hey techies, a little help here, please

[btcruzer]

I've found some LED light fixtures at Harbor Freight that my wife likes. They provide plenty of light where we need it and look like they belong there. They operate on 6 AA batteries but are actually 2 banks of three cells to provide 4.5 volts. I've installed 2 of these in one area and rewired them to connect to a step down adjustable power supply module that reduces the "house" 12volts to the required 4.5 volts. Problem is --- I only ordered one voltage reducer and now the wife wants more lights. On the other side of the room. I'm thinking a resistor could be used to drop the voltage. BUT what value of resistor? I'm assuming the step down adjustable power supply module that I used will keep the output voltage at the 4.5 volts even if the input varies from 12 volts to 14.2 volts. I figure that if a resistor is used, that the supply voltage should be considered as 14.2 when figuring he resistor value to obtain the desired 4.5 volts. But would that then make the lights too dim when on batteries only?

[JimBollman]

btcruzer wrote:

Anyway, now I have the fixture and the second power supply ready to go but encountered another question. I had planned to wire the power supply to the circuit that supplies the range hood fan/light. That circuit is "hot" any time the battery is connected or the converter is on. That means the power supply is "hot" almost always. What kind of problems might this cause other than just a very small constant current draw? We boondock a bunch and need as much battery conservation as possible. I'm starting to think that two out of three ain't bad and that she could cook with a flashlight.

Since you are in there rebuilding the fixture anyway move the switch to the 12 volt side of the regulator.

[JiminDenver]

I can't think of anything outside of the battery drain other than the unit may wear out faster.

[Clay_L]

btcruzer wrote:
SNIP The converter is an LM2596 --is it a "switching regulator"?

Yes it is.
LM2596 3.0 A, Step-Down Switching Regulator per ON Semiconductor Data Sheet

[btcruzer]

JiminDenver - You're right. I have practically rebuilt every other fixture - just had enough of that experience and wanting a simple solution. In case the things throws craps, just slip them out of the mounting and slap in a new one. Guess I'd better visit HF for some backups, before these disappear from inventory. Isn't it amazing how you can find something neat then go back for more and never find the same item again?

Anyway, now I have the fixture and the second power supply ready to go but encountered another question. I had planned to wire the power supply to the circuit that supplies the range hood fan/light. That circuit is "hot" any time the battery is connected or the converter is on. That means the power supply is "hot" almost always. What kind of problems might this cause other than just a very small constant current draw? We boondock a bunch and need as much battery conservation as possible. I'm starting to think that two out of three ain't bad and that she could cook with a flashlight.

[JiminDenver]

You could always take the guts out and replace them with 12v LEDs and then wire them into the trailers system. She would get the fixtures she likes and you wouldn't have to worry about the voltage.

[btcruzer]

Thanks, guys, - as it turns out, I had ordered a second step down converter and then promptly forgotten I had it. Found it in my E-bay purchased list last night when preparing to order another. And actually found it here at the house. So, all might be good. The converter is an LM2596 --is it a "switching regulator"?
here are the specs:
Module property:Non-isolation buck
Rectification mode:non-Synchronous rectifier
Input voltage:4V-40V
Output voltage:1.3V-35V
Output current:Rated current is 2A,maximum 3A(Additional heatsink is required)
Output ripple:30mV( maximum )
Load regulation:±0.5%
Voltage regulation:±2.5%
Work temperature:-40 ? - +85 ?
dimension:43mm*21mm*14mm(L*W*G)
NOTE:This can't be used to step up Voltage

I have one of these in use now and haven't found any problems - what kind of haoc might I expect?

on the other ideas-- I've used 12 volt LEDS all around the trailer, about 20 of them and all work fine. These battery powered things are exactly what she wanted in 3 particular places EXCEPT for being battery powered. The wiring in series idea won't because each unit is switchable on/off and wouldn't all be used at the same time.

[WyoTraveler]

Do you have a multimeter? If the led light is 4.5 volts hook it up to 3 flashlight batteries and measure the current. Divide that current into 12.6 volts to give you resistance value. However, using 4.5 volt leds will not be efficient because 8.1 volts times the current will be power dissapated across the resistor and doing nothing but generating heat. Putting 2 of the leds in series plus the resistor would help or try 3 leds in series but using 12 volt led would be a better idea. Check out 1000bulbs.com. They have fairly good prices on leds.

[RVUSA]

you cant beat these for price...

clicky

If the current draw is low you could get away with using just a 7805 voltage regulator, but you need to make sure it doesnt get any hotter than warm when powering the lights. Otherwise the link is a decent design for next to nothing in cost.

Edit: One other point, dont use a switching regulator. They cause too much havoc with the rest of the rv's stuff unless they are properly filtered.