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DC Fan to run off Capacitors for Night use

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MichaelG

New Member
New member here. I'm trying to run a dc fan during the night using capacitors that will be charging during the day with a solar panel. Would really appreciate any help.

To simplify what I am using it will be a computer fan around 130mm and a 10 or 25 watt solar panel for charging capacitors during the day. I'm guessing I will also need a resistor and no idea on how many capacitors. The idea is to use spare parts around the house and have the fan only working at night until daylight is available to charge the capacitors up again. I was thinking I could maybe even use a solar lawn light.

Michael
 

ronsimpson

Well-Known Member
Most Helpful Member
Why not use batteries?
A capacitor has 0 volts with 0 charge and maximum voltage full charge. The fan will not run below some voltage. Maybe 10V on a 12V fan. So the fan stops when there is charge in the cap.
A battery has a voltage near 12V All the time. Maybe 11V to 13V from 0 to full charge.
 

audioguru

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A capacitor discharge is completely different to a battery discharge:
1) A capacitor discharge begins dropping the voltage quickly then drops slower and slower as it nears 0V. Then your fan would begin running fast then get slower and slower and would probably stop when the capacitor still has a lot of charge in it.
2) A rechargeable battery voltage stays fairly high when it is discharging until it is almost dead then its voltage drops quickly. Then the fan runs at a steady speed until the battery is almost dead.
3) Compare the capacities. A capacitor must be enormous and expensive to do what you want to replace a cheap little rechargeable battery.

My solar garden lights work for about 8 hours at night. The LED in them uses only 10mA but your fan motor uses at least 10 times as much current but the solar garden light circuit cannot supply that much current and will do nothing.
 

MichaelG

New Member
Thanks for the info. I will scratch the use of the solar garden circuit. It would be pretty cool if they made more powerful circuit boards like the solar garden light. I could see many uses for them. I understand that a battery would be easiest to use. How would I setup a battery to charge by solar panel and run the fan only at night? What type of batteries and configuration would it require?

Do you think it's possible to get working using capacitors? Let's say it's a smaller fan say a 80mm and it's only 5 volts to run it and I have a bunch of spare capacitors.
How would I set it up? I would use rechargeable batteries if I could charge them with the sun.

Thanks again for any help.
 

gophert

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Most Helpful Member
Thanks for the info. I will scratch the use of the solar garden circuit. It would be pretty cool if they made more powerful circuit boards like the solar garden light. I could see many uses for them. I understand that a battery would be easiest to use. How would I setup a battery to charge by solar panel and run the fan only at night? What type of batteries and configuration would it require?

Do you think it's possible to get working using capacitors? Let's say it's a smaller fan say a 80mm and it's only 5 volts to run it and I have a bunch of spare capacitors.
How would I set it up? I would use rechargeable batteries if I could charge them with the sun.

Thanks again for any help.

What's the fascination with capacitors? What's with the fear of batteries?
 

Pommie

Well-Known Member
Most Helpful Member
Lets go big. A 100,000μF 25v capacitor is huge and can hold about 30 joules of energy. A AAA battery can hold around 5000 joules.

Still want to use capacitors?

Mike.
Edit maths in head so maybe wrong.
BTW, a cap of that capacity will be around 2" diameter and 4" long.
 
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audioguru

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Most Helpful Member
You need to try a fan with a power supply to feel if it blows enough. Maybe you need a much bigger fan or a bunch of smaller ones. You might need to select a quiet fan.
Then you need to know how much current the fan uses and find a battery with enough mAh capacity to run it for one charge and discharge.
Then you need to find a solar panel that will charge the battery in a little less than one day.
The battery and its charger must be much more powerful if you want the fan to run the night following a cloudy day.

