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# How to calculate size of capacitor needed?

#1
12-04-11, 09:54 PM
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How to calculate size of capacitor needed?

Hi, Sorry about this newbie question, but I am trying to understand how to determine the size of a capacitor required for a specific voltage needed.

I was watching a youtube video about a calculator solar cell, and wondered why it needed a capacitor (other than for storing energy). If a capacitor was absent in this video, would the LED still light up?

How would you come to the conclusion the size of the capacitor? I understand that Capacitance = Charge/Voltage, but I though that charge was constant (as in, an electron charge). If not, how would I determine how much charge is passing through a system in order to calculate capacitance?

Hopefully this is in the right forum.

Thanks.

#2
12-05-11, 06:02 AM
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Originally Posted by Questionz
If a capacitor was absent in this video, would the LED still light up?

Thanks.
You are correct in that he is using the cap for an energy storage device. He can continue to light the led after the light to the solar cell is removed. The cap is not required as long as the light is present and allowing the solar cell to covert lumens to mA.
Cap energy is 1/2 CV*2. Larger cap will take longer to charge but allow more total stored energy.

#3
12-05-11, 06:51 AM
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Hi z, from what I can catch from the video, the capacitor he is using is doing very little. Note that when he attaches the light the voltage quickly drops to 2.3 volts or so, that rapid drop is the capacitor giving up what little energy it had stored between 2.3 volts and 3 volts. The stable voltage that remains is what is being generated by the solar cell while under load. I don't see where his capacitor is serving any purpose at all in his circuit. As a storage device it would hold for only a few seconds.

One additional comment, when experimenting with diodes of any sort, understand that once you exceed the voltage where they begin to conduct, the current will increase rapidly and something in the circuit needs to become the limiter. Often you will see a resistor in series to serve this purpose.

Bud

#4
12-07-11, 12:09 AM
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So heres a silly question, I was thinking of whether it would be possible to charge a capacitor using solar energy. I know how inefficient solar energy is, but just as an interest, I wanted to do it, and possibly have it power a small toy motor. So here are the "items".

2700KV Outrunner Brushless Motor, 10V, 14-25A draw, 72% efficiency (i believe)
Solar cell at 0.8W (I'm assuming in this case its per second? - if not, my calculations will be horribly wrong).
56F capacitor

How I arrived at 56F Capacitor (Hopefully its correct)

Motor:
P= V*I=10V x 25A*(0.72) = 180W(J/s)

Solar Cell = 0.8J/s=2.880kWh

W=0.5*C*(V^2)
2880=0.5*C*(10V)^2
C=56F.

In hindsight, I believe the solar cell is not 0.8J/s, more like 0.8J/Hr. Therefore I guess this won't be able to work?

What would happen if I had 20 solar cells in series, is it correct to assume that it would work out to be 20cellsx0.8=16J/hr?

Basically I am trying to figure out a way to have a toy motor be self-sustaining without using an outlet. Be it from a rechargeable battery that is being recharged via solar cell or whatnot. And the solar cell is kind of what you see at home type, not specific high grade material that you buy for \$100+

Please let me know if my theory is correct.

EDIT: I just saw that some solar cells show a voltage and amperage, (5.5V, 80mA), which means that if P=V*I, then the unit IS in Joules/sec...?

#5
12-07-11, 01:10 AM
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Too early in the morning to get into details, but several problems with your numbers and your objective. A 56 farad capacitor is unrealistic, maybe the size of a real car, never seen one. Watts is volts times amps, no time involved, that's joules or work.

Nothing wrong with learning, but you have a long ways to go before you spend any money.

Bud

#6
12-07-11, 08:19 AM
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Thats right, watts is volt times amps, which is what i have down. But i also thought that work is joules per second?

What would be an ideal capacitor for this scenario?

#7
12-07-11, 09:53 AM
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Originally Posted by Questionz
Thats right, watts is volt times amps, which is what i have down. But i also thought that work is joules per second?

