Led amplifier help

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

Simply look at the datasheet of the logic-level Mosfet to see what is its max allowed current and how much cooling it needs.

audioguru said:

Simply look at the datasheet of the logic-level Mosfet to see what is its max allowed current and how much cooling it needs.

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Hi I don't quite understand? I don't really need to know what the allowable current is, Im needing help with designing a simple circuit to take the output from the channels of the decoder and amplifying them so I can power more LEDs. Remember each output from the decoder can only handle around 100mA max (5 small 20mA LEDs) and I want to power around at least 10-20 20mA LEDs from each channel and would like the room to power more than that if I want to

Thanks

You are talking about paralleling 5 small 20mA LEDs to get 100mA.
Next, paralleling 10 to 20 LEDs for 200 to 400mA.
The DMX 512 board can drive 24 volts!!
Series the LEDs! 5 LEDs draw 20mA at 5x the voltage.
Series 10 LEDs for 20mA at 10x the voltage.

You can do parallel / Series . Two string of 10 LEDs (20mAx2=40mA) (10 LEDs in series for 10X the voltage)

ronsimpson said:

You are talking about paralleling 5 small 20mA LEDs to get 100mA.
Next, paralleling 10 to 20 LEDs for 200 to 400mA.
The DMX 512 board can drive 24 volts!!
Series the LEDs! 5 LEDs draw 20mA at 5x the voltage.
Series 10 LEDs for 20mA at 10x the voltage.

You can do parallel / Series . Two string of 10 LEDs (20mAx2=40mA) (10 LEDs in series for 10X the voltage)

Click to expand...

Ok think I understand. I'm not disputing what you say at all as I don't have a vast understanding of electronics it's Just I had the impresion that if the decoder properties state that you can drive 300mA max per pixel that mean for each rgb cluster (3 of the 96 output channels) I have 100mA available for every red green and blue LED. So i was under the impression that I can only run up to 100mAms worth of LEDs per channel (5x 20mA LEDs per channel)

How many 20mA LEDs are you saying I can run from each 100mA output?

Sorry fining it hard to understand

Two LEDs side by side. 20mA + 20mA=40mA at 1.5 volts.
Two LEDs one on top of the other. current is 20mA but the voltage is 1.5+1.5=3 volts.

You can parallel LEDs so the current adds up until the current adds up to 100mA but lets be safe and stop at 60mA.
You can stack up LEDs so the voltage adds up until you get to 15 volts. (24V max)

Assume 1.5Volt 20mA red LEDs. You can have a block of 3 wide and 10 high LEDs and use 60mA and 15 volts.
I see I need to draw a picture but no time now.

ronsimpson said:

Two LEDs side by side. 20mA + 20mA=40mA at 1.5 volts.
Two LEDs one on top of the other. current is 20mA but the voltage is 1.5+1.5=3 volts.

You can parallel LEDs so the current adds up until the current adds up to 100mA but lets be safe and stop at 60mA.
You can stack up LEDs so the voltage adds up until you get to 15 volts. (24V max)

Assume 1.5Volt 20mA red LEDs. You can have a block of 3 wide and 10 high LEDs and use 60mA and 15 volts.
I see I need to draw a picture but no time now.

Click to expand...

No I understand sorry. No picture required

Didn't think about the fact LEDs are low voltage and was basing current draw on full voltage available through circuit, sorry.

So assuming 1.5v 20mA LEDs, if circuit properties are correct

Per channel I could run 5 wide (100mA) and 16 heigh (1.5v x 16 = 24v) max values obviously.
Correct?

YES! (and no)
You will need a resistor. LEDs should not have voltage applied without a resistor or some other way to limit the current. The voltage (1.5V) is dependent on temperature, current, LED color, and each batch of LEDs will be different. Many people think that if you place 1.5V on the LED it will use 20mA (like a light bulb). LEDs are a current device not a voltage device.

I have never seen a 1.5V red LED. Most barely light with 1.7V and can be fairly bright or burn out with 1.8V to 2.0V.

audioguru said:

I have never seen a 1.5V red LED. Most barely light with 1.7V and can be fairly bright or burn out with 1.8V to 2.0V.

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OK you are right. I did this from memory. Digikey shows red LEDs from 1.6 to 2.5V.
I was trying to show that a 100mA 24Volt output can drive more than 5 LEDs.

audioguru said:

I have never seen a 1.5V red LED. Most barely light with 1.7V and can be fairly bright or burn out with 1.8V to 2.0V.

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Yes ok that's fine I was just using 1.5v as an example. It's the maths used to work out the amount of LEDs I can use that Im bothered about. I'll use the specs of the LEDs I use to work it out correctly

ronsimpson said:

I was trying to show that a 100mA 24Volt output can drive more than 5 LEDs.

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My 120VAC Christmas tree lights have so many LEDs in series that I can't count them all (not enough fingers).

eleven?

ronsimpson said:

eleven?

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My 120VAC Christmas tree lights have 35 red LEDs that look like they are all in series. They flicker when I move my head so maybe they are half-wave rectified and not filtered (then they flicker at only 30Hz).
35 x 2V= 70V. Maybe there is a series capacitor to reduce the current but there is no space for one. There is also no space for a resistor. There might be a hidden rectifier.