Running LED's w/ a wall wart?

[mmelnick]

I just bought a LED desk lamp that has a single high powered LED. It has a .7 amp, 3.5V DC power supply that directly poweres the LED. This got me thinking.

 

Why couldn't you do something like this with a small string of LED's? I know that LED's do better with a fixed current, but I can get a DC power supply for $20 at RS that can be switched between several voltages ranging from 3V up to 12V and has a max current of 1 amp. Wouldn't this keep the LED's from maxing above 1 amp?

 

If I ran 4 LED's on this wouldn't they be running at the designated range of 3 watts each (1 amp at 3 volts DC)? And couldn't I use the switch on the wall wart to dim them in incriments?

[FZhang]

I'm running 2 3W LED's will just a wall wart. Using them as moonlights though so I spliced in a resistor to dim the output

[mmelnick]

I'm running 2 3W LED's will just a wall wart. Using them as moonlights though so I spliced in a resistor to dim the output

 

Very interesting? At least I'm not the only one trying to cut some corners. What wall wart are you using? (Voltage and amperage?)

 

The only thing I can find about why we use a CCD is:

when the current increases on an LED, so does the heat. Once an LED gets too hot, it can be destroyed, or have it's overall life and performance severely reduced

 

But with a regulated 1 amp supply it can't get over 1 amp can it? So what would I have to worry about? The only thing I can think of would be that it could run at LESS THAN 1 amp. But people do that all the time.

[TinyGiant]

check out the bulbs in my nano adventures sig link i put links to the ones i have on my nano that i bought from ebay. cheap they just screw into a light bulb and they look great and i have had good growth

[mmelnick]

check out the bulbs in my nano adventures sig link i put links to the ones i have on my nano that i bought from ebay. cheap they just screw into a light bulb and they look great and i have had good growth

 

I actually already read that a day or so ago. The only problem is that I only have a 4"X4" opening above my tank (it's a 1/2 gal), so I can't use 2 bulbs. I could buy 2 of those and swap out the LED's from one to another, but I'd rather have the flexibility of building my own. I could never run more than 3 LED's w/ a bulb like that.

 

But that's a great solution for a slightly larger tank.

[TinyGiant]

got ya.

 

well if a different ebay buyer accepts my bid i will have 20 white 3w cree equivalent whites and 20 blues

 

i figured i would get them so someone can test the par on them. i wont need many so i can sell them cheap enough

[phalanx]

But with a regulated 1 amp supply it can't get over 1 amp can it? So what would I have to worry about?

 

 

Regulated wall warts regulate their voltage output, not their current. Their current limit is usually a function of the internal magnetics. If you exceed the rated current, the voltage from the wall wart will start to drop but not necessarily enough to protect your LEDs from serious damage due to the current that can be sourced even after it is out of regulation.

 

Some wall warts have overcurrent protection but unless you are buying them from Digikey or Mouser where the datasheets are available, there is really now way to know for sure. You will still have to protect your LEDs with current limiting resistors.

[mmelnick]

You will still have to protect your LEDs with current limiting resistors.

I see.

 

So a switchable wall wart with a resistor would work for a small simple setup?

 

I'm really not too worried about getting the LED's to last 10 years. If I burn out a $6 LED every year or so that's really not a big deal like it would be with an array of a few dozen LED's.

[doctaq]

phalanx saves the day

 

i have the means for a bunch of transistor based drivers, they only drop the voltage by .5v and are cheap, i wish you were closer it wouldnt even be worth it to ship it to you as it only cost me like a couple dollars to build

[mmelnick]

phalanx saves the day

 

i have the means for a bunch of transistor based drivers, they only drop the voltage by .5v and are cheap, i wish you were closer it wouldnt even be worth it to ship it to you as it only cost me like a couple dollars to build

 

Do you have any info on this? Or a parts list?

[jm82792]

I'd be interested in getting a schematic for a driver that would be PWM controlled and drive 4 crees.

I am trying to save money and wouldn't mind building something from scratch.

I'm wary of anything nobody has tried because I don't want to fry a bunch of $6 LEDs.

 

phalanx saves the day

 

i have the means for a bunch of transistor based drivers, they only drop the voltage by .5v and are cheap, i wish you were closer it wouldnt even be worth it to ship it to you as it only cost me like a couple dollars to build

[FZhang]

Very interesting? At least I'm not the only one trying to cut some corners. What wall wart are you using? (Voltage and amperage?)

