Need help with LEDs and pwm

Hi all I've been trying to dimme my 3w LEDs with the jarduino reef control but can't get it to work.

I'm yousing a irf520n mosfit but my problem is that I'm runing a old 400w pc psu for the +12v and then run to the blue -12v wire so that the psu puts out +24v. How do I get both the LEDs and the arduino to run on the same grounds or am I just massing this up

Most PSU's can't have a lot of load on the -12V rail, be aware.

Easiest solution is to use a second transistor to drive the MOSFET. But why do you need 24V?

I have 8 strings of 6 3w LEDs over a full reef tank my strings are as

Rb 3w 3.2-4.0v at 700ma W 3w 3.2-3.6v at 700ma Tv 3w 3.2-4.0v at 700ma Green 3w 3.2-3.6v at 700ma Red 3w 2.2-3.6v at 450ma

My problem is that the LEDs have a -12v dc ground that needs to go to ground on the arduino but the arduino won't tack it so what can I do ? Ordinary I just get a 500w 48v psu? But then I would still have the same problem with the grounds

Why not make the strings run @ 12V? Instead of 6 in series make 3 in series. Wayyyyyy easier. Because I don't know if you already tested it but most of my ATX PSU can only supply 500mA on the -12V rail... 8 x 700mA = 5,6A is a lot for most -12V rails...

Or use 8 step up converters to current drive the string. Better regulation as the, I guess, resistor now.

All led psu are from +24v up so we can run moor then 3 LEDs as I would need 24 led drivers to be able to run in 12v so by running in 24v I can put two lots of 3 on one string and cut back on drivers so I would only need 14 of them. But is there a way to stop the -12v passing through to the ov on the arduino?

How do you put pics up ?

Cutting down on drivers means investing on the PSU... Have you tested it without PWM? I doubt the ATX can deliver the 5,6A needed to drive it...

But yeay, you can, use a transistor to pull the gate of the mosfet (on the low side of the led's) high. Or, even easier, use the -12V as the GND of the Arduino and the 0V/GND of the PSU as the Vin.

But again, I very much doubt that the PSU can drive all the LED's from the -12V rail!

on my sit drivers I have

Irf520n Triode 2n3904 0.82ohm resistor And a resistor on the pwm line

And the psu is rated at 400w 30a on the 12v

As I said, the 12V rail is not a problem. Like you confirmed it can source 30A. But the -12V line is a complete differed story. All ATX PSU around here can handle around 500mA and I have never seen a ATX PSU that can handle more than 2A.

So I guess you havn't tested it yet ;)

Will it works with out pwm lol so just so I'm getting this right my -12v to ground not the arduino and the -0v to vcc. Do I power the arduino of the usb or a 12v and the ground that is going to the vcc?

First, try to test the LED without MOSFET or Arduino. This should turn on the LEDs at full brightness. Just connect the driver and discover it won't work...

Can’t run the LEDs with out the mosfit as that’s part of the driver and it would fry the LEDs

If the MOSFET is part of the driver/is the driver than you really need to make a drawing/schematic.

Ov just don a drawing but can fifer out how to put it it in this post

When you hit Reply there is a link at the bottom with "Attachments and other options". You can attach the drawing/photo. Or upload it to Drive/Dropbox en share the link.

Hope this is right

Let me start by saying, look at the spec of the PSU! What does it say about the -12V rail?

So, it took me some time to figure out your schematic. Dude, if you want help, make a clear drawing!

But I think you are making this circuit

First of all, this is supposed to go on the low side of the LED’s aka the Cathode side of the LED’s or -LED.

And although it will work the set current can be a bit high. If Vbe really is 0,7V then the current will be 854mA. But checking the datasheet I would say it’s around 0,6V but this would still give 732mA and that’s higher then the maximum rating. And as all datasheet say, it’s the absolute maximum rating. So it’s better to run them @ 600mA or so (use a 1ohm resistor instead)

And you mentioned the spec’s of your LEDs. For example the “Rb 3w 3.2-4.0v at 700mA”. (And I have no idea what Rb means, is it a color?) When you run them towards the maximum current the forward voltages tents to go to the maximum value as well. In this case 4.0V. And if you have 6 LED’s in series this gives 6 x 4.0 = 24.0V. But the current driver also has a voltage drop but there is no room for that. It will probably sort of work because the current is lowered by that but it’s not a good practice, you nullify the purpose of the constant current driver.

But if you want to test this circuit, just connect the PWM pin to positive (12V) and the constant current driver should work without Arduino. But I don’t think the LED’s will light up because the -12V rail can’t supply the current (of maybe it can just light one string).

In my opinion, drop this constant current driver and replace it with 8 of these.
Modify them a bit like I did here. Use a 1ohm 1/2W resistor and you are done. You can connect it to 12V rail without a problem up until a combined forward voltage of 27V.

hi all I have 4 strings of 6 3w LEDs on 4 drivers all running pwm but when you dimme them 3 of them dimme to 0% and one dimmes to the 15% that all 4 should be. What have I done wrong lol