Hmm.. I know these should have a current limiting resistor, but I don't have any datasheet for the leds used on the pcb. To be honest all I did was connecting these to my power supply, and slowly cranked up the voltage until I saw light, then one pcb seemed to run great at 15v... But how do I calculate a resistor when all I really know about them is that they draw 75mA from my power supply when supplied with 15V? 
My power supply does output full voltage when the load is connected. I adjust my power supply to 15V, connect the load and it still says 15V. My meter is a Fluke 179.
I brought these led pcbs at Ebay, the seller doesn't really list any useful specifications, it says "Voltage 110-220V" which I find a little weird, but then again, I haven't even been able to calculate the resistor.. I clearly missed something here. I'm glad for all the help I receive
I appreciate your time.
Best regards
Joe