What would be an ideal capacitor for this scenario?
You primary mistake in post 4 is Watts= ... the equation for the energy stored in a cap. Energy is Joules, not watts. This is leading you to wild, orders of magnitude errors.

Sooo, do you want to power the toy directly with solar, or do you want to charge the battery with solar?

And, a small toy motor is not going to be 180 watts. I'm envisioning a slot car motor, but reading about a hand mixer sized motor.

#8
12-07-11, 12:59 PM
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In my mind, I am envisioning a small motor being powered by solar, but once the sun is down, I'd like the motor to be running through the majority of the night via the use of a capacitor, and eventually having solar energy to power the unit again in the morning

Solar ==> Charges capacitor, and powers small motor (Sunny)
Capacitor ==> motor (during the night, or when cloudy)
Solar ==>recharges capacitor, and powers the motor.

My mistake on the Watts calculation. Then since it is in Joules, what is the duration I can expect it to be? per day? Not sure how to utilize total energy to power.

#9
12-07-11, 01:15 PM
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Traditionally, batteries are used to store electrical energy, since you can store so much more Joules per \$. Why the interest in capacitors? One big issue with capacitors is that they will directly drop voltage, linear with charge; this is usually not a feature for most all loads. But, yes, it is possible to do what you want. 180 watt load day and night will demand orders of magnitude more solar cells than what you have in a handheld calculator. The devil is in the details, and so far you are a bit "out of bed" with the quantities of the various items needed.

Easy to power a tiny motor spinning a checked disc; same idea, but far more \$\$ to spin a ceiling fan all day/nite. What exactly is the motor doing?

#10
12-07-11, 01:27 PM
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Well....I got a few different hits for the motor you listed...but the best fit seemed to be one for RC toys like helos. At first I thought the amp draw was wrong but apparently not.

No way will you be able to provide even a few amps from a capacitor of any reasonable size. In my experience...caps are used to provide voltage over a longer period at a very, very low current draw....or a high initial boost of current over a very short time.

Just looking at a small 12V battery for a jump start kit I have here..it is listed at 18 amp hours....that means it could theoretically provide 1 amp for 18 hrs....no way a cap could do that. And a solar panel that could run the motor and charge a big enough battery to run the motor for 12 hrs of darkness would be much larger than the types you are talking about.

The solar cell you mentioned puts out .8w....it won't even run the motor by itself. Maybe it would put a charge on the cap...which would spin the motor for a few seconds...but thats it.

#11
12-07-11, 06:13 PM
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Originally Posted by telecom guy
180 watt load day and night will demand orders of magnitude more solar cells than what you have in a handheld calculator. The devil is in the details, and so far you are a bit "out of bed" with the quantities of the various items needed.

Easy to power a tiny motor spinning a checked disc; same idea, but far more \$\$ to spin a ceiling fan all day/nite. What exactly is the motor doing?
If I were to have 20 solar cells, that are 0.8W (0.8J/s right??), then the amount of watts inrease 20 x 0.8 right? If i were to have more, then this could allow the motor to run for a period longer than a few seconds right?

Originally Posted by Gunguy45
Just looking at a small 12V battery for a jump start kit I have here..it is listed at 18 amp hours....that means it could theoretically provide 1 amp for 18 hrs....no way a cap could do that. And a solar panel that could run the motor and charge a big enough battery to run the motor for 12 hrs of darkness would be much larger than the types you are talking about.

The solar cell you mentioned puts out .8w....it won't even run the motor by itself. Maybe it would put a charge on the cap...which would spin the motor for a few seconds...but thats it.
If I had more than one solar cell (0.8W x n) this could work right? How would one store energy then? If a capacitor doesn't work, would it be possible to store it in a rechargable battery? I think in this case I'm going way overboard, I don't know if 100 solar cells could charge an AA battery.

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