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Super Capacitor in solar power supply - a few queries

I intend to build a remote 'weather station'

The station will:

  • Be completley independant of any external power, ie, it will produce its own power via solar cell(s)
  • Measure temperature only
  • Transmit its finding over an RF link to an indoors reciever

Instead of using a rechargeable battery supply with charger circuit, i have decided to use a 35F, 2.7V super capacitor to smooth and store power.

The power scheme will run along these lines:

Panel Array* ===> Diode ===> Super Cap ===> Voltage boost to 5V via pololu voltage boost ===> Remainder of circuit

*The output voltage will preferably be ~1.5V, as my super cap is rated at 2.7V and i hear a general rule of thumb is to keep the impressed voltage at around half the rating for long capacitor life.

I have several questions concerning this setup:

How can I ensure that the total power output by the panel(s) and capacitor will not cause the voltage boost (and subsequently my logic circuit) to "hiccup."That is  

"The power supply will shutdown, then restart itself. This will repeat until either the overload (?) goes away, or a component in the power supply fails from trying."

- from http://www.powerstream.com/supercap.htm

This hiccup theoretically occurs at the voltage boosts minium input, which is rated 0.8V.

I have tested this with a panel, the volt boost, a picaxe 08M, and a simple program to flash an LED on for a second, off for a second, repeat. I didn't use a large cap as it hasn't arrived in the mail yet, but that shouldn't matter as this would mimic a real life situation cap or no cap anyway.(?) My results were that when the power produced by the panel was on the border of the volt boost's specs, the LED flashed several times a second, implying that the volt boost was rippling on and off many times a second. I expect this kind of situation to occur during twilight hours when the panels + cap may not be able to support power production, or at night when the panel has ceased functioning, but the cap's voltage has decayed to the edge of the voltage boosts limitations.

After all that background, i can finally ask: Is this 'hiccuping' actually damaging to the power supply, picaxe or RF transmitter?

If so, how can I prevent it?

Wherever i have made a bracketed question mark in my post, i am hoping someone will come along and b*tch slap me with some knowledge.


Thanks all!

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• The article you linked to is talking about charging a supercap from a switch-mode power supply, which is the opposite to what you're doing. That diode between the solar panel and supercap is all you need in terms of protection there, no need to worry about overloading anything.

• You're likely to get better results when using the actual supercap.
What is happening here is that the solar panel provides enough voltage to fire up the voltage booster, which then applies power to the PICAXE. The PICAXE then draws current from the voltage booster, which in turn draws current from the solar panel, causing the solar panel's output voltage to sag a little. This drop in voltage brings the voltage booster below the minimum working threshold, and therefore it turns off. The cycle will repeat like this until the solar panel can provide enough power to maintain a sufficient output voltage.
When you add the supercap into the circuit you have a much bigger buffer, whereby the solar panel is supplemented by the charged supercap, which helps prevent this little 'hiccup' as you have put it. Another smaller capacitor can be added to the output of the voltage booster to further increase reliability.

• Your voltage booster is designed to run a battery down as low as it can go, so there's nothing happening here it isn't built for.
Likewise your PICAXE is probably resetting due to a built-in feature called a brown-out reset, which is a safety mechanism that turns the PICAXE off when the supply voltage is too low, so it will also continue to work happily. In fact the brown-out reset feature is important for applications where the PICAXE will need to turn itself back on automatically, like when your solar panel starts charging up again in the morning.
Lastly your RF module is probably pretty robust, and in any case it should be controlled by your PICAXE to only turn on when a transmission is to be made. As voodoobot mentioned in shout earlier the RF module will be eating a relatively large amount of power, so you'll want to keep it turned off as much as possible.

Hope this all helps, will be watching for future updates =)