Gut check - wiring LEDs @ 4.5v from 12v battery

[bzb]

I got some rope lights that run off 3 AA batteries (4.5V). To step down from the 12V battery, I just wire up a 360ohm resistor on the positive wire, right? What wattage would it need to be?

[Bill Fitzmaurice]

I got some rope lights that run off 3 AA batteries (4.5V). To step down from the 12V battery, I just wire up a 360ohm resistor on the positive wire, right?

I don't know, what's the resistance of the rope lights?

[RubiconProSound]

You could always just use a simple 3 pin "LM series" voltage regulator... no matter what voltage you put into it you'll always get the desired output voltage (within reason of course) and they are super easy to use.




I use them all the time for stepping voltage down on 12 volt car projects. There are fixed voltage varieties like the example above or you could always go with the adjustable version...

http://www.radioshack.com/product/index ... =107596962

I've soldered the wires directly to the legs and used heat shrink, no PC board required. If you're not handy with a soldering iron you could always buy the ready made version that plugs into a cigarette lighter outlet.

http://www.radioshack.com/product/index ... PerPage=60

[bzb]

Bill - no idea on that one. Can I get a "close enough" reading from a multimeter?

Rubicon - looks like that may be the thing I need! Thanks!

[Rune Bivrin]

Just as long as you remember that the 7805 will need some kind of cooling if the current is large enough. If you're going from 15V to 5V that's a 10V voltage drop. At 100mA of current you're looking at 1W of dissipated power. While that's not terribly much, it will still heat the regulator too much unless you affix it to some kind of heat sink. A 2" square piece of aluminium will probably suffice.

You should start by measuring how much current the light rope uses at 4.5V.

[cheapbasslovin]

Couldn't you just run a voltage dividing circuit across the battery? 800ohms in series with 400 ohms across the battery (or values similar to that), but then run the LED's across the 400 ohm resistor? If I'm as smart as I think that should put 4 volts on the rope (unless the rope itself has a very low resistance).

[RubiconProSound]

Just as long as you remember that the 7805 will need some kind of cooling if the current is large enough. If you're going from 15V to 5V that's a 10V voltage drop. At 100mA of current you're looking at 1W of dissipated power. While that's not terribly much, it will still heat the regulator too much unless you affix it to some kind of heat sink. A 2" square piece of aluminium will probably suffice.

You should start by measuring how much current the light rope uses at 4.5V.

He mentioned they were LED lights so I figured the current draw would be low...

But yes,
Rune is right, and Radio Shack even has the small heat sink if the current draw heats the regulator too much.

http://www.radioshack.com/product/index ... Id=2102856