Need help with creating switching circuit for solar system

System

Hey all,

I'm trying to create a switching circuit that'll allow me to charge a set of 4 AA batteries with a solar panel rated with Vmax=18.8V, Imax=140A, and Pmax=3.2W. Basically, I'd like my set up to function alot like those solar powered lawn lights that only light when the sun isn't shining. So, when it's daytime, the batteries would charge, and at night they would discharge (and light the LEDs)

Do I need a Buck converter (or voltage regulator) to step down the 18.8V to 12V to feed into my Battery Charger? Also, does anyone know where I can get a circuit schematic that'll detail what I need to do to create the switching circuit? I imagine I need a photoresistor to detect when light is present, however, I'm not sure where to go from there.

I am in my senior year as an EE, so I have a decent understanding of building circuits. Any direction would be greatly appreciated.

System2

Re: Need help with creating switching circuit for solar system

Does your battery charger have a 12 volt input" If so, you could use an LM7812, which will reduce and regulate the higher voltage from the PV's and deliver a regulated 12 volt output, one amp max.
I expect your panel does NOT have an output of 140 amps.
Good luck
Wayne

mike95490

Re: Need help with creating switching circuit for solar system

I would look into a DC-DC converter, in the +90% efficient range, to convert your 15V, to something that can charge the batteries.

Look in the MAXIUM semiconductor website for some DC-DC converters, and IC building blocks for charge control for your batteries


Mfg: http://www.maxim-ic.com/ TI power management http://www.ti.com/

vendors: http://www.digikey.com/

System2

Re: Need help with creating switching circuit for solar system

Thanks guys, I'll look into those regulators/converters.

Oh, i meant milliamps also haha. Also, my battery charger does indeed have a 12 input (via wall wart).

System2

Re: Need help with creating switching circuit for solar system

With this simple settup, if a more efficient and complex circuit (over the 7812) was used, the "wasted" power would not be saved, or used for anything else. However, if one of the purposes in building the circuit is to expand one's knowledge and experience in designing such things, then by all means go for the more complex.
Cheers
Wayne

Roderick

Re: Need help with creating switching circuit for solar system

If you are charging NiMH, you will need a pulse charger to get good results. If you google for the Texas Instruments IC, "bq2004", you will see a sample of what is required. The idea is, even for a AA battery, that you put a 5 or 7-amp pulse of current into the battery, but at a duty cycle that makes the average current low, like maybe 500 mA, or 100 mA. See the data sheet, where they talk about a charging rate, C.

This may all seem irrelevant when you can simply buy an off-the-shelf charger that takes 12V input, but my concern would be that the off-the-shelf charger might demand more average current than the solar panel can supply, in which case, you would have to adjust the charging rate downward, by reducing the duty cycle, not the current during the pulse. You might also need to add a capacitor at the 12V input to help supply the current pulse. As an EE, I'm sure you can take it from there.

If you're charging NiCd, then you might get away with a simpler trickle charge. In fact, you might be able to harvest the circuit from a discarded solar garden light.

niel

Re: Need help with creating switching circuit for solar system

maybe you could rework this schematic: http://www.talkingelectronics.com/projects/SolarLight/SolarLight.html
personally it would be easier to get a 4aa charger not made to plug straight into the wall (but uses a wall wort to convert to other voltage) and feed a regulated voltage to the charger with a regulator ic from a small pv that'll deliver enough current to charge all of the cells. good luck either way.

Robin Gudgel

Re: Need help with creating switching circuit for solar system

Adrian, I am happy to see a senior EE student actually wanting to create something on his own. I have hired many EE graduates at Trace and OutBack that just did not have the knack to design anything. You have the ambition, that sometimes is the most important part. The rest of it can be learned. The charger you want to design can be simple of complicated. Heck, you could even make it an MPPT if you are really interested in this stuff. I must say that depending on how your project goes, it could have far reaching effects come job hunting time, so do it well. By the way, you do not need a photo cell to tell when the sun is shining. You have a big one called a PV array. Use the output voltage or lack of in order to tell daylight from night time.
Keep in touch.
Robin Gudgel
MidNite Solar

crewzer

Re: Need help with creating switching circuit for solar system

I'm trying to create a switching circuit that'll allow me to charge a set of 4 AA batteries with a solar panel rated with Vmax=18.8V, Imax=140A, and Pmax=3.2W. Basically, I'd like my set up to function alot like those solar powered lawn lights that only light when the sun isn't shining. So, when it's daytime, the batteries would charge, and at night they would discharge (and light the LEDs) .. does anyone know where I can get a circuit schematic that'll detail what I need to do to create the switching circuit?

Something like this? http://www.talkingelectronics.com/projects/SolarLight/SolarLight.html

As Robin indicated, the PV module is the light sensor. Larger applications of this basic concept include the Morningstar SunLight controller. This controller, along with a 40 W PV module and a Group 27 size 12 V battery (~105 Ah), is what I use to power and control my ~24 LED night light clusters for my driveway, front walkway, back deck, and shed door.

HTH,
Jim / crewzer

System2

Robin Gudgel wrote: »

Adrian, I am happy to see a senior EE student actually wanting to create something on his own. I have hired many EE graduates at Trace and OutBack that just did not have the knack to design anything. You have the ambition, that sometimes is the most important part. The rest of it can be learned. The charger you want to design can be simple of complicated. Heck, you could even make it an MPPT if you are really interested in this stuff. I must say that depending on how your project goes, it could have far reaching effects come job hunting time, so do it well. By the way, you do not need a photo cell to tell when the sun is shining. You have a big one called a PV array. Use the output voltage or lack of in order to tell daylight from night time.
Keep in touch.
Robin Gudgel
MidNite Solar

---

I have to say Robin, I love your post, and I really mean that. It's wonderful to see the encouragement you are giving.
Wayne

System2

Re: Need help with creating switching circuit for solar system

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=90599




Take a look at this PDF

http://www.harborfreight.com/manuals/90000-90999/90599.pdf

You still need to suppy your own storage battery