on-topic : Ben Krasnow on Youtube has a very good videos in regards to transistors :
http://www.youtube.com/watch?v=8DMZSxS-xVcsideways-topic : he also has a couple of videos that explains very well what "impedance" is (I found these two videos made it very easy for me to understand the concept):
http://www.youtube.com/watch?v=xyMH8wKK-Aghttp://www.youtube.com/watch?v=tZBMfDvWF4Uoff topic :
birrbert .... supercapacitors are working just like capacitors.
To reply to your questions... and I'm trying to make it very easy for you to understand.
Just like batteries, supercapacitors store energy in them.
Batteries store that energy and release it back through a chemical process so they can store a lot of energy for the volume.
For example, an alkaline AA battery when is fully charged can have a voltage of 1.65v and can release up 1-2 A of current for short periods of time and as it discharges the voltages goes down to about 1.4v
A rechargeable AA battery will be about 1.35-1.4v when fully charged and will go down to about 1.18-1.2v when it gets discharged.
So notice that a battery will no longer be able to provide current even though the voltage will still be about 1.2 volts.
The total energy in the battery is given back to you between a range of voltages, let's say 1.4v and 1.65 in the case of regular alkaline batteries.
Most batteries have a rating on them, saying for example 2500 mAh - while not 100% correct you could say that if something uses 1.5v and consumes 10 mA, then the battery will be discharged in 2500mAh / 10 mA = 250 hours
Capacitors and supercapacitors store energy differently, and they can charge up to the maximum voltage rating, but can also discharge themselves all the way down to 0 volts, unlike batteries.
Unlike batteries, they have a much lower resistance which means they can release all the energy inside as a high current burst, let's say 5-10A of current - batteries are limited by the chemical processes inside and the internal resistance ... if there's too much current they heat up inside and can get damaged.
For the same reason, they can charge up much faster and the charging process is very efficient .. you can charge a supercapacitor within seconds with high current. Batteries have to be charged slowly (minutes to hours) due to that internal resistance, so they won't overheat and get damaged.
There's a simple formula to determine how much energy can be stored in a supercapacitor :
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.htmlThe capacitance value is
amount of Charge /
Voltage The energy is 0.5 x Charge
2/Capacitance or 0.5 x Capacitance x Voltage
2.
So if you have a 1F 5v capacitor, the amount of energy would be 0.5 x 1 x 5 x 5 = 12.5 Joules.
How do you use this ... well,
Power is basically
Energy /
Time or
Energy =
Power x
Time.
If you have a device that uses 20 mA constantly when powered from 1.5v, you could say that the power used is 1.5v x 0.020 A = 0.03 Watts per second. The energy will then be 0.03 x 1s = 0.03 Joules or 0.03 x 3600 = 108 Joules in an hour.
Now, you have to understand that LEDs are actually current devices. This means that the LED needs to receive a voltage higher than a certain threshold, usually about 3v for while leds and then no matter how high the voltage (up to a certain limit of course) the led will keep working as long as you limit the current going through it.
When the voltage goes lower, then the LED will no longer work, even if there may still be energy.
So knowing this, let's take the two again with our 3v 20mA led .
Let's say you have 2 alkaline batteries, each with 1500 mAh, 1.65v charged, 1.4v completely discharged. This means the voltage will be between 2.8v (2x1.4v) and 3.3v (2x1.65v) and the total energy they can provide is still 1500 mAh.
But since your LED will stop working at around 3v, the batteries become unusuable when they still have about 25-30% life in them, so in reality you only have about 1200 mAh to work with.
So now, you can make it simple and divide 1200 mAh / 20 mA and you get about 120 hours of led working at 20 mA.
With the supercapacitor, you have up to 5v and 1F. But since the LED stops working at 3v, you can only use the energy between 3v and 5v to light the LED without some "tricks".
So if you have a 1 F 5v supercapacitor , the energy you can use is just the one between 3v and 5v, so for just 2 volts in total. The led will stay lit up for about C x (Vmax - Vmin) / I = 1F x 2v / 0.02 = 100 seconds.
But it's a shame to have a 5v supercapacitor and only use the energy between 3v and 5v so you can resort to BOOST REGULATORS which take lower voltages and output a higher voltage.
Such regulators can work with as low as 0.6-0.7v and output 2.8-3.3v at anything between 25mA and 1-2 amps of current. So you could get such a chip and pair it with a 2.3-2.7v super capacitor and the chip will use the energy between 0.6v and 2.7v (basically more than 70-80%) to output 3-3.3v you need at 20mA.
This way, you'd have the led run for much longer time, but still just minutes of use. 1F is not that much when you need 20mA.
To give you an example, I've just used a LT1307 boost regulator to produce 5.5v at around 5mA with a 25F 2.7v supercapacitor. This particular regulator can work with voltages as low as 1v but at high output voltages like the 5.5v I needed, it stops working at around 1.3v.
So with just the energy stored in the 25F supercapacitor between 1.3v and 2.7v, I was able to get around 5.5v @ 5mA for about one hour and 10 minutes. You would need only 3-3.2v but at 4 times the current, so you would probably get about 30 minutes of operation.
With a very efficient boost regulator that could do 0.6v - 2.7v with 90% efficiency you could probably get close to one hour or even more. Of course, you can put several capacitors in parallel for more capacitance, or put them in series for higher voltage.
I can't decide which energy storage method (capacitors or rechargeable batteries) to choose for my solar mobile phone charger project, so a tutorial that includes a test like this, would be excellent.
You would need about 100 to 500mA at least to charge phones. The best solution would probably be a mix of super capacitors and a lithium battery. Have a specialized chip of the energy harvesting variety which takes energy from the solar cells and charges a super capacitor. As the super capacitor gets close to being full, have another lithium battery charging chip use that energy to charge the battery at a constant flow for a few seconds until the super capacitor discharges.
The energy from solar cells can vary and it won't be a good idea to charge lithium batteries or ni-mh batteries directly from the solar cells.
For charging the actual phone, it would be best to simply output 5v at about 500mA-1A from the lithium battery - supercapacitors will be too big to give you a full phone charge and some phones may refuse to even charge if they aren't able to get at least 100-200mA from the source (something which your solar cells may not be able to provide constantly).
See Linear's energy harvesting page :
www.linear.com/products/Energy_Harvesting#featuredAs an example, see LTC3105 :
http://www.linear.com/product/LTC3105
You can connect a 4.2v lithium battery directly to it and it will slowly charge it using power from solar cells. Then you can use another chip to get 4.2v+ from the lithium battery and output 5v @ 0.5-1A to the phone.
