Evox_Rider

I have a CCM headlight that uses 4 AAA batteries. I just added a rear LED asembly, that came from a car's spoiler and picked up a 12 volt gell cell to run it.

I figure I will need a reisistor to drop off 6 volts before I can run that headlamp from the battery. Trick is how to figure out what size of resistor...I don't know how much current is flowing thru the lamp's bulb.

Any help appreciated.

wirehead

V = IR

You need to know the current in order to size a resistor. And you need to make sure that a resistor is properly sized for the amount of wattage going through it.

It's not a good idea. What you want is a DC-DC converter. If you use a resistor, it's going to convert half of the energy going into the headlight into heat. A DC-DC converter will convert only a few percent into heat.

twobikes

Using a resistor is inefficient. It turns unneeded current to heat. I would be inclined to get a LM317 variable voltage regulator chip and configure it for a drop to 6 volts. Diagrams on how to add the needed extra components are easy to find on the Internet. There is a formula on the back of the bubble pack that you can factor for the values of the components at the output you need.

wirehead

LM317 is just as bad as a resistor in terms of efficiency.

Pig_Chaser

The only advantage to the 317 is regulation, ie Vout will be 6 volts for any Vin as long as Vin doesn't drop below a certain threshold voltage (typically 2 volts for a linear regulator) but the excess voltage is essentially dissipated as heat.

If you want efficiency a switching regulator is the way to go: nationals LM25574 will get the job done, but they're not the easiest thing in the world for a layperson. If you're interested they have an online calc that will design the circuit for you given your inputs.

I don't know if there are any simple/inexpensive prebuilt dc-dc converts for your application, check google.

So for all the time and effort that resistor just might be the way to go

n4zou

Anything you use to drop the voltage is just going to convert the extra voltage to heat and blow it off in the wind. I would just keep using the AAA batteries. It's better to have the headlight and taillight powered by there own batteries so both do not go out of something happens to a single battery supplying both lights. Have you looked at opening up the LED lamp and possibility modifying it to operate from 6 volts instead of 12 volts? Most just have resistors to drop the vehicle voltage to whatever the LED's require. Some are easy to get at but some other lights are sealed in clear epoxy making it impossible to mod or repair them.

Pig_Chaser

That's true of linear regulators, but not of swicthing regulators. Switching regulators will convert the voltage with typically better than 90% efficiency. IE Power out = Power in * 0.90

They're easy to build for anyone with an electronics background, but like i said, i wouldn't reccommend it for a layperson.

n4zou

Very true and I've built a few but it's way overkill for a simple light, especially when you need reliability. The last thing you want is an unrepairable light in the dark on a bicycle.

Evox_Rider

I will try and measure the current in the lamp system somehow, I have a multimeter (took electronics in college, but that was 21 years ago and never did get a job in it...so am vaguely familiar with certain terms etc and very basic functions of things)