energy harvesting

[davecampbell]

new and learning about electronics:
application: have a micro generator capable of producing from 0 - 10 VAC

problem: trying to harvest energy to trickle charge 4 AAA NiCad batteries

Have run the Ac through a Schottky diode bridge, then into a 4 farad ultracap, then onto a 8 VDC boost chip, into the Nicad pack.

Need to develop a circuit that compares the charge on the ultracap, to the output of the generator, when equal, connect to the boost regulator, as well as the Nicad pack. When voltage drops to 1.0 VDC dissconnect the regulator, and the Nicad pack. Lastly go back to compare mode and wait for the ultracap to recharge back to the input voltage. Dual N-mosfets ?

Any help, or direction is greatly appreciated

 

[stevez]

Have you estimated the amount of energy you might harvest? How does that compare to the requirements for charging the batteries?

 

[audioguru]

My solar garden lights use a small 2V solar panel to charge one AA Ni-Cad cell.
The charging current is low and the load current from the voltage step-up circuit that drives an RGB fading LED is also low.

What will drive your generator, a hamster wheel?

 

[davecampbell]

charging circuit

First off, yes the generator is mounted to a hamster wheel, trying to make the little guy pay for his food, by charging my daughters night light.

the generator produces around10 - 25 ma, and the little guy runs for around 4 hours per night.

problem is that he runs at variable speed, so I need to monitor the generator ouput to the capacitor charge, when equal discharge, to the boost regulator, and then Nicad pack

the other issue, is the charging circuit, and Nicad lose power if left connected, and at only 10 - 25 ma, easy to lose all, the little guy has generated.

 

[audioguru]

The math does not add up.
A single wide angle LED is not very bright at 25mA. A white one is 3.3V.
You need it to light for 8 hours so 200mAh is needed.

The hamster generator produces an average of 15mA which is not enough to light the LED plus charge the battery cell.

You need an army of hamsters.
Two light the LED and more of them charge the battery.

 

[davecampbell]

not running the light continuously, when batteries get full, whenever that may be, will turn them on.
only concerned with harvesting the energy, and trickle charging the batteries, just need to monitor generator to capacitor voltage, and have a DPST mosfet type switch to engage and disengagethe regulator, and Nicad pack when the cap reaches full

 

[ronsimpson]

What micro generator are you using? I have a project much like yours. I need to save power whenever it happens and use it at night.

 

[audioguru]

I have a night light in my hallway that uses 36 Luxeon SuperFlux LEDs each at 53mA. It is too bright to look at but lights the hallway pretty well. It gets pretty hot.
It is powered from a 12V/500mA adapter operating at about 430mA.

I have another night light in my bedroom with 16 Luxeon SuperFlux LEDs at 53mA each. My wife says, "It is too bright, turn it off".

 

[duffy]

First off, yes the generator is mounted to a hamster wheel,

AWESOME! Got pics? Ever see this video?

YouTube - My hamster can not manage to stop spinning in his wheel

 

[davecampbell]

built the generator myself single phase, with neo magnet

 

[Roff]

Have you tried running the hamster under load? I suspect he won't run as fast or as long when he's generating a few tens of milliwatts.

 

[skyrock]

Have you tried running the hamster under load? I suspect he won't run as fast or as long when he's generating a few tens of milliwatts.

maybe bigger wheel can help?

 

[Leftyretro]

What does the super cap brings what to the table? The battery will charge just as well with the pulsating DC from the bridge rectifier. Also current cannot flow 'backwards' from the battery to the generator because of the rectifier. So the only thing one might want to add is a battery overcharge detector that disconnects the battery from the charger after obtaining a full charge.

Lefty

 

[duffy]

Yeah, if you tax the hamster too much, he probably won't like to run in the wheel. Hmm... this leads to an interesting question about the source impedance of a hamster...

 

[Roff]

maybe bigger wheel can help?

You might get a better impedance match between the hamster and the load. Other than that, power out<power in.

 

[confounded]

I have an idea, i've no idea if you could do it, hopefully the brains on here could help.
What if you seperate the hamster from the light.
The battery powers the light when needed (night)
The hamsters rotation creates a different potential energy e.g raises water.
In the morning the water raised (i guess youd have to manually release water?) is released, powering a turbine which charges the battery. The turbine could produce a higher current for short duration. This has advantage that hamster can pedal at any speed, and doesnt have to simultanously power light and charge battery.
Are the maths possible?

 

[davecampbell]

OK, have a generator able to produce 0- 10 VAC, rectified it to DC. Am charging a 4 pack of AAA Nicads, needing approx 7.2 VDC, at whatever milliamp.

Have a Buck/Boost regulator chip set at 8VDC. Chip faults on and off, is common. Super capacitor constantly accepts the charge, even when chip is in fault. Supercap is also removing ripples and variations in generator voltage.

Problem is even though chip is in fault still pulling power, and also getting some reverse pull from the nicad pack.

Want to design a window comparator, attached to 2 N-type mosfet switches, that will disconnect the generator/supercap, from the boost chip and battery pack. lower limit will be set for 1.0 VDC, and upper limit is current output of the generator.

 

[audioguru]

Did you measure the output voltage and current from the generator?
Is it a few mA at a few volts to light an LED?
Will it take weeks of generating to charge a battery that can light an LED for a couple of hours?

 

[duffy]

Still worth it! Well worth it, even for a few minutes of hamster light a week.

And there's another valuable goal in this project - the output impedance of a hamster.

 

[MrAl]

Yeah, if you tax the hamster too much, he probably won't like to run in the wheel. Hmm... this leads to an interesting question about the source impedance of a hamster...

I thought it was a widely known fact that one hamster has a source
impedance of 1 hamps. <chuckle>

On the more serious side, i noticed that in this thread it was stated
by the OP that the generator puts out 10 to 25ma, and the animal
runs for up to 4 hours a night.
Well, given the max of 25ma and 4 hours that totals up to 100maH
(one hundred milliampere hours) and so in order to get 8 hours of light
from a small white LED we can only drive the LED at a current of
100/8/1.4 milliamps, which comes out to almost 9ma. This includes
inefficiencies of NiCd cells, but a super cap would do better.

As far as switching on and off when the little guy runs, a single diode
might just be enough to handle this task using a dc generator.
The diode will only allow current flow from the generator to the
super cap or NiCd cells, with only a tiny loss.
The diode would connect with its anode to the generator plus (+)
output, and its cathode to the NiCd (+) terminal. Generator ground
to the negative of the NiCd cells.

There is some chance he might actually like having to try a little harder
to run in the wheel once the generator is connected. It may take
him some time to adjust however, and he might actually require more
food intake in order to put out the increased energy.
Should be interesting.

Once you get this going it is required that you post videos so we can
all see him run and make light