Hi guys,

I've been thinking of powering a project I'm working on using a super capacitor (www.jaycar.co.nz). I'm thinking of keeping the capacitor charged by using a solar panel and this Mini Boost Buck DC Board (www.electrodragon.com). The output of this module is 3.3V, 150 mA however my project will draw more current than that (up to 320mA). I know that a capacitors voltage will never exceed the voltage of the power supply it's charged with but is this the same with current? Will the capacitor be able to provide the project with more than 150mA despite it being charged with only 150mA once it has had time to charge?

Also, will the capacitor draw more than 150mA from Mini Boost Buck DC Board (as trying to power the project directly from the board ruined one of the Mini Boost Buck DC Boards as it drew more than 150mA) and if so how can I prevent this?

Thanks

 

You will require one hefty big super cap or you will be very disappointed with the capabilities of a super cap. Do the math.
At 320mA @ 3.3V, that represents a resistive load of 10Ω.

Time constant is calculated using the formula tau = R x C.
After one time constant, the voltage on the capacitor would have fallen to 37% of its initial value, i.e. 1.2V
Hence, a 1F super cap will fall from 3.3V to 1.2V in 10 seconds.

How long do you want to maintain usable voltage on your project?

 

I had a bunch of these at one time 2 would maybe get a you a hour

 

Um... Did you know that batteries were invented so that you would not need to use capacitors?

 

Thanks for the replies. Valid points, but was hoping the solar panel would keep the capacitor charged. So I'll need to look for a larger capacitor like be80be's or I'll have to be looking at other options for a power supply.

The reason I'm hoping to not have to use batteries (like Lipos) is that from my research they seem quite complex to charge with solar panels. If I did acquire a large enough capacitor would the current problem still be an issue?

 

A super cap will look like a dead short to your charger if there is no charge on the capacitor to begin.
If you want to limit the current, you need to put a 22Ω resistor in series with the charger.

But now you have another problem. It will take a very long time to bring the super cap up to full charge.

Have you priced a 1000F super cap?

Let's suppose you can find a 1000F /3.3V super cap. If you try to charge it at 3.3V with a 22Ω series resistor, the time constant will be 22000 seconds = 6 hours.

To charge it to about 3V will take about five time constants, i.e. 30 hours.

Now you have a misunderstanding about current. A capacitor charged to 3V can supply an infinite amount current for short period of time. The problem is that a capacitor is not a battery. The voltage will start to fall instantly as you draw a current from it.

You have not stated how long you want to run your project. What minimum voltage you must supply?
Let's say your minimum voltage is 2.5V. A 1000F capacitor will supply 320mA @ 3-2.5Vfor about 1200 seconds = 20 minutes.

Use a battery instead.

 

Thanks for the replies. Valid points, but was hoping the solar panel would keep the capacitor charged. So I'll need to look for a larger capacitor like be80be's or I'll have to be looking at other options for a power supply.

The reason I'm hoping to not have to use batteries (like Lipos) is that from my research they seem quite complex to charge with solar panels. If I did acquire a large enough capacitor would the current problem still be an issue?

And what are you going to do at night??

Charging batteries is not complicated at all if you purchase the right charger. There are plenty of them available at very reasonable prices.

 

The premises you ask about a cap being charged at a low current and whether it can deliver a higher current is correct. HOWEVER: If you're charging it with 150 mA while you're using 320 mA you'll be using more current than your supply can deliver. Your capacitor will never take a charge.

Now: If you wanted to use the PVP (Photo Voltaic Solar Panel) to charge the cap and be READY to deliver current at a given moment then yes, that would work. That means that your system is doing nothing until it's commanded to. And it will operate for a very short time. Someone already did the time calc's with that cap and the specs you've outlined. They said 10 seconds was all you could expect of reasonable power from the cap.

Think of it like a bank account. Every day you put 150 dollars into the account and every day you withdraw 320 dollars. Won't be long before you're bankrupt. In fact, the first time you try to withdraw you'll be busted.

I have a system going right now in my back yard. It's an Ultrasonic Dog Bark device that under normal operation runs on a 9 volt battery. However, because the dogs in my neighborhood bark so often, every two weeks I'd be changing batteries. So I bought an 18 volt PV rated at 5 watts and stuck it on the roof. Wired that to a buck converter and down-converted the power to 10 volts. When the dogs bark it emits a short burst of ultrasonic sound to distract them. It works. At night I don't want to prohibit dogs from barking. If there's something to bark at then interfering with that would be foolish; which, when operated from a 9 volt battery ALL DAY AND ALL NIGHT the dogs would not bark at night when there might be good reason for them to do so. MY system operates intermittently, and there's enough power from the PVP and Buck so that I don't need any super capacitance or rechargeable batteries. True, I COULD put a rechargeable device up there, but there's no need for it because the PVP handles the job nicely. Back to the bank analogy: I'm not depositing and withdrawing, I have a credit. When I need, the money is there. When not used, the card is paid off by the sun. And I never draw more than my credit rating, so my system is sound and stable.

You need to redesign your project. In short, you'll need a bigger PVP.

 

I have a solar powered clock that uses a 1F capacitor as my battery. The cap can run my project for 10 days with no sunlight.
What's the difference here? My current draw is 3μA.

 

What's the difference here? My current draw is 3μA.

3 microamps versus 320 milliamps? Only an order of 106,666.7 times higher draw.

 

Hi guys,

Thanks for the feedback. I'll drop the idea of using a capacitor and work on using a battery.
Thanks