There is a advertising sign in a field that has no electricity near it. The owner has both a solar panel and a windmill generator charging its battery.
 

tomizett

Active Member
You can get portable solar panels designed for keeping car batteries topped up - once of those would be a nice match for a small 12v sealed lead acid alarm battery. Then you have 2 control problems - how to detect when it's dark and start the fan, and how to stop the fan when the battery voltage drops, to avoid spoiling the battery.
 

audioguru

Well-Known Member
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Use a solar panel designed to charge a 12V battery, a rechargeable battery and a charger circuit for the battery. There is nothing that needs a booster. Even a regulator is not needed.
Calculate the sizes of the solar panel and battery by using the current spec's of the fan and how long you want a charge to run it.
 

gary350

Well-Known Member
I have an LED light that runs for about 10 minutes on a capacitor. I think an electric motor might run a few seconds on this same capacitor. You will need a pick up truck full of capacitors to run your motor. You will do better to use a solar panel to charge a car battery to run your fan at night.
 

audioguru

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Most Helpful Member
It is a little computer fan. It blows very little air. He will need many of them or a much larger one, and a larger solar panel and larger battery.
I wonder why he doesn't use a normal electric fan? Maybe he lives in a cave?
 

MichaelG

New Member
I have two choices capacitors or battery. I'm fine using a solar panel, batteries or capacitor to recharge my energy source or anything.

I don't have a way to use an extension cord and I want to see what some of you more experienced people would recommend. I don't want to pay to have an outlet put outside for this project.

It's crucial for my test to have this automated and turn on when it becomes dark. I was told that lawn solar lamps are too weak to handle a load to power a small computer fan for 8-12 hours. A normal fan is too big and not small enough for this project. Plus it needs to run off dc.

Suggestions are appreciated for this.
 

dknguyen

Well-Known Member
Most Helpful Member
I have two choices capacitors or battery. I'm fine using a solar panel, batteries or capacitor to recharge my energy source or anything.

I don't have a way to use an extension cord and I want to see what some of you more experienced people would recommend. I don't want to pay to have an outlet put outside for this project.

It's crucial for my test to have this automated and turn on when it becomes dark. I was told that lawn solar lamps are too weak to handle a load to power a small computer fan for 8-12 hours. A normal fan is too big and not small enough for this project. Plus it needs to run off dc.

Suggestions are appreciated for this.
A large battery will do it, 3-5Ah about the average 12V, 250mA fan. Assuming yous spec the 25W panel properly current/voltage wise and there is good sun, that should charge the battery in 2-3 hours.
 

AnalogKid

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Most Helpful Member
You can run your own calculations using "the other" capacitor equation. With this, if you know the motors rated voltage and current, and the desired run time, you can calculate the capacitor size needed. Note that this is an approximation, good in this application to approx. +/-5%. The tradeoff is that it is multiplication and division, not exponentials and natural logarithms. The equation is based on the capacitor being discharged by a constant-current load, a load whose current draw does not change even though the capacitor voltage is steadily decreasing. Since a real fan's current does change with the applied voltage, we approximate the average current over the run time.

ec=it

Voltage drop times capacitor value equals average current times the run time
Example:
Fan - 5 V nominal, stops running at 3 V -- e = 2 V
Current at 5 V is 50 mA, current at 3 V is 40 mA -- average current i = .045 A
6 hour run time -- t = 21,600 seconds

Rearranging:

c = (i x t) / e = (.045 x 21,600) / 2 = 486 F

So, you would need about 500 farads (not microfarads) of capacitance charged up to 5 V to run the fan for 6 hours.

ak
 

audioguru

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Most Helpful Member
But the voltage of a discharging capacitor drops steeply at the beginning. You will hear and feel the fan slowing down. A battery holds up its voltage pretty well until the end of the discharge when it drops.
 

Pommie

Well-Known Member
Most Helpful Member
A typical fan uses about 5W. At 5V that is 1A. You need to store around 10AH (50WH) to keep it running. You could use a 12V 7AH (84WH) lead acid battery charged by a 15W solar panel and a 5V regulator. A relay connected to the solar panel will switch off when it goes dark - use the normally open contacts to power the fan. You will need a diode to stop the battery from powering the relay.

Mike.
 
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