1 Square Inch of Power

Today I had a nice sunny day, so I plugged the solar panel in and left it in the sun, then waited 15 minutes......

Still no good!!!! Something has to be wrong. So I removed the second LTC3105 so I could check that the first one is working. Back into to sun for 15 mins....

Success! The supercap voltage was just over 2.5v! So there must be an issue with the second LTC3105. I tried resoldering using the solder left on the pad.....Ba Bong...(don't do that) so I removed all the old solder using solder wick and then got some fresh solder paste and got to work with the hot air gun. I put a bit too much on, so a couple of traces were bridges next to the DFN package. A quick cleanup with the wick and try again....

Nope! No output from second LTC3105.

Back to the schematic to see what I've done wrong. I noticed a couple of things. Because I was using a supercap between the 2 ICs, I thought I wouldn't need a big 10uf cap on the input to the LTC3105, like the first one in the chain does. I used a 1uF. Maybe if I change that.

Nope. That didn't work.

The only other "extra" part on the board is a 33pF filtering cap between the output and the feedback pin. I may as well try removing that. Can't hurt right???

Right!!! It now works!!

After leaving it in the sun for 15 mins, the supercap is full and 3.3V on the output!

Let's add a load to the circuit. I have an Arduino Pro Mini 3.3v lying around with the default Blink sketch on it. I plugged it in to the Vin pin (not Raw) and the power light came on and the second led started blinking!

In full sun, the tiny solar panel could only just continue to charge with both LEDs on, but charge it did!

Even when it was moved into the shade, the supercap and the second LTC3105 still produced 3.3v output, with the supercap voltage slowly going down.

I let it fully discharge then put it back into the sun. The supercap voltage got back up to 0.4v.....then dropped back to .3..... The output was only .2v.

So looks like if there's a load connected that draws too much power on startup, the board gets stuck in a loop.

Methinks it's time for a voltage supervisor or similar that either enables the second LTC3105 only when the supercap voltage is over a certain threshold (1v maybe?). I also only had a 4.7uF cap on the output of the second LTC3105. Looking back at the datasheet, 10uF is recommended, so I'll replace that and try again.

Regardless, the board works! It could be used to power a small micro and sensor. Maybe even BTLE or a low power RF module?

To be continued ......

Just as predicted last project log, the supercaps arrived!

I soldered one of the 2.7v/5F caps onto the board, then setup the solar panel under the halogen light source ( because night time). I connected the multimeter up to the positive terminal of the supercap, which is also the output of the first LTC3105. The voltage rose to .3v..........but no higher.....

Now I know its a tiny solar panel, so I left it under the light for 5 minutes. I had previously figured out that having the solar cell really close to the light is a good approximation to sunlight.

Or is it?

No increase in voltage.

As I don't have a lab power supply, I can't provide an alternative input that is current limited. I thought I'd connect an AA up directly to the supercap to charge it up to 1.5V. And yes, this worked!

I then put the lamp back onto the solar cell, but no increase in voltage even after 5 minutes.... What's going on?

I checked the solar panel voltage again. It's outputting 1.48V. Exactly what it should be with the MPPT resistor setting. Maybe I have a short? I desoldered the second LTC3105 so I can isolate the first one. Tried again. No increase.

I went back to one of the reference web pages that are similar to this project - http://wiesel.ece.utah.edu/redmine/projects/essc/wiki

I decided to do the math to see how long it should take the solar panel to charge the supercap.

From the website:

• Charge time:
• Ic = C dv/dt
• Ic/C dt = dv
• V = int(Ic/C dt)
• V = Ic/C * t
• V/Ic/C = t

Theoretically, solar output is 1.5V @ 44.6mA. So 0.0669W. Using the LTC3105 datasheet, it shows a graph for efficiency and power loss for certain input voltages and output currents. Now I'm just using the basic Ohms law calculations for the output current.

So assuming 100% efficient, output power is .0669W. At 2.5V, the output current is 26.76mA. Efficiency at this current vs the 1.5v input is a smidge over 70%. So output power is .04683W. At 2.5V, that's 18.732mA. (

V = 2.5V C = 6F Ic=.018732

I had to check the math, so (V*C)/Ic works to get their results ( and using the math from another source)

2.5V/.018A/6F = 800 s = 13 minutes

Looks like I'll have to test during the day to make sure the solar panels are actually outputting the required current, as I also found that the halogen lamp easily makes the cell output the right voltage, but output current is questionable.

To be continued....

Just a quick update......but nothing useful really.

The Supercaps that were ordered from AliExpress on 22nd Dec still haven't arrived, although other items ordered on the same day have arrived. So it shouldn't be too long before I can complete the module and test it properly.

Originally, I was going to solder the solar panel directly onto the power supply board using regular pin headers. The problem is that it's hard to get access to the board to switch out the MPPT resistors ( If a different panel is used).

I might get some tall headers like these:

Lets see what the postman brings next week!

Right on the last day to enter the comp, the PCBs arrived from Oshpark! So a quick assembly and test (at night, but using a halogen light source). 3.249v output (not 3.3v) That's because I don't have the exact resistors I used in the schematic, so I got as close as I could get with the ones I had available ! The Supercaps haven't arrived yet, so I can't perform a full test. The little solar panel performs as expected. 1.5v @ approx 45mA.

Silly me forgot that everything in USA is closed for the Thanksgiving weekend. I sent off the board designs to get made by OSHpark last week.....

Anyway, while I'm waiting, I uploaded the gerbers, BOM, schematic and added links to the OSHpark board ( That should qualify me for the next round :-) ) I created a second square inch board (although not part of the comp) to attach the solar cells. The cells I'm using are IXYS KXOB22-12x1L cells. Each one is 0.5V, and there's 3 in series. So a whole .07W (ish). The link to the project on OSHpark is there too.

The schematic has the resistors to set the MPPT voltage to 1.5V, and the output of the second DC-DC converter is set to 3.3V. These can be changed using the info from the datasheet, in case the solar cell you use is different, or if you want 5V out.

At the last minute, I thought I'd enter the square inch challenge. So I did a quick layout, and found that a full square inch is waaaaaaay to big. So any suggestions on what else to fit in there