2AA Batteries and one supercap cell

[foxOnTheRun]

Hi to all, let's start simple: what should I do if I wanted to play with a couple of AA batteries 1,5v and 1 or 2 supercap cell (2.7v each) to power up a small LED?

I'd like to build a small demo board where, given a programmable device to implement some logic (example like a mid range PIC like 16F690, 16F684) I can set the % or the balance of power flowing to the LED from these 2 sources (or a motor if that suit better as example) - I'd like to balance the power absorption from these two reservoir. Example, if I would attach a trimmer to the PIC, I could set the flow like:

_ 100% from battery
_ 20% from battery, 80% from supercap
_ 100% from supercap
.. and so on

As sidenote, I can stabilize the working DC level to 5v it via a simple switching circuit like max756, so 2AA and 1 supercap cell would be stepped up to 5v independently (or down, depending on power sources voltage, or even a SEPIC..but..). But then I'm missing the building step to implement the power flow control :(

I feel I really miss some building blocks in electronics, one in particular being the capability to actually ""control"" power flows (double brackets necessary..).

I've posted this simple example, not to be solved by some cheap tricks (diodes, limiting resistors, relays..), rather to really get the point: how do you treat paraller power sources? adding more, at which level do you stabilize, regulate and control power sources?

I was thinking of having just one switching regulator, then I could feed it with the supercap cell and 2AA.. but the problem is still the same, I've only shifted the place where to balance the flow of power.. what's the keypoint?

Hope I explained the problem clearly, have you got any ideas?
Tnx.

[Hero999]

There's no simple way to control the power comming into a circuit in the way you've described. If you use diodes, the power supply with the highest voltage will supply all the current. You need an SMPS and controling power is not easy since P = VI and would require an analogue multiplier, controlling the current is easier.

Just one question: why?

A super capacitor hardly stores any energy compared to a couple to AA batteries, for example a 10F capacitor charged to 3V stores 45J and a couple of rechargable AA cells can store 21.6kJ which is 480 times the amount of energy stored in the capacitor.

[foxOnTheRun]

There's no simple way to control the power coming into a circuit in the way you've described...

Just one question: why?

All right, why is simple, I want to build a small demo board for a school project and I wanted to investigate in such kind f control - I found barely nothing on books.

A super capacitor hardly stores any energy compared to a couple to AA batteries, for example a 10F capacitor charged to 3V stores 45J and a couple of rechargeable AA cells can store 21.6kJ which is 480 times the amount of energy stored in the capacitor.

Right! same numbers I got:

_ in the first test I can also drop the supercap requirement - my objective is find a control schema to control the currents flowing outside my single elements

_ in a second time, I could put the supercap back and programming a little I could play with some nice control schema like, keeping the current from the battery stable/low and draw from the supercap during peaks in power consumption

But, before going on telling wonders of supercaps and so on, from where I could start looking for control methods (circuit) for such currents ?

II was thinking of playing with the resistor output network found in many switching regulator, if I can cheat em and force their output voltage to change.. but I think it's not much feasible the way they tell you in datasheet to not parallelize multiple unit.. (traco, densei-lambda). Any suggestion? tnx :)