I have a LED PCB attached to a water cooling block, and I want to use several in-series strings within a parallel circuit. There are several kinds of LEDs of different forward voltages that I would like to connect in series this way. Can I do this? As long as each string has the same total Vf? I assume thermal runaway is no issue since water cooling is used? There are about 30 in-series strings

 

It's not the Voltage across the led but it's Current, most LEDs do have a VF, say 2.8 to 3.6v , it's better to drive them with Constant Current then they will be the same brightness, so obviously more LEDs in series means a larger supply voltage.

If you want to put all Leds in Parallel with each series resistor, then you have to get the correct resistor value for each led.

 

hi mart,
You can connect LED's of different Vforward in series, but consider that the same current will be flowing thru each LED.
Add a series resistor to each series LED string, in order to limit the current thru the LED string.
E

 

I would say that the number and type of LEDs in each string should be identical, not just the forward voltage. If two strings have the same nominal forward voltage, but different numbers of LEDs then the change of forward voltage with temperature will not track.