Working with high power LEDs/maxing out computer power supplies

[RobinAmerica]

Hi,

I hope i'm posting this to the right area, it seemed to fit in a few different areas, but since the main issue is related to drawing max power allowed from the PSU, i thought this the best fit.

I'm interested in high power LEDs and how far they've come to minimize power expenditure, and i've been learning a ton over the past year about how to implement simple circuits to take advantage of them. I do have some very basic experience with electronics and troubleshooting problems, mainly of the most basic type. There are some things that seem to go right over my head, i was hoping to get some help. Safety has always been the #1 topic for me, and so i'm also interested in doing things correctly.

I bought quite a few of the inexpensive "generic chinese led emitters" off ebay. That might seem to contradict the last sentence, but i've found them very safe, even if not very efficient. But being so inexpensive, i thought it better to start with these, this way when i screw something up, i don't take the bank account out with it.

My main interest right now is in using computer power supplies as the main power source- they're relatively inexpensive, reliable and safe (assuming name brand/ul listed etc).

Here is one of the LEDs i am using:

**broken link removed**

There's a lot to say, but without writing a book.. this led has been hard to learn with. I didn't notice until recently (and after i had spent a ton of time learning to work out the math for resistors etc..) that different sellers on ebay have completely different specs for what appears to be the same LED. Unsafe to start with. One seller will have it listed as 300mA per channel, another says 700mA per channel. I know there is some wiggle room with the specs, but that's rediculous, and hard to know how to wire it up safely, while getting the most light out of it possible.

Anyway, i do have nine of them wired up to a controller, which is powered by a computer power supply. This was the first project i made, and i did the usual beginner attitude- who needs a constant current controller? Don't be rediculous, a simple resistor does the job and seems to work fantastic. And it's still working just fine. The obvious problem, the resistors are trashing the majority of the current through heat (well not to mention that these LEDs are inefficient to begin with, but that's beside the point). The system works fine as it is, i can do everything i was hoping to with it, but being so inefficient, i started looking at constant current controllers.

That's what i'm using now. I have one 3 channel controller for each LED. The constant voltage controller i used before had several channels, so i just hooked the leds to it and didn't have to worry much about circuit design. But since these wire together differently, i'm finding out how much i thought i knew but didn't.

The controllers have +/- power in, work with a min of 12vdc, have a negative out for each channel, and a common (for all of the + channels).

What i would like to do is have for example, have two white LEDs and two of these colored RGB LEDs hooked into the same fixture under the same lens for options in color. The two white and the two colored would each be controlled as if the pair were one LED.

I think that means that the two white LEDs would be hooked in series, and the two colored LEDs would be hooked in series, but i'm a little confused how to hook them up other than that. I'm finding that there are a lot of other things to think about too, but determined to walk slowly through this and learn my way through it.

I was hoping to, at some point, have more than one of these fixtures powered off the same PSU. I've been able to power 9 rgb LEDs so far off the same supply. How do you calculate how many can be safely powered? I've looked, and there is some bad info out there! If i have a PSU that supplies 20 amps at 12v, it's not as simple as using 20 amps to calculate it apparently because power supplies will divide their power differently between rails, or is this correct? In other words, can a single molex coming out of the PSU supply the full 500watts (or whatever) that the PSU is rated to? I know that you should keep constant loads down to 80% of the rated max..

I've heard it both ways, but that leads me to the next question, all of the wires coming out of the PSU indicate they are 18gauge.

Some sites say 18 gauge can carry 2.3 amps safely:
http://www.powerstream.com/Wire_Size.htm

Some say it can only carry 10 amps:
http://wiki.xtronics.com/index.php/Wire-Gauge_Ampacity

Here's one that says 16 amps:
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/wirega.html

How do i know which table to go by?

Thanks for reading, any help would be appreciated.

Rob

 

[ashugtiwari]

to solve the problems in electronics or any branch of physics, learn and use mathematics, in this case network analysis and theory with basic of solid state electronics. That's enough for theory.