 

sorry for the late response. but the wall wart I'm using is 800ma and 5.25V not regulated. Basically just enough to run 2 LED's. The fact that I'm powering moonlights means that I'm not really picky about the light output. They also run really cool because I'm not running much through them. After about 10 hours without a heatsink they were still cold to the touch.

 

Again, these are for moonlights. Wouldn't use this method for main lighting

[doctaq]

evils ultimate guide, transistor based driver

look up the guy who wrote it, he has a few more models that include one pwm dimmable one

[jm82792]

Evil seems to have been busy for a while.

I have not yet seen anything that PWM that has some efficiency even on evil's page that's not SMT.

So I Want to give the STCS1 shot.

However I've never touched SMT,

never etched my own PCBs but the savings would be huge.

So when he is around I'd like to ask a question or two.

 

evils ultimate guide, transistor based driver

look up the guy who wrote it, he has a few more models that include one pwm dimmable one

[mmelnick]

Regulated wall warts regulate their voltage output, not their current. Their current limit is usually a function of the internal magnetics. If you exceed the rated current, the voltage from the wall wart will start to drop but not necessarily enough to protect your LEDs from serious damage due to the current that can be sourced even after it is out of regulation.

 

Some wall warts have overcurrent protection but unless you are buying them from Digikey or Mouser where the datasheets are available, there is really now way to know for sure. You will still have to protect your LEDs with current limiting resistors.

 

Some of this is getting a little bit over my head. So what will happen if the rated current is exceeded?

 

Are you saying that if the current is exceeded the voltage will drop, so that the current can go higher to run at the same watteage? This exceeding the rated input for the LED's?

 

Can I just put a resistor in line with the output wire of the wall wart?

[blasterman]

I know that LED's do better with a fixed current,

 

 

No, they don't. Current regulated supplies tend to be a tad more efficient and more design flexible given they deliver the same amount of current as long as LEDs in series don't exceed the total voltage of the supply. That means a 24volt current regulated LED driver handles one LED just as efficiently as five.

 

With a fixed voltage supply the math has to work out to the range of Vf for each emitter. For instance, I'm using 18.5 volt laptop bricks on one of my tanks. Five LEDs in series equals 3.7 volts to each LED, which is perfect. I then just add another parallel series for as much light as I need. No resistors needed. I don't dim, nor have the need to dim because as far as I'm concerned it's a boutique requirement that series reefers don't need (IMHO - save the flames). I've never seen an SPS tanks with dimmable metal halides.

 

I utterly disagree with anybody that says a regulated current supply is safer and better than a dedicated fixed voltage supply like a Mean Well - if both are designed properly. Commercial designs have to use current regulated supplies for reasons that don't apply to DIY. By all means, if you have it all worked out and the extra money get the dedicated LED drivers, but fixed voltage supplies do work fine with proper considerations.

 

I've built big arrays using MeanWells (both fixed current and voltage), Xitaniums, laptop bricks, computer PSUs, etc. Also, $25.00 buys you a lot more Mean Well than a Wall Wart, and most Mean Wells have have a voltage (or current) adjustment range to play with. There's alomost no need to use a current resistor with a fixed voltage Mean Well given they have so many voltage options.

 

Exceeding the current of a fixed voltage supply = bad. Good ones will detect it as a short and stop working to prevent transient voltage drops from damaging electronics. So, for safety, it's best not to exceed 2/3 that rated current of a fixed voltgae supply. For instance, I have some big, 20watt LEDs that run at 2amps at 18.5 volts, and when hooked up to my 18.5 volt laptop brick the brick goes into safe mode. A Wall Wart likely won't do this and do screwy things with voltage.

[pazhope]

I just started a thread with this, and don't want to steal yours, but the idea is the same.

 

I can't spend 40 dlls per driver and want to build my own, in the process I found this one..

Sure Electronics driver

 

Its under 7 dlls and it appears it can handle 8 LEDS, you will need a power supply. I am just looking for a second opinion before pulling the trigger with 2 of these.

[doctaq]

how do you figure what current you will get and if the current stays consistant throughout the day? what if say for instance you want to have 500ma, run about 3.4v per led? cree xr-e

its not so much a requirement but dimming is a novelty that leds add to lighting that other kinds of lighting generally do not, changing the color temp, yada yada,

 

people dont even do this for low powered leds, reccomended at least 30% voltage over the resistor.

 

No, they don't. Current regulated supplies tend to be a tad more efficient and more design flexible given they deliver the same amount of current as long as LEDs in series don't exceed the total voltage of the supply. That means a 24volt current regulated LED driver handles one LED just as efficiently as five.

