As a photographer in the post-industrial age of darkroom equipment sometimes you have to take a problem into your own hands. That was the case last weekend when I finally had it with my two Beseler enlarger heads. My condenser head is too weak to do 16×20 prints, and I hate how it picks up dust and scratches. My dichro 45 has a loose connection somewhere which causes it to suddenly stop working, and the fact that it has a built-in vibrating fan on the enlarger head just seems to me like a terrible design choice.
So I did a little Internet searching and found these, 500 lumen EndorStar LEDs! I ordered 10 to be on the safe side. Now something to power them with. Certainly you can just hook up a power supply and a resistor and call it good, but really LEDs want to powered by a constant current source. The circuit I used is simple enough and should keep the LEDs from drawing too much current. I mounted the LEDs with thermal conducting compound to the bottom of an aluminum project box, and attached a heatsink to the opposite side. I needed to build two copies of the above circuit to drive 10 LEDs. The reason is the power supply I chose was a 48V power supply, and each LED has a voltage drop of 9.6V at 700mA, my target current. So 9.6V times 5 is 48V. Of course the drive circuit has some overhead, so my end current won’t quite get to 700mA. One note on choice of power supply. The power supply I ended up using was a Vicor Flatpak switching power supply as it was the easiest to get that was 48V (my second choice was a standard power supply, but was only 12V which would have needed more circuitry). The problem with the Vicor is it takes approx 500ms to reach 48V which doesn’t work for my burst method of enlarger timing. So I had to add an always on AC connection for the power supply, and then another AC connection to a relay to switch the 48V instantaneously.
Once I got the circuit built and the LEDs mounted I used the diffuser collar from my dichro45 mounted to the underside of the project box. The light looked nice and even so I gave it a trial run last night. For a 16×20 print on Ilford MG my exposure was 10 seconds at f16, much shorter than I would prefer! 5600 lumens is equivalent to more than 3 100W light bulbs. Of course that generates a fair amount of heat, my heatsink was hot to touch after 3 minutes of constant on. My choice of two separate driver circuits made this an easy fix, I just put a switch on the second driver circuit so now my enlarger head has a high and low.
Overall a pretty easy project, not my cleanest, but will certainly be heavily used.