Second version of the Victory over the Sun project.
After the sketch version (see post https://myfablab.wordpress.com/2015/03/17/victory-over-the-sun-sketch-version/) the recycled jeans dress is totally modified. The front is reordered in horitontal strips, where the big LED’s form two curved lines. These 8 LED’s plus one at the bottom will report about the game between the solar cell and the dynamo which is operated and powered by your hand and arm.
The back side is decorated with a pattern using the laser cutter.
The material is recycled jeans. The idea is that there should be a match between the concept of energy generating and the material that is recycled. The jeans were worn for a few years and the worn out parts were removed. From the left over parts, which seemed still ok the dress was assembled. But in handling the fabric it was noted that also the seemingly good parts were deteriorating fast. In total contrast with the energy of course. Energy is energy, even if it is transformed many times. There is no comparison between fabric and energy, although we humans tend to associate green, “nice” energy with green recycling of goods and fabrics. Our tender ideas are not appreciated by reality.
The electronics is improved after the first skethc version. The LED’s are extended to 8. In the dress there are 9 LED’s. The two bottom ones will be used to indicate that a game is going on.
Inside of the dress, connecting the LED’s. The parts of the electronics, LED’s, circuitry, supercaps are all connected using connectors, so that testing is made easier.
In the electronics the supercap of 1.0 for each of the participants was replaced by two supercap’s of 10F at 2.5V in series. Now a player has to work to get to 5V!
One problem remained and remains. After the game a lot of energy is stored in the supercaps of 10F at 5.0V. You can calculate this energy: the formula is Energy of a capacitor = .5*C*V*V.
See for instance: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng.html
This means 125 J. In the first version this was only 12.5J because the Cap was 1.0F instead of 10F.
After the game this energy has to be dissipated to start a new game. But this getting rid of the energy costs time. The first try was using a ventilator. Even a cooling ven for a computer running normally at 12V could be used and took minutes to get the voltage of the supercaps down.
The ven is visible at the left bottom.
The last experiment was to charge the lipo used for the circuitry of the game.
During dissipation the voltage of the supercaps drops from 5.0V to hopefully 0V. The lipo battery is between 4.1V and 3.6V. So to charge the lipo a special LTC chip is needed (LTC3105) , to keep the voltage at 4.2V during charging back the energy of the game into the lipo. This LTC is actually a harvesting chip studied during a research on energy harvesting:
So the supercap should be connected to this chip and the output of the chip fed back into the lipo.
Expected was a fairly normal exponential behavior while discharging. Not quite so!
This is a typical graph of the Voltage over the supercap and the charging current to the lipo:
The time axis is not linear, noted were values of tens of mA of current and the corresponding remaining Voltage over supercaps. The LTC3588 took care of keeping the charging voltage to the lipo n 4.5V, even till the supercaps were at 1.5V.
There is a spike in the middle of the graph which was reproduced.
The drop-off is not exponential, probably because the charge pump is being cleverly regulated, this LTC 3588 chip is fairly complex.
The discharging is made interesting, but the last problem remains: the charge on the supercaps will not be brought back to 0V, which is the ideal starting point for the game. On the other hand with the 10F caps, the game takes long enough for the human arm to get tired. So maybe we keep this way of discharging.
One might get anxious about the lipo. If the lipo is charged to quick, things may get nasty. But the voltage of the lipo changed during charging only 1mV, from 3.81 to 3.82. The energy density of the lipo is far more bigger than the supercaps:
Lipo battery of 1400mAh * 3.8V = 3600 * 1.4 * 3.8 =~ 19000 J. So the 120 of the supercap does not really impress the lipo.
Adding an oscilloscope picture:
Yellow line: voltage over the supercap,
Blue line: inverted voltage over a shunt of .5 Ohm. For a square this means 10 mA. Maximum current 24mA. The current is different from the former graph because of the shunt
Start: first 4.9V being above the voltage fed to the lipo 4.2V (so the chip DC-DC downwards), then you see the crossing of this set voltage, the chip has to DC-DC upwards to 4.2V, there is a discontinuity, the current delivered is bigger -more efficent? After that the exponential. The last part is still interesting, the chip really keeps the voltage up to 4.2V while the supercaps ar at 1.5V.