# Question about safely using super capacitors

Hello,

I'm working on a minimal arduino circuit (by minimal I mean a flashed ATMega328p chip and a few other components, but nothing else). It all works just fine, and I'm finally ready for my the last step: solar power + charging circuit. At this point, I do not not care about efficiency of the circuit. I expect it will run for just a few seconds at a time before losing power, and that's totally fine. Here are the relevant components for my question:

• 6v solar panel (outputs a maximum of ~350mA)
• 2.5v 10F super capacitor (x2)
• 3.3v voltage regulator (x2)
• 10 ohm resistor (x2)

The calculations:
I've determined my circuit will draw about 230mA of power at peak usage. I believe connecting the caps in parallel will give me 5F of storage with 5v of power. I'll use the 10ohm resistors in series to limit the output current to 250mA.

The plan:
I want to hook the solar panel to a 3.3v voltage regulator (so I don't provide too much power to the capacitors) and I'll have the caps wired in parallel. From there, I want to send power from the capacitor to my 10ohm resistors (wired in series) which should provide ~ 20ohms of resistance. From there I need to drop the power back to 3.3v I think, so I will use another voltage regulator. And finally, pipe the output of that to my circuit board.

Questions:

• When providing power to the 2x capacitors in parallel, do I need to make sure the voltage is less than 5v (the "total" of the "pack") or less than 2.5v (the rating of a single cell in the "pack")?
• Do I need to drop the power from 6v to 3.3v and then from the super capacitor to 3.3v? Or do I just need to do that once?
• I am worried that something will overheat. I have read countless places that voltage drops cause a lot of heat. And so I'm wondering how I can calculate the estimated temperature (or at least how to know whether the regulator will require a heatsink, I guess).
• Should I be concerned about the resistors overheating?

Thanks so much for your help! I've done a ton of googling and blog reading and even attempts with smaller capacitors, but these questions are what I haven't been able to puzzle out quite myself.

Capacitors in PARALLEL add capacitance (2 x 10 = 20F) but operate at the same voltage (V = 2.5)

If I interpret your requirements correctly, you need to connect the capacitors in SERIES such that the effective capacitance is 5F and the operating voltage is 5Volts

Your solar charger must never exceed 5 volts so you will require some form of in-line regulator

You do not need to limit current to your micro. Circuits draw whatever current they require.

If you want 3.3v for your micro your simplest method is to limit the charge voltage on the capacitors to 3.3v and this can be done by the regulator from the solar panel.

Heat across resistors is the product of current and voltage so in your case a 10ohm resistor carrying 250mA would develop 0.625watts hardly enough to feel warm.

A solar panel might , at best, be capable on average of outputting between 5 and 10% of its rated capacity in any 24 hours. So your 350ma unit might be expected to output 0.8Ah or 2.5Wh per day

Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html

It will be best if you post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

That way we can see what you want to build.

What is the application that only needs it to run for a couple of seconds at a time?

Thanks .. Tom...

@Jackrae Awesome information, thanks! That pretty much answers everything. I see now I was confused about capacitors in parallel vs. series. Thanks for catching that. Just to clarify, if I send 3.3v to the capacitors (which will be rated for 5v in this configuration) will they pass on that 3.3v to the circuit? So I won't have to limit it beyond that, right?

And will the temperature from the voltage regulator (6v solar panel to 3.3v regulator) be any concern? Since it's such a low amperage, I am hoping not. But I still have to ask, for safety

@TomGeorge Thanks for the welcome! I did read the first post, but maybe I missed a blurb about including a drawn circuit. Since my question is answered, I'll keep this in mind for next time - but I appreciate the heads up!

I've made a small arduino circuit that uses an ESP8266 WiFi module which sends a ping to Amazon IoT every few minutes. The output will be metrics I can use to make a visualization of the solar energy trends, uptime of my project, etc. It just needs to stay alive long enough to send a ping, which is about 200 bytes of data and happens really fast.

It's not useful, but the real goal of this project is to learn more about solar energy, capacitors, charging circuits, and maybe even give me a playground from which to experiment with better ways to keep the thing alive longer!

Thanks,
Josh

Thanks for catching that. Just to clarify, if I send 3.3v to the capacitors (which will be rated for 5v in this configuration) will they pass on that 3.3v to the circuit? So I won't have to limit it beyond that, right?

It's not the best design.. In theory the voltage will divide equally but in the real world the capacitors will charge at different rates and the voltage will NOT divide evenly (because there is tolerance and the capacitance won't be equal), and after the capacitors are charged the voltage will divide unequally again because the leakage resistance in the capacitors will be different.

Maybe you can get away with it and maybe with 3.3V the voltage won't exceed 2.5V across either capacitor...

And will the temperature from the voltage regulator (6v solar panel to 3.3v regulator) be any concern? Since it's such a low amperage, I am hoping not. But I still have to ask, for safety

I think you'll be OK. I assume you're using a regulator rated at about 1A? 2.7V x 230mA is 0.621 Watts, so that's not a lot of heat and you're saying it's for a short time.

@DVDdoug I appreciate the info, thanks! I see what you mean... I'll try a few different configurations then if I have to, but now I feel reasonably comfortable that my general approach isn't going to explode or fry. So that answers my questions, thanks everyone!

but now I feel reasonably comfortable that my general approach isn't going to explode or fry.

Well that is more than I feel. Good luck, and use eye protectors when playing with this stuff.

sharpcoder:
@DVDdoug I'll try a few different configurations

Such as ??

You really don't need "farads" for a few seconds run time and I think a simply NiMh of Nicad battery would serve you better

What do you need 230mA for? Can you reduce this power requirement?

I take it you appreciate that capacitors discharge exponentially and therefore do not give any form of stable/constant discharge voltage

Hi everyone,
This may not relate to this topic directly. but I think I may get the response I am looking for.
I am running some Arduino ATtiny85 modules, and have decided to power them with a 2.5v Solar Cell via a 500F Super Capacitor. I did a voltage test on the components after assembly and I have noticed a significant voltage drop when hooked up to the Super-Cap. See attachment.
I am a novice at Arduino and Electronics and bemused with the drop in voltage when providing power to the Super-Cap.
With the Super-Cap disconnected I am getting 2.37v from the Solar-cell and 2.34v at the Schottky Diode. With the Super-Cap connected I get 1.13v at the Solar-cell and 0.73v from the Diode to the Super-Cap.

I can understand the voltage drop after the Super-cap, but can someone explain why I see a large voltage drop at the Solar-cell with the Super-cap connected?

Cheers,

Lance

It's the internal resistance of the solar diodes that is causing the voltage droop under load. Irrespective of what manufacturers claim small solar cells do not give what you might expect. They often give voltage rating as the "open circuit" voltage, ie no load applied, and the current rating as the "short circuit" current, ie maximum load with no measured voltage output. I suspect this is the case with your array.

Out of interest, why would you expect to see a voltage drop "after the Super-cap". You show no resistive devices to cause a voltage loss, unless you mean a voltage drop after the diode.

Are you giving the supercap time to charge? Depending on your light conditions that thing might take hours to get up to a usable voltage. 500F is a monster.

5F on drawing 500F in text which is it ?