[quote author=Stuart Faust link=topic=178319.msg1322141#msg1322141 date=1374243091]
Im planning to use a 12v 8a dimmer per string of 12.
If anyone can recommend a better way of doing it please let me know.[/quote]
How about 3 strings of 4 LEDs? No resistor, but they won't be quite as bright.
(3V instead of 3.4V). Power consumption would then be 3* 0.7A =2.1A @ 12V = 24.12W, well within the range of a 400W ATX PSUs 12V supply.
You'll still need a transistor to control them, as the arduino pins cannot supply that much current.
Im looking to have different coloured whites a few greens and purples.
the blues will be kept on a separate dimmer for night time mood lighting.
i also had thought about using a timing circuit to turn the leds on and off at high frequencies so the eye does not see but conserves power etc.
Use the PWM outputs on the arduino. No timing circuits needed and you can dim the LEDs by varying the PWM output.
Could i use this to turn them on and off quickly to save power drain and led life?
Also if your mentioning using 12v for 3x4w.
this would mean i get more of a power drain as i would need alot more power as 2.1a per 4 leds instead of using the resistors giving more per similar power drain?
You are referring to PWM dimming. You would get a significant voltage drop in the TIP122, which would put the resistor calculations off. I suggest you use a logic level mosfet instead.
I think you should stick to 3 LEDs per string, otherwise the LED brightness will be extremely sensitive to small changes in the power supply voltage. You might in any case want to want to replace the resistors with constant current sources, to make the LED current more predictable. See attached for one way of doing this. You can drive 4 of these from a single Arduino PWM pin. Instead of dissipating 1.3W in each resistor, this arrangement dissipates 0.5W in each 1 ohm resistor and about 0.8W in the mosfet.
PC power supplies are very economical because they are made in large numbers and it is a competitive market. To drive 48 LEDs @ 0.7A in strings of 3, you will need a PC power supply that can provide around 12A continuously on the 12V output. Be aware that the 12V output is typically not as well regulated as the 5V, also that these supplies are intended for use with a significant load on the 5V output.
If you choose not to use a PC power supply, then you could choose a higher voltage supply to make switching the LEDs easier. For example, you could use a 24V 6A power supply with the LEDs in strings of 6, such as this http://uk.farnell.com/xp-power/vft150ps24/psu-150w-low-cost-24v-4-2a/dp/1821499 (caution: that one isn’t enclosed, so it’s only safe to use when it’s inside an enclosure, because of the high voltages in it).
Marine lighting? Are you using batteries and an inverter or shore power (110V)? If using batteries and inverter, skip the power supply and run the LEDs from the boat's 12V batteries or else get a 12V or 24V supply instead of a computer supply where you're paying for things (5V output) you don't use.
Marine lighting? Are you using batteries and an inverter or shore power (110V)?
Depending on where he is, shore power could be 240V. Not everyone is in the US.
If using batteries and inverter, skip the power supply and run the LEDs from the boat's 12V batteries or else get a 12V or 24V supply instead of a computer supply where you're paying for things (5V output) you don't use.
Who pays for ATX PSUs? I've several that I've recovered from defunct computers. He could be using the 5V output to power his Arduino.
I think you will find that by 'Marine' the OP means a marine aquarium
For this application I think you will be much better off using a Constant Current LED driver, such as the Meanwell LDD series, together with a higher voltage power supply i.e. 36v or 48v.
Then you will just need to run 4 or 5 strings of leds in series rather than confusing the issue by using parallel strings and the inherent problems they have i.e. load sharing issues and the need to install fuses to protect the leds.
im in the Uk , trying to set it up without having to spend too much.
thought that the ATX psu would be a good place to start as have spare ones and they are used to being on all day etc.
Dont even mind using 2 of them if it helps.
cant get right in my head how to run the strings discussed could someone put a basic diagram up for me. Say if i wanted to do 12 x 3 strings etc then i could duplicated this out to the different psus / lines.
5v im planning to use for cooling on the LEDS using 80mm case fans etc
If you intend to use resistors to control the LED current rather than constant current sources, then the diagram you posted originally is OK. Alternatively, replace the 2.7 ohm resistors by constant current controllers, which (with a suitable design) can also be used to switch the lights on/off from an Arduino, or dim them using PWM.
PC case fans are usually designed to run from 12V, not 5V.
As I said before, I wouldn't recommend running 5 of them in series with just a resistor. Try calculating the series resistor using that wizard. Then change the supply voltage by 0.3V to allow for imperfect regulation, or change the LED forward voltage by 0.1V each, and see how much the current changes.
You could drive two strings of 5 using a one mosfet and one BJT per string as a combined on/off control and constant current driver. Then you could feed them both from 1 or 2 Arduino PWM pins to do PWM dimming.