World on this. Useage is a very powerful badge of authority in language.