# How do you reduce voltage and make a current last longer?

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I don't know a lot about electrical engineering but I'm trying to design a simple circuit system. Lets say I have a source generating electricity in a circuit and say just as a random number 30 amps from one second of electrical current are stored in a capacitor. How would I then release that electrical energy more slowly to power a small light at 10 amps a second for 3 seconds?

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"Amps a second" is not a valid unit. Amp = coulomb/second, which means a coulomb = amps*seconds

You can think of a capacitor as storing a charge at some voltage, which you can express in your example as 30 amp-seconds. So what you want is 10 amps for 3 seconds. But capacitors don't work like that. The discharge undergoes exponential decay.

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

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As swansont already explained, there is no simple circuit that will do it. One way would be to use some sort of DC-DC converter that will be able to keep constant output despite the fact that its input voltage decays, but DC-DC converters are not simple circuits. (with a very good DC-DC converter, I would say, you could get 80 - 90% of the energy stored in capacitor).

Much more important to note is (in addition to problem with units that swansont mentioned):

- a standard capacitor that can take/give 30A for 1 sec is a very very large capacitor. With these numbers, more practical would be to use a supercapacitor or a battery.

- a lamp that takes 10 amps is rarely called a 'small light'

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What about some kind of rechargeable battery? But then I have the same problem of controlling the current, but at least it wouldn't automatically decay exponentially.

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The current will be determined by the resistance of the circuit it flows through. One simple way of limiting the current is to put a resistor in series, but this is only practical for quite low currents because otherwise a significant part of the energy is lost in the resistor as heat.

Otherwise you might need to design some sort of constant current source.

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Resistors are sort of the opposite of what I'm going for. I want to be able to collect large amounts of energy in a short time, store that energy, then release it over a longer period of time to power a lower-power-necessity series of lights. I'm sure there's ways this is done otherwise every device would explode as soon as the circuit was completed with a charged battery.

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Charge two capacitors (or batteries) in series then connect them in parallel.

Of course, doing this electronically and fast enough is another matter.

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Well lets say I have a small generator with a stator and rotor, now how easy is it?

I'm still not seeing an answer to my question. I want to store energy, then, without losing a bunch of it fro resistors, release that energy a little bit at a time.

Edited by MWresearch
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Well lets say I have a small generator with a stator and rotor, now how easy is it?

I'm still not seeing an answer to my question. I want to store energy, then, without losing a bunch of it fro resistors, release that energy a little bit at a time.

Use a battery.
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I didn't intend to bother with this thread from past experience, but the OP seems serious this time.

Further he has been short changed in answers, even though he was a bit lax in terminology.

Firstly the situation of charging a store with large short term current pulses is exactly how a standard rectifier / filter capacitor works after a transformer.

So there is nothing wrong or unusual there.

Similarly the current and power to lighting if resistive or especially if LED can be dramatically reduced (saved) by pulsing that supply.

A superbight LED will still be well visible with a 1% duty cycle.

So what is required is a chopper ciercuit driving either thyristors or powerFETS supplying the lights.

Without proper details of the requirements I cannot be more specific, but standard off the shelf integrated circuits are cheaply and widely available for this.

Edited by studiot
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... Lets say I have a source generating electricity in a circuit and say just as a random number 30 amps from one second of electrical current are stored in a capacitor. How would I then release that electrical energy more slowly to power a small light at 10 amps a second for 3 seconds?

The light bulb should be replaced with one of smaller wattage (or power ) (or greater resistance) to obtain 3 seconds of 10 Amperes current instead of 1 second of 30 amperes current for the original higher power and lower resistance one.

The light intensity will diminish accordingly, duration will be longer

Your example of 30 Amperes and 1 second is equivalent to 30x1= 30 Coulombs of useable energy in the capacitor.

A common and abundant voltage converter of the DC-to-DC type can also lower the voltage and extend time. A lower voltage than a lamp is rated for will emit less luminosity.

Capacitor discharging exhibits a behavior like releasing air from a balloon; at the beginning can supply pressure and volume, rapidly decreasing after a short time.

It has been set a standard of 63% of a capacitor voltage discharge as useable.

More precise figures and details about your project can provide better suggestions and guidance.

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Resistors are sort of the opposite of what I'm going for. I want to be able to collect large amounts of energy in a short time, store that energy, then release it over a longer period of time to power a lower-power-necessity series of lights. I'm sure there's ways this is done otherwise every device would explode as soon as the circuit was completed with a charged battery.

The lights will only draw as much power as they need - assuming they are driven at the correct voltage. If your power source/store is at a higher voltage then, as someone else said, you need some sort of DC-DC converter.

If you are storing the energy in batteries then you will also need something to control charging. The complexity of this depends on the type of battery. For lead-acid batteries it is pretty simple. For large multi-cell Li-ion batteries (or similar) it will need complex charge balancing and battery management hardware.

But all this should be available off the shelf.

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Ok, so have the wires from the stator hook up to a series of parallel batteries, then have those batteries hook up to a DC-DC converter and have that hook up to the lower voltage lights. But, the last thing to consider is overcharging. A generator on its own can overcharge a battery given enough power and time, I need a way to first: have the circuitry recognize when the batteries are full, then second, only let excess energy out in some way so that the batteries stay full after energy is taken from them, maybe heat through specific resistor-vents that lead into the air or a light source emits the energy into higher frequency photons.

