Hi.
I want to dim 50 24v LED's, which expander can i use for this?
I have used MCP23017 to other projects, but i cn understand this are not good for PWM, sp what to choose.
All the best
You can start by posting an annotated schematic showing exactly how this is wired. I can think of many ways to do this. You can use voltage to dim but that will give you very crappy results. Be sure to show your driver circuits and links to 24V LED data sheets.
Hello FHansen
Take a view to gain knowledge:
I have not wired anything yet. 
An SX1509 board could be another option.
But probably SX1509 and PCA9685 can't drive your LEDs directly.
Post a link to the specs of your LEDs.
We have been looking at GW QSLPS1.EM from OSRAM.
The SX1509 looks to be a good option, we will have a FET to drive the 24v.
Because of the instrument, it would be the best to use the 24v there is all ready present in the instrument.
Link please...
Can you please tell us your electronics, programming, arduino, hardware experience?
Thanks.. Tom.... 
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Hi,
Do you want to dim them all at the same time, or individually?
Tom....
Individually.
It's a kind off a keyboard, where the velocity is going control the dim, of a corresponding key
That is a very very bright LED for a keyboard.
Can I suggest you start with say 6 LEDs, not the ones you are looking at but standard 20mA domed LEDs and a controller and begin to develop your project.
How are you going to detect key press velocity, probably one of the first things to get to work.
Build in stages...
The module linked by @paulpaulson would be ideal for your project, but not just yet.
Tom....
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I have all the midi worked out, and it's working if connect a LED to devboard
I need a direction for the expander.
I have ordered the module that paulpaulson suggested.
Thanks
LEDs are "current driven". You supply the specified current with a constant current or controlled current source and the voltage "magically falls into place". That's the opposite of how most electronics works with a constant or controlled voltage.
...A constant current power supply jumps to its maximum voltage with nothing connected as it tries to push current through infinite resistance. If you short the output it simply supplies current at (nearly) zero volts. (Bad things can happen if you short a regular power supply.)
Here are some LED driver boards. I assume most of them are PWM dimmable.
With regular little LEDs you use a resistor to limit/control current. Although you can do that higher power LEDs, you need a power resistor, it wastes energy as heat, and it's not the normal way of doing it.
High power LEDs also generate heat and you may need a heatsink to keep them from burning-up. They don't run as hot as an incandescent bulb but the silicon chip can't handle as anything close to the heat of a white-hot tungsten filament .
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And being that the chips are small, the power density (W / cm^3) can be quite high.
That is a shame because it won't work with the LEDs you have selected.
Apparently no one here has bothered to read your LED data sheet.
jim-p, I'm ready to use another board, which board will you propose.
It's allmost working for me 
except for the resolution.
I'm driving short 24v LED strips, with this
It's a circuit i found somewhere on the web, and i have used it as a motor driver.
At this point i feel my problem is the resolution.
I need 49 outputs for LED.
Thanks for all your time
Q1 is backwards
D1 is backwards
Make D2 a 15-16V zener
R2 should be between R1 and Q2
You need a current limiting resistor in the base of Q1
Make R1 and R2 4.7K
Use a 680 ohm resistor in the base of Q2
I'm ready to use another board, which board will you propose.
Not sure what board you mean
I mean a expander board, i have ordered Sparkfun TLC5940.
Your solutions are quiet different, not i will love to try them out.
I'm by no means a expert in this.