LED's are P-N junction diode which has normal ON state voltage drop of 0.7V, in case of power LED's it goes to 1.5V or more and that depends on manufacturer sheet. I suggest to find LED's with specific part numbers, bcoz datasheets of parts from manufacturer are more accurate than distributor specifications.

When using LED's, the usual power supply used is 3.3V to 5V, in which case considering LED drop of 1.5V. The series current limiting resistor is required to limit max current in LED. Let's say LED is rated at 500mA i.e. 0.5A. SO, using Ohm's law, Resistance = (5-1.5)/0.5=7 Ohm. SO, a resistor of 10 Ohm rated at I^2*R = (0.5)^2*10 = 2.5W is good enough. Now, this is good design and all but look at power dissipation, the LED will be alright but resistor will be too hot and hence not an ideal design. In such case a PWM circuit is used to limit the current to LED and is a good substitute to save power. You can use simple 555 timer to generate PWM and lit the LED's.

SMPS that is found in computers have multiple power outputs i.e. +/-12V, +/-5V, +/-3.3V, +/-1.8V. And every power output has some current rating. In case of LAPTOP or xbox360 power brick, the output is constant at 12V or 20V at say 10A. This power is then divided and steeped down in different power supplies into those respective systems. for eg. 1.8V (1.5V in modern computers) is used for microprocessor core supply i.e. for core i5 processor etc. 12V and 5V is used for analog circuits like audio amplifier, charging circuit for RTC battery, for USB (its digital peripheral) etc.

Now, SMPS is already regulated, so when using LED's directly for powering, use 5V or 3.3V which is enough to light the LED's until the output current from those points is insufficient for LED's. In such case, use step down converter or linear drop out voltage regulator to power LED's. Sometimes, even a simple resistor is enough to limit the current. IF you see LED's being dim with direct connection through current limiting resistor, simply decrease the resistor but before that calculate the resistance value mathematically first. Also, use of PWM is suggested, which will decrease the power consumption.

For wires, suppose if 10 gauge wire can carry 1A current. This wire can also be used for current's less than 1A but not for greater than 1A. And when you have multiple current ratings for same wire, i suggest you go to manufacturer website, enter the part number for wire and find the specification sheet or you can contact their support team for spec sheet which is always accurate.

Hope i understood your problem and helped you.

 

[RobinAmerica]

Thanks, mostly this is stuff i have, or am getting anyway, a fairly solid grasp on. The theory is becoming easier to understand, but applying it is where i'm having a problem. Doing the math is straightforward, finding the specs usually fairly easy. But the main problems i'm having, as i was saying,

Different web sites say a wire of gauge X can carry a current of Y amps. The thing that i don't understand is one website will say a 18 gauge wire can carry one current, and another website will say something completely different. (see the last msg for examples)

Also, i'm not sure how to calculate how many of a given LED i can run on a computer power supply. If a 10watt RGB led takes 700mA on each channel, the total is 3*.7= 2.1 amps, shouldn't i only be able to run nine of the leds on a power supply that's rated at 20amps? (2.1amps goes into 20 9 times, leaving 1.1A). I am able to run 12 of those LEDs off this power supply with no problem.

And, i looked at the molex wires on the power supply, they say 18 gauge. This page:

**broken link removed**

Says 18 gauge wire can carry 2.3 amps, or 16 amps for chassis wiring, which i think doesn't apply, or does it? Even if it does, 16 amps is less than 20.

I think i am missing something simple, but i've been going over it for weeks as i've had time. Well i've learned a lot along the way. The LEDs are quite a bit brighter than i expected.

Also, i mentioned, i'm using constant current controllers now. I was using a resistor to drop voltage but that was so cumbersome in comparison- lots of power wasted as heat, which negates the whole point of the project.

Thanks,

Rob

 

[bluelasers]

So wire is a resistor, in theory it is a perfect conductor. But in the real world it is not. Therefore it is a resistor, and like all resistors it has power rating. If you pass to much current over a wire the friction from the electrons moving through the wire create enough heat to melt or damage the wire. Or a space heater/toaster oven.