 

With a fixed voltage supply the math has to work out to the range of Vf for each emitter. For instance, I'm using 18.5 volt laptop bricks on one of my tanks. Five LEDs in series equals 3.7 volts to each LED, which is perfect. I then just add another parallel series for as much light as I need. No resistors needed. I don't dim, nor have the need to dim because as far as I'm concerned it's a boutique requirement that series reefers don't need (IMHO - save the flames). I've never seen an SPS tanks with dimmable metal halides.

 

I utterly disagree with anybody that says a regulated current supply is safer and better than a dedicated fixed voltage supply like a Mean Well - if both are designed properly. Commercial designs have to use current regulated supplies for reasons that don't apply to DIY. By all means, if you have it all worked out and the extra money get the dedicated LED drivers, but fixed voltage supplies do work fine with proper considerations.

 

I've built big arrays using MeanWells (both fixed current and voltage), Xitaniums, laptop bricks, computer PSUs, etc. Also, $25.00 buys you a lot more Mean Well than a Wall Wart, and most Mean Wells have have a voltage (or current) adjustment range to play with. There's alomost no need to use a current resistor with a fixed voltage Mean Well given they have so many voltage options.

 

Exceeding the current of a fixed voltage supply = bad. Good ones will detect it as a short and stop working to prevent transient voltage drops from damaging electronics. So, for safety, it's best not to exceed 2/3 that rated current of a fixed voltgae supply. For instance, I have some big, 20watt LEDs that run at 2amps at 18.5 volts, and when hooked up to my 18.5 volt laptop brick the brick goes into safe mode. A Wall Wart likely won't do this and do screwy things with voltage.

[mmelnick]

With a fixed voltage supply the math has to work out to the range of Vf for each emitter. For instance, I'm using 18.5 volt laptop bricks on one of my tanks. Five LEDs in series equals 3.7 volts to each LED

 

So 5 LED's on a 12V supply would run at about 2.4V each. Or 4 would run at 3V each right? That's the way I understand it.

 

How would I overload the wall wart then? If I add more LED's wouldn't it just drop the wattage to each LED? That's what i'm trying to figure out.

 

And if I do a quick test setup and it's running at a certain current and voltage, can I assume that it will always run there? Or could I come bace in a month and find that it has randomly shifted?

[phalanx]

With a fixed voltage supply... No resistors needed.

That is dangerous misinformation you are spreading. LEDs are current controlled devices which require regulated current to operate safely and reliably. A purpose built regulated current power supply or a regulated voltage supply with a current limiting resistor in series with the LEDs are both examples of a proper solution. Using a voltage regulated supply adjusted to sit right on the knife edge that is the "ON" voltage of an LED is setting yourself up for future failure. In this application, it only takes a small shift in string voltage to cause large changes in current through it which could result in the LEDs extinguishing (current drops) or buring up (current rises). Just because it work in that one instance, doesn't make it a proper solution.

 

 

I utterly disagree with anybody that says a regulated current supply is safer and better than a dedicated fixed voltage supply like a Mean Well - if both are designed properly.

What do you base this assertion on?

 

If you are connecting LEDs to a current regulated (CR) or a voltage regulated (VR) power supply without anything else in the string, the CR supply is ALWAYS a safer option since it can prevent run away currents where the VR can not.

 

Between a CR or a VR supply with a resistor in the string, they are both safe but a switch mode CR supply can be much more efficient over a much wider input and string voltage range giving it a significant advantage over the alternative.

 

 

Commercial designs have to use current regulated supplies for reasons that don't apply to DIY.

Commercial designs are bound the same physical properties of LEDs as the DIYer. The exponential voltage to current function of an LED doesn't become linear for DIYers which is why it is important to either use a CR supply or a VR supply with a current limiting resistor.

 

 

I've built big arrays using MeanWells (both fixed current and voltage), Xitaniums, laptop bricks, computer PSUs, etc. Also, $25.00 buys you a lot more Mean Well than a Wall Wart, and most Mean Wells have have a voltage (or current) adjustment range to play with. There's alomost no need to use a current resistor with a fixed voltage Mean Well given they have so many voltage options.

Once again, just because it works doesn't mean it's right. A proper design will never leave an LED string in a position where there is no way to control the current through them. A VR power supply alone does not have the means to control the current through an LED.

 

 

Exceeding the current of a fixed voltage supply = bad. Good ones will detect it as a short and stop working to prevent transient voltage drops from damaging electronics. So, for safety, it's best not to exceed 2/3 that rated current of a fixed voltgae supply. For instance, I have some big, 20watt LEDs that run at 2amps at 18.5 volts, and when hooked up to my 18.5 volt laptop brick the brick goes into safe mode. A Wall Wart likely won't do this and do screwy things with voltage.

