12 volt lighting

Re: 12 volt lighting

The short answer is "it depends"...

Basically, LED's are probably the most efficient, followed by florescent and bringing up the rear, standard filament bulbs (halogen bulbs can be very efficient).

The reason "it depends" is because of the circuits that drive the LEDs and Florescent bulbs... "The ballast".

For LED's, the brighter ones need to have a specific current driven into them (like 1/2 amp) instead of 4 volts--so, for a simple 12 volt LED fixture, they may place an LED that requires, roughly, 3.5 volt and a simple resistor to drop the other (12v-3.5v=) 8.5 volts:

V=IR
R=V/I=8.5 volts / 0.5 amps = 17 ohms

P=I*V=V^2/R=I^2R

P=V^2/R=3.5v^2 / 17 ohms = 0.72 watts
P=V^2/R=8.5v^2 / 17 ohms = 4.25 watts

So, for a simple LED fixture--it is possible that 4.25 WATTS is wasted driving a 0.72 watt LED.

For a Florescent Fixture--they require higher voltage (and sometimes, depending on the design an even higher starting voltage), but a constant current (rather than voltage) once they get started.

In the end, whether a 12 volt DC florescent (with a step-up circuit and current limiter) vs a 120 volt AC florescent fixture (with a cheaper ballast circuit) but requiring an inverter--you almost have to measure the 12 volt power for the DC fixture and the AC fixture to see which is more efficient. And weigh the cost issues between a $15 DC CFL vs a $2 AC CFL---and the wiring issues of sending 10 amps of 12 VDC 30' to drive a 120 watts of lighting vs sending 1 amp of 120 VAC 30' to drive 120 watts of lighting...

In the end--if you have a small lighting requirement, small DC powered LED/CFL lamps (checked with a meter for efficient use of power, and as used in your application to make sure the light is sufficient and has the correct color rendering for your needs) will work for the one room cabin...

However, as soon as you need more than a few fixtures, and you have other needs (small TV, charging your cell phone, compter, refrigerator, etc.)--you probably will end up with more panels, batteries, and a true sine wave inverter.

There is a problem with running "cheap" 12 volt appliances (i.e., 12 volt car adapters) on a 12 volt solar RE system with storage batteries.

A car, typically runs between 12 to 14.2 volts (engine off, engine running). A solar RE system can run from 10.5 volts (dead batteries--which you don't want and/or long wire runs) to 15-15.5 volts when equalizing your batteries (especially in very cold weather)--which many car adapters appear to have problems with.

So--you eventually end up with an inverter (and everyone buys a MSW--Modified Square Wave Inverter). And MSW inverters have their own issues (can damage small equipment like cheap battery chargers, overheat motors like refrigerators, or work OK for 80% of your other equipment--who knows for sure which will be OK).

And TSW inverters are 10x more expensive vs an MSW inverter... But, you eventually purchase one of those--because you bought the cabin 50 miles from any store/RFD and can't afford the issues that come with an MSW.

-Bill

Re: 12 volt lighting

Craig,

You are correct--it was supposed to be WATTS wasted--not volts. (Fixed in original post).

You can string LED in series and spread the voltage drop across the string (say 4 diodes * 3 volts = 12 volt drop)...

However, the problem is that LEDs (and diodes) are not linear devices... For example, using the 12 volt example above--when the battery is charging at 15 volts.

If it was linear, then the current through the LEDs would increase 15/12=1.25x... but in reality--the current can go up several times--and overheat / kill the LEDs. Also, LEDs (and diodes) shift their operating point with temperature too.

Now--instead of Resistors or series string of LEDs--There are lots of electronic circuits out there that run in current mode--basically, you "program" the device to only pass 0.72 amps, regardless of the input voltage or the LED's temperature.

Basically, a switch mode power supply setup to regulate current can be better than 90% efficient works well with LEDs. But--either you need to build your own, or find a fixture that does the regulation for you.

There are also companies that sell regulators (and kits) for LEDs (intended for flashlights) if you want to build your own fixtures. And there are off-the-shelf small regulators (and components) too.

There are other ways of limiting current (such as using a series capacitor with LED's at 120 VAC 60Hz)--although--that can have its own issues (such as with MSW inverters causing too much current flow).

One place you can look for information on driving LEDs is called the "Candle Power Forums" (started as a flashlight hobby site)--more than enough there to keep you busy for a few weeks. :roll:

http://www.candlepowerforums.com/vb/
http://www.candlepowerforums.com/vb/forumdisplay.php?f=38 (Sandwich Shop--small regulators)

Otherwise, you are kind of left with trying to find efficient fixtures by asking around and doing your own measurements.

-Bill