I am attempting to make an LED flashlight utilizing 23-28 LEDs. The info on the LEDs is as follows: 25mA, 3.2-3.6v, 5mm, through hole. I am powering them with 4 c batteries in series for 6 volts and roughly 7800mAh. To limit current I am using 100 ohm, 1/4W resistors. Here is my issue:

The LEDs are to be mounted on a round circuit board(copper pads on one side, plastic on the other). However, since each LED requires a resistor it is near impossible to solder and connect the LEDs and resistors as required on the same round board because things are crammed in so tightly.

My questions: Would it be possible to have the resistors on a different board entirely? If so, would the extra wire needed to wire them together draw too much current(or would that be voltage?)? Also, if I were to use an LED driver, how many of these LEDs can be connected to one driver? By using an LED driver would I still need to use resistors(if so than using the driver would be useless for this application, wouldn't it?) Also, I have read the LED tutorial and was unable to glean much from it, so please don't post that link as an answer because it doesn't really answer my questions. Any help would be much appreciated!
Sincerely,

SBH

 

You can mount the resistors anywhere you have room. The extra wire length is no problem for such low currents and you can use small wire.

To save space you can mount the resistors vertically in "cordwood" fashion.

The problem with drivers is that you can't operate LEDs in parallel so you would need one driver for each LED.

 

Have you thought about using some more modern high power LED's (say 350mA - 700mA variety)? You could just use one or a few to get a really awesome flash light.

You can put the resistors on another board no problem. The loss for short 'chassis' style wiring would be hard to measure. This site suggests you could use 40 guage wire (which would be so small you'd have trouble working with it). http://www.powerstream.com/Wire_Size.htm

I don't think a current regulated LED driver would help you since the LED's need to be in series for a 'buck style driver' to work. Which would require voltage in the 80V range. Maybe you can find a boost style driver that could work but I'm not familiar with any.

Check out LEDSupply.com for some ideas on flashlights with modern high power LED's and drivers. They've got some cool stuff.

 

A much better approach is a classic buck puck circuit. It will power the LEDs (or 1 3W LED) with much greater efficiency, and when the batteries are fully discharged will die quickly, as opposed to a slow and gradual death.

I had something like that on this project...

High Power LED Flasher

 

The info on the LEDs is as follows: 25mA, 3.2-3.6v,

Have you tried putting two LEDs in series, first with a low ohms resistor, then maybe without any resistor? They may not light, but on the other hand they just might. You might also try 5 C cells in series with 2 LEDs. Again you'll have to be careful in your tests to not fry your LEDs, but you might arrive at a combination that doesn't require resistors, just like the LED flashlights you see at the hardware store.

Otherwise, I'm with Bill that a buck puck would be more effective than a pile of resistors. You'll have more consistent brightness during the life of the battery, more light for a given amount of battery capacity (higher efficiency) and the battery will be fully spent when it's done.

 

One thing beginners tend to ignore while building projects in confined spaces, is to always use stranded wire instead of single-core wire. It bends much more easily, whereas the single-core wire, in large numbers, might refuse to bend at all.

Yes, I have done that too.

 

Using a resistor for each LED is hideously inefficient, has a lot of bulk and will involve a lot of wiring too. You really need a circuit with an inductor, that buck-puck thing or its equivalent. If you're running off batteries, you have to conserve power, not throw it away.