Needing help with RGB LED chip PSU

[musht]

How should I calculate that?

 

http://www.reuk.co.uk/LM317-Current-Calculator.htm

[Roderick]

Time for an update. And thank you to all that have commented, I am learning a lot, very quickly!

 

Couple of things: I am limited to 24VDC because of the DMX decoder. (Tried 30VDC and blew up one decoder...)

I can't have different drivers for different colours because of the same.

 

First observation: Parallel vs Series.

24V to DMX controller.

Two chips in series: 23V over two chips, 11.5V over one chip

This is way below the required voltages.

 

Single chip:

Red - 13.4V LM317 @ 350mA

Grn - 18.5V LM317 @350mA

Blue - 23.1V - no current limiter

 

Two chips in parallel:

Red - 12.4V LM317 @ 350mA

Grn - 16.9V LM317 @ 350mA

Blue - 18.0V LM317 @ 350mA

 

Just to get maximum light output, could I reduce the LM317 control resistor to increase the voltage over the parallel chips?

I could wire each chip individually but from a cabling point of view I'd rather not do that.

[timsabre]

There's not much point parallelling LEDs unless you are going to increase the drive current. You won't get a lot more light out but it costs more money.

 

The problem with parallel LEDs, with a single LM317 driving both, is that you're relying on the 2 chips having exactly the same forward voltage. If they don't then one will take a lot more of the current (putting it over its rating) and one will take a lot less.

 

Fixture manufacturers pay a lot of extra money for "binned" LEDs which the manufacturer has tested and sorted to have matching forward voltages, but you may well get a random selection, so it's not a good idea to parallel them if you're thinking of increasing the drive current above 350mA. You need a separate LM317 for each parallel chip, this is why manufacturers bump up the supply voltage and put chips in series.

 

Couldn't you put a regulator on the DMX decoder to allow use of a higher PSU voltage?

[alistermorton]

Sounds like the problem is that that the DMX decoding circuitry is powered from the same rail that supplies the LEDs. If the decoder was powered separately (or seprately regualted) then increasing the LED drive voltage wouldn't be a problem - driving a 24V (or higher) load from a 5V logic level isn't exactly uncharted territory.

[Tomo]

LEDs have two types of driver - Constant Voltage (CV) and Constant Current (CC).

 

Constant Voltage drivers are simple and mostly work like you'd expect.

You can connect up the LEDs in parallel, series or both.

Differences lead to individual diodes being a bit brighter. If any LEDs fail then only the affected series line goes out.

 

Constant Current should only ever be used in Series.

 

A CC driver pushes a certain number of mA out, regardless of the load. (As long as the supply voltage is sufficient).

 

With LEDs in parallel, if any fail the CC driver will push all the current through the remainder - rapidly destroying them all.

 

Additionally, manufacturing tolerances mean some of the LEDs will use less current than others.

If you're driving a parallel set at the total rated current, at least one of the LEDs is going to be getting more than the rated current and will die young - and destroy the rest.

 

You're using the LM317 as a Constant Current supply, so you should only connect the LEDs in series.

 

- It so happens that your LED chips are already series sets.