First off, hello. I'm the latest new guy. I know enough about electronics to only be dangerous. I worked on electronic warfare systems in the USAF for about 20 years but to be honest, the job mainly consisted of swaptronics and there were limiting rules about what maintenance could be performed on these systems. Component replacement was not permitted. Since I retired however, I've successfully built about a dozen vacume tube guitar amplifiers from scratch with point to point wiring and constructed terminal lug circuit boards.

I've been playing around with LEDs. I'm an avid motorcycle rider and am always looking into custom lighting including off road auxiliary lighting. The use of very bright LEDs is still quite new and the cost of the best packaged manufactured components is high. Just like the early CFLs, these new LED lights are very expensive and will remain so until there is more competition in the market. LEDs are perfect for off-road motorcycles due to the very limited available current from the alternator.

Car and motorcycle voltage is 12 to 15VDC so when using LED components, I need to reduce the voltage most of the time unless I configure the LEDs into some sort of multiple LED, in series, idea. The bad thing here is that I'm restricted to using multiple LEDs and if a single component dies, everything dies from either an open circuit or a totally blown circuit if the death of a single component creates a situation of a short -- the over current wipes out everything. That is really bad news if the LED light is the only illumination you have on a motorcycle.

Some LED lighting is becoming affordable with many inexpensive hand held flashlights manufactured in China. My son just purchased a 1000 lumen flashlight for less than $20 on ebay and its performance is awesome considering that it operates on three C-cell batteries. It has a single LED chip installed in a quite decent projector style mirror and lens assembly. The width of the beam is fully adjustable from a wide angle to a very in-focus square of about a 5 degree arc. This flashlight is all aluminum and waterproof. I could easily hack this thing apart and in a very tidy fashion, mount it on a motorcycle.

Here is the question. Is it reasonable to use just a simple power resistor to control the current when the power source is a fairly unfiltered and roughly regulated 12 to 15 VDC output of an alternator output? Should I employ filter caps? Or, is the voltage source probably so uncontrolled that I need to get into an actual complicated semiconductor voltage controller so the longevity of my LEDs is not compromised? "Simple Is Better", is a rule that I'd like to follow. On the other hand, I'm probably wasting my limited available current in a power resistor which defeats the use of an LED. Is there a simple solution of an available voltage control device or circuit? Thoughts???

By the way, I am now a professional luthier and have been so for the last 15 years. If anyone needs assistance with guitar electronics or anything related to stringed instruments, I'd be happy to assist.

 

Well WB, I'm sad to say you've run up against one of the terms of service here with your very first post. Looking for the link...

Here it is. http://www.allaboutcircuits.com/l_tos.html

6. Restricted topics. The following topics are regularly raised however are considered "off-topic" at all times and will result in Your thread being closed without question:

• Any kind of over-unity devices and systems
• Automotive modifications
• Devices designed to electrocute or shock another person
• LEDs to mains
• Phone jammers
• Rail guns and high-energy projectile devices
• Transformer-less power supplies

 

Hi Wrench Bender,

Welcome to the Forum!

Is it reasonable to use just a simple power resistor to control the current when the power source is a fairly unfiltered and roughly regulated 12 to 15 VDC output of an alternator output? [snip] "Simple Is Better", is a rule that I'd like to follow. On the other hand, I'm probably wasting my limited available current in a power resistor which defeats the use of an LED.

For single LED or fixed number of series LEDs, it's okay to use a resistor. In your case, you probably want to run a number in series so you minimize the power wasted in the resistor.

Forward voltage depends on color and current, white LEDs typically have a Vf of 3-5V.

EDIT: I'm new too and forgot about the automotive restriction...

 

You might have to change your question to a more general one regarding driving high power LEDs as automotive stuff is not allowed here; it's not too late to edit and save the thread.

Driving LEDs is a popular topic so you will probably find that your questions have already been answered.

 

Is it reasonable to use just a simple power resistor to control the current when the power source is a fairly unfiltered and roughly regulated 12 to 15 VDC output of an alternator output? Should I employ filter caps? Or, is the voltage source probably so uncontrolled that I need to get into an actual complicated semiconductor voltage controller so the longevity of my LEDs is not compromised? "Simple Is Better", is a rule that I'd like to follow. On the other hand, I'm probably wasting my limited available current in a power resistor which defeats the use of an LED. Is there a simple solution of an available voltage control device or circuit? Thoughts???

The resistor strategy is simple and works but has drawbacks. The resistor must be sized to protect the LED under worst-case conditions. So you might choose for the LED to be at or near maximum current (and brightness) at 16V. That means the LED will be drawing lower current and will be less bright at, say, 12V while the generator isn't running. This may or may not be a problem. And the resistor simply burns off excess voltage and thus wastes that power.

The fancy solution is a constant current controller using a DC-DC converter. It can tolerate a wide range of input conditions while producing a rock steady output. So your LED is at a steady brightness. And these things are more power efficient. They're not so simple, but you can go to E-Bay and find all sorts of them very inexpensively.

 

You can also use a linear electronic constant-current regulator (consisting of a few parts), which maintains an almost constant-current through the LED(s) as the bus voltage varies from 11.5V to 14.5V.

I avoid switch-mode current regulators (and their attendant RFI) like the plague in aircraft and automotive applications that also have a variety of radios installed. Besides, in aircraft and automotive applications, you have an infinite heat sink to bolt the LEDs and regulators to...

 

The last converter I bought delivers 3A at 5V (at constant voltage) and cost something like $4. While in use charging my iPhone, it does produce an audible hiss on the AM radio, with nothing on the FM. I might be reluctant to use it in an airplane, but it's not a problem in the car.

 

To clarify, and if it helps,...
The application is a non-street legal off-road dirt bike not designed to ride on paved roads and prohibited by the DOT from being utilized on public roadways. It is not an automobile or a vehicle subject to any federal or state regulations or typical safety rules applicable to what would be typically considered an automobile.
My apologies to the forum.

 

Well, you've made it this far. Maybe the mods are taking a wait and see.

As Mike pointed out, in between the extremes of a simple resistor and a full-blown DC-DC current controller is a linear solution. Compared to a resistor, it has the advantage of smooth control over the current and brightness of the LEDs. Think of it as a smart resistor. It shares the same disadvantage of all linear solutions - the power loss. But maybe this is not so much of a problem.

I mean, the linear regulator might give you 75% efficiency compared to the 85% you might get with a DC-DC converter. In some applications that boost in efficiency might matter. On your machine, probably not.

 

If you're concerned about voltage fluctuation causing the LEDs to brighten/dim, you can build a simple current source. As drawn, I = 1A/V. Use a rail-to-rail opamp if you need to get closer to battery voltage.

EDIT: Obviously the LED(s) go between the collector and battery...

 

Because available current is very limited I suppose I'll use a DC to DC converter. I've found a 5A unit for about $8. This could also be handy for creating a USB charging/power source for my phone and GPS -- two items that I really need when I'm out in the middle of nowhere. These chargers and power sources have been terribly unreliable from eBay and if it's three hours to a road, it could be a matter of life or death.