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Subject: Re: simple DC light design

Date: 03/18/01 at 11:38 AM
Posted by: Ray
E-mail: rrogers@plaidheron.com
Message Posted:

In Reply to: simple DC light design posted by Paul Williams on 03/14/01 at 11:21 AM:

This is lot of questions and the reply will have to be long.
**********
(I plan on putting the bulbs in series...or would I be better off with a 6V and the bulbs in parallel?).
In parrallel is my definite _feeling_.
Bulbs are rated at definite voltage and the lifetime goes down radically with overvoltage, on the order of the twelth power. Any time I have seen/tried that it has been roughly true. In one case a ceramic heater went from a lifetime of years to weeks.
Unfortunatly thinking about series connection leads to the opposite conclusion. The series connection would self regulate since resistance going down with temperature (typicall) would lead to lower voltage across the hotter bulb. Perhaps somebody else with experience with this problem can give a more definitive answer.
*********************
I would like the lights to come on at dusk and turn off after a set time limit (say, 4 or 5 hours)so I will need a photo cell of some sort AND I need some device to then turn off the lights after the alloted time period. Whew. Sorry it's so long.

No help with the Photocell; I am very discouraged by the development of Solar Cells over the last 15 years. Hopefully the development will pick up in the next decade. For the 4-5 hour timeout I would suggest one of those Single Board Computers (does anybody have experience with the "stamp"?) that run basic. A little expensive but probably the simplest route and the programing would allow detailed control/enhancements. If you get the correct board you could even get Ethernet interfacing built in. I would suggest a Dallas Semi board but the last time I looked it wasn't plug and go like a basic board would be. The stamp modules are at:
http://www.parallaxinc.com/html_files/products/stamp_modules.asp
I have not used them but they have endured for a couple of years.
**************************************
How do they rate batteries per their output? I see 600mAh, etc, how does this factor into the equation? I assume using the bulbs I have in mind, they will consume approx. 18 Watts each (4.8V x .75A x 5Hours=18Watts)correct?
As I recall 600maH means you can pull 60ma for 10hours, and you would have to replenish that. Read the battery sheets resonably carefully. My last go round I pick up a standard lead acid battery from Allied but you could try
www.digikey.com
search on batteries. If the total current is to be 1.5A for 5 hours then you need at least a
7,500maH (7.5AH) battery.
P075-ND at digikey gives 10AH.
For charging I would suggest a Maxim or Linear Technology, although with Lead Acid you can be pretty sloppy in the charging circuit (essentially a diode) but I think you would prefer some more precision.
BTW: The total power is Watt-Hours (or Joules). Watts is the Energy/sec units.
You must get Solar Cells that will replenish the Amp-Hours during your typicall/worst-case daylight. There are solar calculators on the net somewhere that will give you typicall solar power days for a given latitude/location.
************************
In short: Use a Single board computer/module to get timing/control (watch the power during selection). Use a IRF540 to drive the lights in Parrallel. Choose a 6v, >7.5AH battery. Select a solar panel based upon your location and the requirement to supply the >7.5AH during you daylight hours.
*********************
Disclaimer: This advise is not warrentied to be of use to anyone, anytime, for anything. In other words the "inclusive or" of all Warrenty exceptions given by software manufactures.
I love to read those disclaimers:) Not usable for any purpose and furthermore you have no rights with respect to it's use. Microsoft's "ability to innovate" is the freedom to dump any collection of bugs on the users, disclaim responsibilty, control use, collect money, and also to squelch anybody who disagrees with them.
You have to excuse me, I have just done a VBA program for Excel and spent 10% of the time on my problem and 90% of my time experimentally determining what MS was doing with my code.
:):):):):):)

Please let me know if I have been to sketchy (or possibly wrong? naw... ) in my advice.


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