Led amplifier help

[si1986]

Hi everyone, I am in need of a little help please.

Im looking to build a low res LED screen for a stage set up that is powered by these http://www.usledsupply.com/shop/rgb-32-spi-dmx-decoder.html

The problem I have is that this board only allows for 300mA output per RGB pixel (100mA per led) and this really isnt enough as I need each pixel to be fairly bright maybe 30 or 60 LEDs (10 or 20 R,G & B)

Is there a simple way I can use the output from the above decoder to control a simple MOSFET circuit to amplify the output. I have in mind something similar to this http://picprojects.org.uk/projects/bigmosfetrgb but I only need the MOSFET power part of the circuit. I would obviously have a MOSFET curcuit for each of the 96 outputs.

Am I correct in thinking I could build a curcuit for each output that has its own power supply that powers the LEDs through a MOSFET and the output from the decoder controls the mosfet (as such)

A little help would be great as you can probably tell.

Many thanks
Simon

 

[OLIVE2222]

Hi Simon,

The Mosfet idea should work to drive your large leds pixels. The most efficient solution is to build pixels clusters with leds connected in serie then to parallelize them. This will limit the amount of current needed. So the higher the voltage the better. Laptops power supplies are great for this job with around 19V and 4.5Amps, they are also cheap and widely avaiable (ebay).

Olivier

 

[si1986]

Hi Simon,

The Mosfet idea should work to drive your large leds pixels. The most efficient solution is to build pixels clusters with leds connected in serie then to parallelize them. This will limit the amount of current needed. So the higher the voltage the better. Laptops power supplies are great for this job with around 19V and 4.5Amps, they are also cheap and widely avaiable (ebay).

Olivier

Laptop charger is a great idea thanks. I was planning to build around a 60 LED RGB board for each rgb output (60 LED RGB pixel)

Any idea how I what MOSFETs I would need?

 

[OLIVE2222]

Simon,

Let's say you use 6 led's in serie with a 19V supply, mean 6*2.5vf = 15volts. R for 12mA = (19-15)/0.012 ~ 330R.
Knowing the 12mA for each cluster you can set maximum 8 cluster in // (48 leds) without additional Mosfet on the decoder board!

Olivier

 

[si1986]

Simon,

Let's say you use 6 led's in serie with a 19V supply, mean 6*2.5vf = 15volts. R for 12mA = (19-15)/0.012 ~ 330R.
Knowing the 12mA for each cluster you can set maximum 8 cluster in // (48 leds) without additional Mosfet on the decoder board!

Olivier

Hi thanks for your reply.

I'm using 20mA 2v LEDs and I have 100mA per channel available (300mA per rgb cluster)

If using a 24v supply I can series 12 LEDs (24v at 20mA) then parallel 5 of them (5 x 20mA = 100mA) that allows 60 LEDs per channel yes? That means 180 LEDs total for each rgb cluster correct?

I won't be doing this obviously as that would be running things far to close to the limits but do I have the sums correct?

 

[OLIVE2222]

Yes Simon the maths are correct but you must use a serie resistor in each branch of serialized led's. So 10 leds and a 200 ohms resistor will be OK. Giving a maximum of 50 leds (and 5 resistors) per channel. Indeed you must appply a margin 2/3 of the max. spec. is a good practice.

Olivier

 

[si1986]

Yes Simon the maths are correct but you must use a serie resistor in each branch of serialized led's. So 10 leds and a 200 ohms resistor will be OK. Giving a maximum of 50 leds (and 5 resistors) per channel. Indeed you must appply a margin 2/3 of the max. spec. is a good practice.

Olivier

Just been thinking the LEDs have a common positive and the intensity is controlled via the neg so surley the 100mA per channel is only down to what the tracks on the pcb are capable of taking? Yes? No?

If I were to connect a positive feed to all the clusters via heavier cable I could use more LEDs on each panel. Or doesn't it work like that?

 

[OLIVE2222]

The amount of current is determined by the driver implemented on the decoder (can't read which one is used from the picture). Sure the track must be wide enought to allow the current flow, but it's not the main limiting part.