Power supply for LDD

[oreo57]

Ok I got it. Thanks for the time and the break down. One more final ? I won't need resistors because the driver will deliver the current exactly?

Yes in the examples the "driver" IS the resistor.. because you are using a constant voltage source it needs to be only 1 value.

When you use constant current drivers it frees you from having the exact voltage to run the LED's... The driver controls the voltage internally to keep amps constant


Amps = Volts/Resistance..

http://www.instructables.com/id/Choosing-The-Resistor-To-Use-With-LEDs/?ALLSTEPS

LEDs are current-driven devices that require current limiting
when driven from a voltage source. In most applications,
it is desirable to drive LEDs with a constant-current source.
The current source is used to regulate the current through
the LED regardless of power supply (voltage) variations or
changes in forward voltage drops, VF , between LEDs

http://www.digikey.com/Web Export/Supplier Content/Allegro_620/PDF/Allegro_an295031.pdf?redirected=1
http://en.wikipedia.org/wiki/Current_source
http://en.wikipedia.org/wiki/LED_circuit
What many people miss

The LED used will have a voltage drop, specified at the intended operating current. Ohm's law and Kirchhoff's circuit laws are used to calculate the resistor that is used to attain the correct current. The resistor value is computed by subtracting the LED voltage drop from the supply voltage, and then dividing by the desired LED operating current. If the supply voltage is equal to the LED's voltage drop, no resistor is needed.

So practically speaking you NEED to know the Voltage drop AT the power (mA) you want to run the LED's at.. in order to correctly determine your power supply voltage need...

 

[Yunghov]

So will any ps work or do I still need at least 32 volts?

 

[oreo57]

So will any ps work or do I still need at least 32 volts?

I just used V(f) at 3...and would require 2 700mA drivers.. 3V(f) may or may not be enough to turn the LED's on......(3.2 x 4 = 12.8)
MORE accurate w/ an easy to find PS.. add one diode (21 total) Would use a 1000mA driver

all sorts of PS will work.. depending on how you construct the string....

 

[oreo57]

So will any ps work or do I still need at least 32 volts?

The strings have to be designed around the ps or the power supply has to match the strings..
Also you have to keep in mind driver availability and how "hard" you want to drive the LED's..

It sounds more complicated than it really is..I guess

 

[Yunghov]

Oreo, let me get this right. On 10 LEDs 350ma I will use a 350ma driver running in series right? Because series strings will add all the voltage together. parallel string will share the voltage but add all the mA together.

 

[oreo57]

Oreo, let me get this right. On 10 LEDs 350ma I will use a 350ma driver running in series right? Because series strings will add all the voltage together. parallel string will share the voltage but add all the mA together.

YEs the mA of the string.. in this case 350 + 350 = 700mA IF you want to use 1 driver to run 2 strings wired in series (20 LEDs total) ..
TECHNICALLY it is safer to just add 1 350mA driver per string BUT it can be done w/ a 700mA driver in a series parallel arrangement..

The problem is IF one of the 2 strings opens all 700mA will go into the remaining string..
Most LED's when heat sink-ed properly can probably take it till you notice it and fix it..

I've ran 5 1W LED x 3 strings in parallel (total of 15 LED's) and had everything from single LEDs short to one whole string (actually had 2 go once) pushing 1000mA into one or 2 strings w/ out frying anything..
YMMV.........

Forward Current: 350mA
Peak Forward Current: 500mA
Reverse Voltage: 5V
Forward Voltage: 3.0V~ 3.6V

From the above example the LED can take more than you think.. and for whatever reason, when I lost 2 strings even at 1000mA (double peak) going into one string nothing went poof.......
Power supply was 24v 5A w/ LDD 1000

ARGUABLY the more parallel strings the less likely one string failure will ruin the rest........

 

[Yunghov]

So what happen? Did a driver go bad? Is there a way to prevent that agin?

 

[NanoReefWanabe]

wow did all that mumbo jumbo just get painfully difficult to read or what?

you want to run 2 strings of 10 leds each with a forward voltage of 3.2-3.6….assume the higher of the two to be safe…3.6x10 is 36v plus a couple for the drivers you will be using, so that brings you to 40V now do you want to run the LEDS at 700mA or 1000mA or 500mA or 350mA it doesn't matter chose the drivers you want and multiply that by 2…so if you want to use 1000mA drivers, then you need a power supply that will supply 40V (minimum, 42 is fine 48 would be a little too much unless you want to add a couple more leds) and it has to have a current rating of 2000mA minimum, though you will likely find one at 2.1 or 2.4A which will both work fine…as long as it is 40-48V

all that other math and stuff is completely unnecessary….running parallel strings is not for the beginner….to be honest i didn't read any of the posts after all the ridiculous math started….so i don't know if you have this resolved or not…but getting a 48V 2.4A power supply and a couple 1000mA drivers is all you need…then wire ten leds in series and connect to one driver and do the same for the other ten, then connect the two drivers to the PS and enjoy your lights…obviously test for ground shorts before applying power.

 

[Yunghov]

wow did all that mumbo jumbo just get painfully difficult to read or what?

you want to run 2 strings of 10 leds each with a forward voltage of 3.2-3.6"¦.assume the higher of the two to be safe"¦3.6x10 is 36v plus a couple for the drivers you will be using, so that brings you to 40V now do you want to run the LEDS at 700mA or 1000mA or 500mA or 350mA it doesn't matter chose the drivers you want and multiply that by 2"¦so if you want to use 1000mA drivers, then you need a power supply that will supply 40V (minimum, 42 is fine 48 would be a little too much unless you want to add a couple more leds) and it has to have a current rating of 2000mA minimum, though you will likely find one at 2.1 or 2.4A which will both work fine"¦as long as it is 40-48V

all that other math and stuff is completely unnecessary"¦.running parallel strings is not for the beginner"¦.to be honest i didn't read any of the posts after all the ridiculous math started"¦.so i don't know if you have this resolved or not"¦but getting a 48V 2.4A power supply and a couple 1000mA drivers is all you need"¦then wire ten leds in series and connect to one driver and do the same for the other ten, then connect the two drivers to the PS and enjoy your lights"¦obviously test for ground shorts before applying power.

I'm going to run 2 350ma drivers. Will 2.4a be too much or will the drivers handle the current?

 

[NanoReefWanabe]

I'm going to run 2 350ma drivers. Will 2.4a be too much or will the drivers handle the current?

i don't think the amperage matters as longs it is over what you need"¦it will just leave room to add more strings down the line...

 

[oreo57]

I'm going to run 2 350ma drivers. Will 2.4a be too much or will the drivers handle the current?

Yes, not a problem..