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LED Wiring, Over Amp Burnout? Answered

Hi everyone, hope someone can help?
I just started playing around with some LED lights, trying to make a really bright Flounder light that can last at least 6-8 hours on the supplied battery.
I was told if I used a "step-down", I wouldn't need resistors, (since the step-down does this in an essence), which I haven't, But the step-down keeps burning up?

Here's the set-up I've been playing with...

- 18vdc (power drill) battery. (unknown amps).
- Step-down driver.... input 8vdc - 36vdc, output 1.5vdc - 36vdc
- 3w LED Cree : 
                          ~ Reverse Voltage: 5.0 V
                          ~ DC Forward Voltage: Typical: 3.8V Max: 4.2 VD
                          ~ DC Forward Current: 700mA
                          ~ Luminous Intensity: 170-190lm                                                       
I use a volt meter to tune the Driver to a constant 4vdc to drive the LED. And the driver burns up and then the bulb after about 5 minutes.
I was told I needed to add Ceramic Resistors, between the driver and battery, and between the driver and LED. (But what size?)
I was also told I don't need extra resistors because of the driver? Which is Right?

Or should I just add resistors between the battery and LED and do away with the Driver completely?

Also, what is meant by "Reverse Voltage"?

I have a 6vdc battery with a 4.5 amp capacity. But I want to use the 18vdc battery longer use, time wise. This is why I have the Driver, when 1 battery deletes, hook up the other one.

I'd like to put 3 - 3w (180L) LED bulbs together, and run them off the 6vdc and/or the 18vdc battery so the LED's will last for at least 6-8 hours on 1 charge.
I want to use either the 6vdc and/or the 18vdc battery because they are lightweight.

I could go with 1w LED's so long as I can get the brightness needed to light up a distance of about 12' underwater to see a Flounder, and get 6-8 hours usage.

Any help would be appreciated.



Best Answer 6 years ago

Oh dear, much wrongness I sense in the Force.....

You have to limit the CURRENT in the LED, not the voltage ACROSS it !!!

You've used the wrong driver. YOu need to buy an LED driver, not a variable constant voltage supply. If you are absolutely desparate, and want to waste a bunch of your precious battery resources as heat, then use a circuit like the one I've attached, made from an LM317 regulator and a 1.8 Ohm resistor - a 1W resistor, so you'll automatically waste 1/4 of the power in your system.

Reverse voltage is the MAXIMUM your LED can stand wired backwards.

Going straight to the battery with resistors will lose even more energy.

Your best bet here for light would be to run three LEDs in series off your 18 V battery, with the circuit I've shown. Anything more than 13V should work nicely.

LED driver.JPGLED driver.PNG

Answer 6 years ago

Thank you Rick and Steve for the replies.
Some of the Drivers have a Max amp input, but if it'll take 36v, how would the amp input play in?

I'm still a little confused on the 3w LED bulb. I thought the 700ma of the Forward Current, meant that was the amount of amperage the bulb will draw per hour?

Steve, I appreciate the diagram, but is this the best set-up for longivity of battery consumption? (I don't wanna waste any power).
Also Steve, could you tell me exactly which LM317 to get? I went to ebay, and there are several different types? (maybe an item # I could look at?)

Rick, as far as the 6v battery I have, it shows; 6v, 4.5ah/20hr

I've seen a Flounder light someone made, using 3 - 1w LED bulbs, which uses 3 C-cell batteries and will last 6+ hours. It's pretty bright too. But I haven't found any Real Bright 1w bulbs, so I'll stick to the 3w bulbs.

I'd like to do something like this with the 18v drill battery or even a 12v motorcycle battery with a higher a/h, but using 3 - 3w bulbs and getting 8-10 hours of usage. These are 3w - 180lumens each, with a total of 540lumens, using all 3 together. That should be bright enough. :)

Am I grasping for straws with this?

So I guess what I'm asking now is, what would be the Best set-up using the 3w bulbs, which will give the least amount of power consumption?

Thanks again


Answer 6 years ago

No, amperage is a measurement if you like of flow rate, not volume of flow.

If you want to try it, A "TO220" canned LM317 would be fine.

The BEST setup is a switch mode "buck" LED driver, which will drive three of your LEDs in series.



6 years ago

You need to limit the current through the LEDs usually by using a resistor.

The reverse voltage is the Max voltage that can be applied in the wrong (non lighting) direction.

To calculate the resistor value you need to know the forward voltage and current of the LED your trying to drive.

The apply ohms law or use this online calculator


You can get an idea of how long the LEDS will last on the battery by working out the current used and dividing that into the battery capacity - usually stated in Amper hours.

To really know you need to try it and see. Reality may differ from theory for a number of reasons.

Here in the UK all the high wattage LED bulbs I have seen are intended for mains voltages - your milage may differ but check.

Watts=Amps x Volts so 3 watts at 6 volts = 2 amps. that would appear to give you the potential for 2 hours from your 4.5 amp battery 9assuming you mean 4.5 amper hours.