Efficiently dimming an LED light - confused!

Hi,

I have put together a lighting setup for my bike using some scavenged parts - RC car 6-cell NiMH battery into a dynamo LED headlight. (I thought it was a bit of an off-chance but to my surprise, it works a treat.)

However, I don't get the battery life I would like. The light is also unnecessarily bright, so my obvious thought is to dim it for the times when I'm not heading down hill, and then open it up for the short blasts.

It seems that the inefficiency of a variable resistor would make this a bad option. There's not much point turning the light into heat! However, I don't have the technical nous to proceed beyond this. It seems to me that a high-efficiency variable voltage regulator for, say, 0-12v must be a really common application, but I don't know what it is I'm trying to search for! The limit of my electronics is basic soldering and multimeter use, plus some half-remembered school physics, I'm afraid...

Any pointers gratefully received!

You really need a LED driver. Taskled are an oft recommended supplier; the h6Flex would allow you several brightness settings but needs 7v minimum to operate. You don't say what your battery produces. Other LED drivers are available. I'd be concerned that you're overdriving the LED in the headlight and it will die quite soon unless you limit the current going to it.

Nick

Dynamo lights unless for the B & M 12 volt system, are usually a nominal 6 volt with a maximum current of about 0.5 A. They are not renowned for being so bright. The light is not likely designed to handle the extra heat of extra power so not only are you likely overdriving the LED(s) but frying it/them with exces heat, too. LED's are current not voltage devices so your sense of a varistor not being the ticket is correct. You could post pics so the LEd(s) might be identified.

I'm going to be a little heretical here, and talk about Voltages. It is true the ideal LED driver is a current source. On the other hand most LEDs come with a typical Vf (forward voltage) and Vfmax (Maximum forward voltage). If I remember my old RC car, that's just 6xAA, yes? If so, then you're running at 9v which is probably a little tasty for something not expecting much more than 6v.

Again, standby for the heresy police to shoot me down, but there's a device called a LM317. It's a 3-pin package and can be used as a current or voltage regulator. Either way would do to be fair but the voltage regulator part is probably where I'd be just to make sure the voltage didn't get out of hand. Example at this link: http://www.maplin.co.uk/lm317-3-terminal-adjustable-voltage-regulators-8067.

Also, as it's a dynamo light set, it'll probably work quite cheerfully down to 2 or 3v so if you get the right resistor combinations, you could try several voltages out and go with the lowest one you can get away with. It'll make your batteries last longer. A prototyping board doesn't cost much either and is well worth it.

Some of the current LED dynamo lights have some sort of regulator circuit within them. Without knowing the specifics of the internal regulator, it's risky to connect it to anything other than a dynamo. If the manufacturer gives any recommendations, I'd recommend following them.

If the dynamo light is simply a rectifier and LED(s), then a current regulator would be ideal. A voltage regulator is better than just hooking the battery directly to the light, but considering how easy it is to put together a current regulator, I don't see any reason to use a voltage regulator.

For any regulator selection, you'll want to know how much voltage is required to power the light at the minimum desired level, how low the battery voltage will go, and ensuring that you select a regulator that can work with these voltage limitations. For instance, if the light needs 5v to operate and the battery can go down to 6v (typical for a 6 cell battery), then you'll need a regulator that can work with only a 1 volt difference between the input and output. This would eliminate popular options like the LM317 as well as many popular switching regulators.

regards,

Steve K.

With a title like Efficiently dimming an LED light you are definately looking for a switching regulator. A linear regulator like for instance the LM317 pointed out CAN be efficient but it is directly dependant on your input voltage and output voltages being as similar as possible. This is much less important with switching or switch mode regulators.

I would imagine your power source is a sub-c battery pack? You should be getting quite a bit of runtime from it. This is a very general statement but what little I know about those dynamo headlights they are pretty low current devices. Whats the battery pack rated at? Over 2AH, or even 3?