Edited by MWresearch
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... But, the last thing to consider is overcharging. A generator on its own can overcharge a battery given enough power and time, I need a way to first: have the circuitry recognize when the batteries are full, then second, only let excess energy out in some way so that the batteries stay full after energy is taken from them, maybe heat through specific resistor-vents that lead into the air or a light source emits the energy into higher frequency photons.

Many varieties commercially available: Charge controller web search

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The stator, generator or energy source does not overcharge a battery if it is regulated. Like the battery in your car receiving charge from an alternator governed by its voltage regulator.

When batteries are fully charged, there is no need to "let excess energy out" Again, like in your car, there is no need to turn lights on in order to prevent overcharging. It was taken care of during the charging process to not exceed rating/voltage 'by not sending more charge in'

To "stay full after energy is taken from them" is the job of the charging regulator, which recognizes a state of acceptance for more charge and 'tops it up'.

A voltage regulator senses a lead-acid battery voltage. If it is say 14.00V, stops 'sending' current. If it is 13.47V "after energy is taken from them", sends more charge.

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I need a way to first: have the circuitry recognize when the batteries are full

That is what a charge controller or battery management system does.

then second, only let excess energy out in some way so that the batteries stay full after energy is taken from them

I'm not sure I understand this. If the batteries are to stay full after energy is taken out, then does that mean you always have a source of energy to recharge them form? In which case, what is the function of the batteries? Why not just take the power directly from the source / generator?

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The stator, generator or energy source does not overcharge a battery if it is regulated. Like the battery in your car receiving charge from an alternator governed by its voltage regulator.

When batteries are fully charged, there is no need to "let excess energy out" Again, like in your car, there is no need to turn lights on in order to prevent overcharging. It was taken care of during the charging process to not exceed rating/voltage 'by not sending more charge in'

To "stay full after energy is taken from them" is the job of the charging regulator, which recognizes a state of acceptance for more charge and 'tops it up'.

A voltage regulator senses a lead-acid battery voltage. If it is say 14.00V, stops 'sending' current. If it is 13.47V "after energy is taken from them", sends more charge.

I'm still not sure by what you mean when you say "there is no need," because electricity and a battery isn't conscious, it doesn't know that a battery is full, how would a battery automatically know its full and not overcharge? How was it already taken care of? Are you saying a charge regulator prevents overcharging in the first place? Then what does it do with all that excess energy? Then, since the only thing I can think of dealing with excess energy is resistors, why wouldn't I just have resistors in place of a charge regulator?

Edited by MWresearch
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I asked a simple civil question in post 10 and would appreciate a simple civil answer.

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I'm still not sure by what you mean when you say "there is no need," because electricity and a battery isn't conscious, it doesn't know that a battery is full, how would a battery automatically know its full and not overcharge? How was it already taken care of? Are you saying a charge regulator prevents overcharging in the first place? Then what does it do with all that excess energy? Then, since the only thing I can think of dealing with excess energy is resistors, why wouldn't I just have resistors in place of a charge regulator?

Consider a variable resistor coupled with sensing the battery's voltage.

Several ways to accomplish charge regulation, but that is the gist of it. You'll want to pick the best one for your battery type and application.

Edited by Endy0816
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I am not convinced any charge regulation is required, but until MW research deigns to respond we shall all remain in the dark about the subject.

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Consider a variable resistor coupled with sensing the battery's voltage.

Several ways to accomplish charge regulation, but that is the gist of it. You'll want to pick the best one for your battery type and application.

Well I guess it seems resistors are inevitable. If I have a generator who's output I want adapted to different voltages while making sure the batteries don't over charge then a variable resistor seems like a good option.

Edited by MWresearch
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Well I guess it seems resistors are inevitable. If I have a generator who's output I want adapted to different voltages while making sure the batteries don't over charge then a variable resistor seems like a good option.

This is a TERRIBLE option.

It depends on the type of battery and the type of power source, but it is very unlikely that a resistor is adequate to provide correct charging control and prevent over-charging.

This is particularly true for modern battery technologies. If you don't know what you are doing you could, at best, shorten the life of the batteries. At worst you could cause a fire or an explosion.

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If you're using lithium batteries you need to build (or I'd suggest to buy) a proper charge controller else you will very likely end up with a fire. Most of what I've done in the last 5 years has been 5v stuff but I know you can get 5v/3.7v controllers (both input and output) from China for a few dollars. It's not worth the risk of not doing it.

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Well I guess it seems resistors are inevitable. If I have a generator who's output I want adapted to different voltages while making sure the batteries don't over charge then a variable resistor seems like a good option.

That was just one of the easier examples to follow, it may not be ideal in your particular case.

You should probably check more typical resources in this case, "generator set up home". Tons of videos and how-to guides freely available.

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Well the explosion is what I wan to avoid, but I'm having trouble figuring out what a charge regulator does differently because doesn't a charge regulator have to do SOMETHING with all that extra energy? I guess a DC-DC converter would do the trick from what you describe, but it would have to be a variable converter, like DCX-DC, no matter what input it had for voltage, the output voltage would be the same which means it would both have to do something with excess energy in the form of resistors but also have capacitors for when a lower voltage is input, I would think at least. Would a DCX-DC converter be the best option for something like a generator with a rotor that could rotate at all sorts of different angular velocities? Otherwise a charge controller is the other option. A charge controller redirects extra energy to some kind of water heater, and I know water has a high specific heat, but couldn't that water heater just overheat? I'm thinking about something that deals with wind and sun and rain and other kinetic movement, energy on a macroscopic scale but not a factory scale, something that's no bigger than a person but no smaller than the palm of your hand.

Edited by MWresearch

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