Introduction
This is my final blog about my journey into the world of supercapacitors.
I must say, it has been quite some journey and I have thoroughly enjoyed this design challenge. In my introductory blog, I had noted that one of my primary goals was to understand the nature of charging versus discharging and how you can incorporate this knowledge into your application design. I believe I have achieved that and can now appreciate what it takes, design wise, to build a circuit capable of charging these really big supercapacitors. Similarly with discharging. With some simple experimentation I can now understand some of the mechanics behind how a supercapacitor works.
My design challenge efforts have been very much application focused. As discussed in my second blog, I had to decide which super capacitors would be best suited to powering LED's. As noted the LED forward voltage plays a key role in choosing which supercapacitor to use and in my third blog I described some of my findings when experimenting with standard LED's and the three DGH supercapacitors.
This had triggered to some learning insights on my part, which I will now share with you.
It's time to put your shades on to avoid getting blinded...
Using high powered LED's
I find that with many projects, where you have a clear vision of what you want, you purchase a bunch of components to achieve your goal before really understanding the mechanics behind your component choices.
I too fell into this trap, and had ordered some high power LED's from Cree (product number: XBHAWT-00-0000-000LT20E7), well before realising how a diode's forward voltage can make a world of difference when used with supercapacitors.
According to the datasheet, these tiny LED's deliver more than 500 lumens at up to the maximum drive current of 1.5 A. At 700mA, the LED has a typical forward voltage of 2.86V. At the maximum drive current, it's forward voltage is 3.08V.
Thus the forward voltage of these LED's was higher than the maximum charge voltage for the DGH supercapacitors, which is 2.7V.
As such, if I wanted to use these LED's I needed to switch to the DSF type supercapacitors, which have a maximum charge voltage of 3V.
Using the 3 Farad 3 Volt DSF Supercapacitor (DSF305Q3R0)
The final capacitor used in my experiments was the 3 Farad 3 Volt DSF Supercapacitor (DSF305Q3R0). I tried charging the 7 Farad DSF supercapacitors with the ESP8266 NodeMCU dev board, but it finally fried the component that had given off some white smoke when charging up the other 10F supercapacitor (see blog 3).
Based on my previous experiments with the DGH Supercapacitors, I made some circuit design changes.
To get my LED's to flash in a controlled, repeatable manner I used a microcontroller and a n-channel MOSFET. As the LED's were now flashing, I potentially could use really low series resistance values of less than 1ohm, if the pulse period was less than a second. In my experiments I used a 200msec pulse. However, the lowest resistor values I had available were 3.3 ohms 500mW resistors, so that is what I used.
I also wanted to see how low the voltage would get to before the LED's actually turned off. To achieve this I added another diode (a BAX16) in parallel which has a much lower forward voltage (typical 1.5V). This seemed to do the trick, although I did have to reduce the series resistor attached to the diode from 100ohms to 50ohms.
| {gallery:autoplay=false}DSF Supercapacitor switching LED Circuit |
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Then while digging around looking for bits and bobs, I discovered an old TSL2516 LUX sensor inside an enclosure, which was still wired up ready for use. So I used that too to track brightness.
Finally, it dawned on me that the DSF supercapacitors could probably work with 2 x 1.5V alkaline batteries. So I grabbed two AAA batteries and attached those to circuit too, to see what happens.
And here are the results.
My experimentation results
The first experiment was simply using the DSF305Q3R0 to power the two high powered LED's. It was interesting to note how the voltage fluctuates when flashing the LED's.
I then reflected on the relevant information I extracted from the Cornell Dubilier datasheets about supercapacitor suitability for different applications and that the DSF and DGH supercapacitors were recommended for applications with very fast discharge rate.
I decided to test this out and decided to try one of the EDC supercapacitors, which offered a much higher maximum charge voltage (5.5V).
Firstly, I found that this self discharge rate is quite high, so by the time I connected this supercapacitor to my circuit its voltage was already below 4V. But the interesting things was how the voltage fluctuated every time the two high powered LED's flashed.
So I felt that the DSF and DGH supercapacitors were in fact the right choice here.
Let's see what happens when I charge the DSF305Q3R0 with 2 x AAA alkaline batteries:
Here it appeared that the AA batteries were starting to struggle to charge the supercapacitor to it's full charge voltage. But it got there. In fact, I let it charge to 3.15V.
Then I let it flash away, blinking every 2 seconds with a pulse of 200 milliseconds. Here is the data I captured over 10 minutes. It too had that "fuzzy" look but I felt it was still better than the EDC supercapacitor
Another interesting insight, was that the LUX values slowly dropped in value as the voltage edged towards the LED's forward voltage.
Here is a video capturing a snippet of the experiment.
Closing Remarks
To close off, I wish to thank Cornell Dubilier and Element14 once again for this fantastic opportunity to learn and experiment with supercapacitors. It was very much appreciated.
I now understand the challenges behind the use of supercapacitors and how that might apply to an LED flashing application. Although I did not test every single capacitor in the kit, I was more than happy with the DGH and DSF options and based on the results, I felt happy with these choices. Certainly more testing and experimentation will prove or possibly disprove this choice for this application. Anyway, I hope the information I shared in these blogs is of value and that it helps you too.