Now P=VI or power equals voltage times current. (Power dissipated across a resistor.) So if you transmit 1000 volts with 1 amp you will melt the wire. But if you sent 120 volts with 1 amp you will not.

Also resistance matters as I can rewrite the equation as P = (I^2)R or power equals current squared time resistance. Again wires are resistors but with very small resistance values. The large the width of a wire the less resistance (so more current capacity at the same voltage. The smaller the wire gauge the wider the wire width.) However the longer the wire the larger the resistance, so less current at the same voltage.

The websites you are looking at are using giving you two numbers transmission and chassis. Transmission is long wire, so less current. Chassis is short distance so more current.

However these wires are more than like rated/calculated a higher voltage than the voltage you will be using. (120 -230 Volts more than likely.) Therefore you may have more wiggle room for current.

Power supplies have current rating for each voltage rail. Make sure you stay under this. 1.1 Amps should be safe, but make sure you are looking only at the one rail and not the whole supply.

The main thing is the wire should not get hot or even warm at full load. If it does stop immediately. You will need a bigger wire or a lower gauge of wire.

I hope this helps.

 

[RobinAmerica]

I did notice that some sites list their gauge ratings in two columns- one for chassis wiring and one for power distro. But if you look at the sites mentioned, some do not, and also the numbers don't match any of the numbers on the other sites.

So, if i use a gauge of wire that's rated to 15 amps, i can pump 15 amps through it, no matter if it's 100V, 1V, or 26V? The numbers vary so widely between the sites i've seen, i figure i'm either missing something, or the variance just gives people values that are good enough and they don't care about accuracy so much. But i haven't seen a direct answer either way, and i've posted the question several times, so maybe it's such a stupid question that noone wants to waste their time answering. Asking stupid questions has been with me for a long time. I once got into trouble in 3rd or so grade for asking a stupid question. Well, *i* thought it was a good question, but after he called it stupid, i asked the teach, if it's such a stupid question and you still can't come up with an answer, then what does that say about you? I became a martyr and an addict, now i ask stupid questions of all sorts. But this is off topic.

On figuring out how many LEDs i can run from a given power supply, if i have a 250watt supply that says it can power 9amps on the 12v rail for example, i can just divide the total power of the led into the 250 watts? i.e. for 10 watt LEDs, 250watts/10watts = 25 LEDs, is that right? I know i should stay at 80% of max ratings, but in theory, is that the way to do it?

 

[ashugtiwari]

Wires are made up of different metallic materials like copper wire, iron wires, alloy wires etc. SO, when you have 3 wires of same gauge or cross sectional area but different material like gold, silver, copper, iron etc., the maximum current these wires can carry is also different. For eg. gold is most conductive, so it can carry more current.
Now, here is the thing, gold has high number of free electrons, hence it can carry high current with just small voltage (and voltage means energy provided by battery or power supply to free electrons in metals to make them flow in metal hence called conductor). And with same voltage, silver can carry less current, copper is somewhere in between, aluminum carry much lesser current. Hence, check also the conductivity/resistivity of the wire conductor before taking int consideration maximum current it can carry. And as i said always prefer manufacturer website for specification.
Now, consider you have copper wire of 10 gauge which can carry 10A of current. The wire will also have some resistance R which is lets say 10 Ohm. So, when you apply 10V to this wire, according to ohm's law I=V/R = 10V/10 = 1A. So, the 1A of current is flowing through the wire. To make wire carry 10 A, V=I*R = 10 A*10 Ohm = 100V. SO, the maximum voltage you can apply to wire directly is 100V. If you cross this voltage of 100V, the current will suddenly increase, for eg if you give 200V, the current will be I=V/R=200/10=20A, which is beyond the copper wire can carry. This will heat up the wire too much and slowly wire will burn or will bend melt. It will loose its characteristics. So, never cross the upper limit. Hence, may be specification say this wire can carry 10A, but it also depends on circuits it is used. For eg. this wire used in circuit with maximum effective resistance of 100000 Ohm. The max voltage you can apply is 100000*10=1000000V. This is how you should calculate max voltage. For this find the value of resistance per unit length from specification sheet of wire.

