DC/DC PWM controlled conveter, charging a capacitor (arduino UNO)

Hi there, firstly a brief explanation of my project, and please be aware i dont want answers just checking im on the right course!

We have been set a task of charging a capacitor with a solar panel via a DC/DC converter (datasheet: http://docs-europe.electrocomponents.com/webdocs/0f8d/0900766b80f8df41.pdf )
Eventually the charge stored in the cap will be used to drive a load so another similar converter will be used but that is a few steps ahead.

My plan is to charge the cap up to a certain voltage (dependant on which cap we eventually choose), detect this voltage using the analogRead command, when the required voltage is met, the arduino sends a PWM signal to the converter to shut it off.

on to the questions:

The datasheet for the converter recommends a PWM frequency of just 20-200Hz whereas the arduino PWM output is at a much higher frequency, what problems may i run into here? what possible solutions/workarounds are there?

what is the maximum voltage that can be detected on the analog in pins?

Any and all help would be greatly appreciated, and please let me know if i can explain my problem any better.

Using a LED driver to charge a capacitor with a solar panel ? I have not heard of that before, and I'm not sure if that will work in all situations. A capacitor as a load is not the same as a led, I don't know if the DC-DC converter can handle that.

The PWM is recommended between 20 and 200Hz, and maximum 2khz.
The Arduino uses about 500Hz (not for every pin), so that is no problem. Perhaps it is not fully linear.

Why do you want to use PWM ? You can use the pin 3 of the DC-DC converter to turn that module on and off.
Use a digital output of the Arduino to that pin. At 0V the converter is on, at 5V the converter is off.

The Arduino can read 0...5V on the analog pins, and no more !
That is why everyone uses a voltage divider with two resistors.
For example one resistor of 10k from A0 to GND, and another resistor of 100k from A0 to the capacitor. The capacitor voltage can be 0...55V, and that is divided for the Arduino to 0...5V and can be read with analogRead(A0).

The resistor of 100k 'leaks' some current away from the capacitor.

I wonder what kind of capacitor you want to use. A goldcap can store a large charge, but it is only for 2.5V. Some supercaps are fake.
Perhaps it is best to use about 10 normal capacitors to start with, for example 10.000uF of 50V or 63V. Ten of those make 0.1F and costs as total between 30 and 50 dollars.

Thank you, very detailed and informative reply, if i knew how to give you karma i would!

As to why we are using the LED driver, not too sure but this and the solar panel are the two things we are given and have to work with.

Thanks for pointing out that i dont have to use PWM, should make things a little simpler.
What do you mean super caps are 'fake'? if i order from a reputable source surely the components should behave as advertised?
I will consider using multiple smaller caps as per your advice but just curious as to what you mean.

On to my next question, powering the arduino board; how could i pull 5v off the converter output without affecting how the cap charges?

Believe it or not im actually studying to be an electrical engineer, as you can probably tell my knowledge is a little lacking, again thank you very much for your help.

I can't find it, but I remember that some 12V super caps for cars are nothing else than normal capacitors in a fancy wrap.

Ofcourse, there are reliable and good super and ultra caps.

If you have for example an Arduino Uno board, you can apply about 7.5V to the DC plug.
I think it is best to have a seperate DC-DC converter for the 7.5V.
If you use a diode from the solar panel and a diode from the capacitor and both to the DC-DC converter, that DC-DC converter is powered from which of the two has the highest voltage.

The solar panel is like a current source. If you like something dangerous, it might be possible to charge the capacitors directly from the solar panel. Perhaps with only a single mosfet in between to switch it on and off.

Hi thanks again for your reply, although charging the cap directly is a nice idea (definately will at least explore the possibilty) im assuming we have to use the dc/dc converter.

As to what else you mentioned im not quite sure what you mean? the arduino uno can be powered by up 7.5volts rather than the recommended 5v? and if take that 5v from the dc/dc converter isnt that taking away energy from the cap charging?

The Arduino Uno can be powered with:

  • USB
  • DC jack, recommended is 7.5V (7 to 12V)
  • Vin, also recommended 7.5V
  • 5V to the Arduino 5V output pin. This is not recommended, since the voltage regulator might be blown due to reverse current, and there is no protection for higher voltages.

If you charge the capacitors during daytime and keep them charged during the night, the power for the Arduino will discharge the capacitor some.
But you have to power the Arduino Uno somehow, so I thought using the solar voltage and the capacitor voltage and a seperate DC-DC converter just for the Arduino. Or do you power the Arduino Uno with its own power supply or battery ?