Help converting cold light Graflarger to LED

[Ari]

This article is from a forum member who rigged his MF Beseler to handle 4x5; in the process, he had to make a new, and larger, light source out of LEDs.
Instructions on making the head are halfway down the page:
http://www.deadbread.com/crumbs/23c.html

[cardiomac]

Let me add to the excellent advice you have gotten already.

I suggest you use Cree warm white XP-E or the XM-L EasyWhite LED's. You want many, medium brightness LED's, not few really bright LED's because you have limited space to diffuse the light. Get them pre-mounted on a star - they will be much easier to handle. Check out the LuxDrive series. The stars can be screwed or epoxied to the heat sink with thermal epoxy. A heat sink is critically important. LED's don't generate much heat but they are very intolerant of what heat they do generate. You want to keep the LED's less than 75 degrees centigrade. Above that the life span is greatly reduced. Also, LED light output is inversely proportional to temperature, in some cases to a significant degree. The trick is to keep the working temperature stable. This means a large thermal mass and a large heat sink. Prewarming the LED's for a few minutes before making your first exposure is good practice. Use a constant current driver - 700mA is the most common. The voltage varies with how many LED's you are driving. Just make sure the total forward voltage drop of your LED's in series is within the range of the driver you have chosen. Recom makes good drivers. Remember, the majority of the power output of the driver will end up as heat so plan your heat sinks accordingly.

Overall, the best single source for components is DigiKey. Also, ReefLEDLights has good values on heatsinks. Good luck.

[Tim Meisburger]

Thanks guys for all this advice. Truthfully, although I'm a fair carpenter, my knowledge of electricity doesn't extend much beyond wiring switches and outlets. We don't have any radio shack here in Thailand, but I did find little shop selling 1.5w leds and I bought 50. Although they didn't speak much English, I was able to gather that they should be wired in parallel (which makes sense) and that if I added a resistor to each positive pole (the long wire) I would change the voltage required from 3 volts to 12 volts. Why that was preferable I couldn't get, but I went ahead and bought fifty resistors as well. Today I was talking to my landlord, who understands electronics, and he essentially laughed at my difficulty, took my leds, and said he would either put them together himself, or if he could find one would buy me an appropriate panel. So, I'll let you know how that goes. It does seem like this would be a good place to establish a cottage industry making replacement enlarger lights!

Cheers, Tim

[Steve Smith]

Your landlord will know this, but here is a simple calculator for the current limiting resistor: http://led.linear1.org/1led.wiz

White LEDs typically need about 3.6 volts and 20mA would be a good current to start with.

With a 12 volt power supply and 20mA through each LED, you will need a 470 ohm resistor in series with each LED

However, it would be better to wire the LEDs into groups of three in series. This would give a combined forward voltage of about 10.8 volts which would need a resistor of 56 or 68 ohms per three LEDs.

This arrangement is more efficient as there is less voltage across the current limiting resistors which means less heat disipated by the resistors.


Steve.