Now, back to LED, yes, you can connect 25 LED's when you have power supply of 250W at 12V considering each LED does take 10W. But, do make sure if your computer SMPS is rated at output of 250W at 12V. When you take 3 LED's rated at 700mA and 10W and connect them in parallel, yes, the total minimum current you will need is 3*0.7=2.1A. But, remember connect these LED's in parallel and also make sure, the power supply can provide more than 2.1A. If the LED's are connected in series, then power supply need not provide more than 700mA. Bcoz, in the series connection, all LED's share same network path and in this path if the current is 700mA, all the LED's will get same current. Its the voltage that will matter in series connection i.e. if one LED drops 1.5V, then 3 LED's together 4.5V is supposed to be supplied from power supply which is well within 12V. In this case, reduce, the voltage to 4.5V by connecting a resistor in series. Higher voltage can increase the current in LED's when it is not limited by resistor. So, when using power supply make sure, LED will only get rated supply i.e. 1.5V at 700mA, not more or less than that. And this can be accomplished using PWM average voltage signal which can be generated by 555 timer. Its a very simple circuit and there is no power wasted. You can try creating this circuit. When PWM is used, even resistance is not required and there is no overheating of either LED's or any component and no power wastage as PWM signals only rated supply not more than that. Make sure, you use 555 in astable mode with a variable potentiometer to change the average voltage.

Hope that helps.

 

[bluelasers]

So, if i use a gauge of wire that's rated to 15 amps, i can pump 15 amps through it, no matter if it's 100V, 1V, or 26V?

No, as I stated in my last post. If I have 100V I may only be able to do 4 Amps. However if I have 26V I may be able to do 15 Amps. While 1V may be able to do 100 Amps. The reason is the wire has a max amount of power it can carry, depending on guage, single conductor vs multi conductor, wire material, wire length, and insulation type. Most ratings are very conservative. (Which does not mean you should ignore them. Just some may be more conservative than others.)

On figuring out how many LEDs i can run from a given power supply, if i have a 250watt supply that says it can power 9amps on the 12v rail for example, i can just divide the total power of the led into the 250 watts? i.e. for 10 watt LEDs, 250watts/10watts = 25 LEDs, is that right? I know i should stay at 80% of max ratings, but in theory, is that the way to do it?

So most power supplies output a couple voltages (12V, 5V, 3.3V,....) each has a certain about of current they can output at that voltage. Each voltage is called a rail. They are not equal (+12V may supply twice the current as +5V.) However is you sum each rails voltage times current, you will get a total of 250W. ((12V * (12V rail current)) + (5V * (5V rail current)) + ..) = 250W) 250W is the total amount of power the supply delivers on all rails. 80 percent is a good load number.

Since each strip I am guessing take 12V, you only need to make sure you power supply can supply enough current. Remember you only what to use 80 percent of the current capacity. The total power consumption will not be near 250W because you are not using the other voltage rails.

 

[RobinAmerica]

Thanks ashugtiwari, i knew that there were variables like material, temperature, whether it was solid or stranded, if stranded, how many strands and how thick each was etc etc.. But i wasn't seeing those notes, thanks for explaining that. I take it that accounts for sometimes 5-10 amps difference, i'll spend more time reading fine print. The length of wire i think shouldn't have too much affect, the distances between power supply and emitter diodes should be 1-2 feet. Thanks for such a detailed answer, it also helps explains why some of my own measurements weren't looking the same in some tutorials as what the author was getting, and that made me just want to give up at times.

Thanks blue for that info. I'm aware of rails, and the 80% of max ratings i'll pay attention to. I'm using high power LED's not strips, but the theory is the same..