This part I agree with!

[mmelnick]

So what about using something like this with a DC input:

http://cgi.ebay.com/DC12V-24V-3W-LED-Drive...=item4aa1fea000

 

 

Or this with an AC input:

http://cgi.ebay.com/3W-AC60V-240V-Waterpro...=item4aa1eeabf1

[phalanx]

So 5 LED's on a 12V supply would run at about 2.4V each. Or 4 would run at 3V each right? That's the way I understand it.

It doesn't work that way. The power LEDs being used by most people have a nominal forward voltage of about 3.5V. This number tells us the voltage required to turn the LED on. Voltages lower than this would result in miniscule amounts of current giving you an LED that is either off or very dimmly lit. Once you exceed 3.5V, current through the LED will increase exponentially which means that if you were to connect a single 3.5V LED to your 12V power supply, the extra 12-3.5 = 8.5V not being used by the LED will cause so much current to flow that the LED will burn up.

 

When you wire LEDs in series, they each take away their forward voltage from the supply so if you used 3 LEDs in series you would get 12-(3 * 3.5) = 1.5V left over which is still enough to cause excessive current through your LEDs. To protect from this we use a current limiting resistor in series with the LEDs. Resistors have a linear voltage to current ratio which ultimately means the current through them is much less sensitive to changes in voltage.

 

At this point we have 1.5V left over with an uncontrolled amount of current flowing through the LEDs. To figure out how to size the resistor we use the formula R=V/I where I is the current in Amps that we want through the LED and V is the voltage left over from our previous calucation. Say we want 750mA to the LEDs. Our formula would look like this R=1.5V/0.75A = 2 ohms. That 2 ohm resistor will dissipate the extra 1.5V at 0.75A as heat. The resistor needs to be sized to hand the power. Power is simply V*I so 1.5V*0.75A = 1.125Watts. Since you always want a resistor rated higher than the power being dissipated, I would at a minimum get a 2Watt resistor although 3 and 5Watt resistors are more common.

 

How would I overload the wall wart then? If I add more LED's wouldn't it just drop the wattage to each LED? That's what i'm trying to figure out.

All LEDs in the same string will be powered by the same current. A string with a single LED will use the same current if a dozen more are connected in series (if the supply voltage is high enough). Since your string size is limited by the forward voltage of the LEDs and the voltage of your power supply, you can add more LEDs by making parallel strings. The current used by all the strings is the sum of their individual currents. 3 strings drawing 0.75A each will draw a total of 2.25A from your power supply.

 

And if I do a quick test setup and it's running at a certain current and voltage, can I assume that it will always run there? Or could I come bace in a month and find that it has randomly shifted?

In an ideal world it should always stay the same. In the real world, it is possible for the forward voltages of the LEDs to vary over time which can affect the current through them. Usually the changes are not enough to hurt anything so long as you aren't running the LEDs at the edge of their capabilities.

 

Since LEDs can change over time, this is where a Current Regulated power supply becomes an asset. The CR supply can adjust the current output automatically (and nearly instantly) as the LEDs change over time so you will always have the proper current flowing through them.

[mmelnick]

^^^

 

OK, that makes sense. Thanks for breaking it down for me. That was really helpful!

 

 

What about the links I posted? Would that be a good option? I think something like that would be a pain for most people with a larger array, but I'm planning on about 3 LED's, so adding 3 of those wouldn't be too bad.

[phalanx]

So what about using something like this with a DC input:

http://cgi.ebay.com/DC12V-24V-3W-LED-Drive...=item4aa1fea000

 

 

Or this with an AC input:

http://cgi.ebay.com/3W-AC60V-240V-Waterpro...=item4aa1eeabf1

 

Those are current regulated supplies that are both designed to power a single 3W LED. The top one uses low DC voltages and outputs 0.75A. The bottom one has a universal AC input (wall power) and outputs around 0.7A. It is possible that they may work with 2 or three LEDs in series but you would be operating them outside of what they were designed to do.

[mmelnick]

Those are current regulated supplies that are both designed to power a single 3W LED. The top one uses low DC voltages and outputs 0.75A. The bottom one has a universal AC input (wall power) and outputs around 0.7A. It is possible that they may work with 2 or three LEDs in series but you would be operating them outside of what they were designed to do.

 

Well I would do one for each LED right? Not one of those for 3 LED's.

 

I thought they were meant to run a single LED. But again, some of this is a bit over my head.