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Confused with watts & amps calculation

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skmdmasud

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we know watts=voltage x amps.

I have 3v leds which runs at 300ma
so its watt = 3 x .3 = 0.9 watts

I have a 2 amps power supply with output 30vdc
so it can handle watts = 2 x 30 = 60 watts

there for i can run 60/0.9 = 66 leds

what i am confused about is if my power supply only provides with 2 amps then i can max run 2/0.3 = 6 leds

which above calculation is correct. I know i am missing something.

Thanks and Regards.
 
You're ignoring the massive voltage difference and the huge losses over the resistors feeding the LED's (27V loss for single LED's).

You need to put multiple LED's in series (probably runes of 8 LED's), with a lower value series resistor to limit the current.
 
Hi,

Your power supply puts out 30v max and 2 amps max. That's 60 watts. If it put out 3v at 20 amps you could run a lot of LEDs in parallel with small resistors in series if they all took say 2.5 volts each.
But your supply does not put out 3v at 20 amps, it puts out 30v at 2 amps, so you've got to change the arrangement of LEDs if you intend to use the full power of the power supply.

To do this you create strings of LEDs that can run with a single relatively small value resistor, then connect the strings in parallel.

Now if the voltages of the LEDs were perfectly 3v all the time, you could run 10 in series connected to the power supply, and that would draw 300ma. Since you have 2 amps available and would then only be using 0.3 amps, you still have 1.7 amps left. 1.7 divided by 0.3 gives us an integer 5, so 5 more strings can be connected. That would be a total of 6 strings of 10 LEDs in series for a grand total of 60 LEDs. Now 60 LEDs times 3v times 0.3 amps gives us 54 watts, so that's close.

But the world isnt that perfect, the LEDs wont be exactly 3v and there could be a variation with temperature and other factors, so we only use 9 LEDs per string instead of 10 and use a dropping resistor to drop the voltage a little and help to ensure the right current goes through the LEDs.
Since 9 times 3 is 27, that leaves 3v to drop at 300ma, which means we need a 10 ohm resistor. 3v at 300ma is 0.9 watts, so we use at least a 2 watt resistor. We might also check to see that with that 10 ohm resistor we get the required current.

But in doing that we lost 6 LEDs, so what if we use 7 strings instead of 6 and underdrive the LEDs slightly.
2 amps divided by 7 gives us 0.2857 amps. Thus, we would adjust the current for each string to this value and we can use 7 strings with 9 LEDs in each string. That's 63 LEDs but since the current is slightly less we still end up with 54 watts worth of LEDs.
Dropping 3v at 0.2857 amps means we need a resistor value of 10.5 ohms, and power dissipation is still close to 1 watt so we should use a 2 watt resistor or better. A 10 watt resistor here would stay cooler though and those sizes are quite common. More than likely though we could get away with a 10 ohm resistor but if we have to we increase it slightly with another smaller value resistor of 0.47 ohms at 1/4 watt.
But things are not always this perfect either, so the 10 ohm resistors might have to be adjusted in value a little unless they provide less current than we wanted, and then we can simply put up with the slightly dimmer light levels.

Obviously if you want to use an LM series three terminal regulator to regulate current you'll have to use one device per string. Theoretically you can use one LM device total for all strings, but if one LED in one string blows open, that will force more current through the remaining good strings which would then stress them too much and probably overheat them. You also might have to drop the number of LEDs in each string so that the LM device has sufficient overhead voltage to work with. If your power supply is well regulated you might get away with 9 in each string.

So that's one way to go about it.

You also want to check to make sure that your power supply can put out 30v at 2 amps continuously.

Also, under driven LEDs have much longer life so you may want to drive them at somewhat lower current levels like 250ma or even 200ma perhaps. The life goes up quickly as the drive current drops it's not even close to proportional. You might have to check for noticeable color shift however.
 
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You need about 3V across the LM regulator for proper operation so you could run 20V / 3V = 6, or 5 leds in series with the regulator. For a 2A supply you could run 2A / 0.3A = 6 strings for a total of 30 leds.. Each string should have its own LM current regulator for proper current sharing.
 
Hi,

Your power supply puts out 30v max and 2 amps max. That's 60 watts. If it put out 3v at 20 amps you could run a lot of LEDs in parallel with small resistors in series if they all took say 2.5 volts each.
But your supply does not put out 3v at 20 amps, it puts out 30v at 2 amps, so you've got to change the arrangement of LEDs if you intend to use the full power of the power supply.

To do this you create strings of LEDs that can run with a single relatively small value resistor, then connect the strings in parallel.

Now if the voltages of the LEDs were perfectly 3v all the time, you could run 10 in series connected to the power supply, and that would draw 300ma. Since you have 2 amps available and would then only be using 0.3 amps, you still have 1.7 amps left. 1.7 divided by 0.3 gives us an integer 5, so 5 more strings can be connected. That would be a total of 6 strings of 10 LEDs in series for a grand total of 60 LEDs. Now 60 LEDs times 3v times 0.3 amps gives us 54 watts, so that's close.

But the world isnt that perfect, the LEDs wont be exactly 3v and there could be a variation with temperature and other factors, so we only use 9 LEDs per string instead of 10 and use a dropping resistor to drop the voltage a little and help to ensure the right current goes through the LEDs.
Since 9 times 3 is 27, that leaves 3v to drop at 300ma, which means we need a 10 ohm resistor. 3v at 300ma is 0.9 watts, so we use at least a 2 watt resistor. We might also check to see that with that 10 ohm resistor we get the required current.

But in doing that we lost 6 LEDs, so what if we use 7 strings instead of 6 and underdrive the LEDs slightly.
2 amps divided by 7 gives us 0.2857 amps. Thus, we would adjust the current for each string to this value and we can use 7 strings with 9 LEDs in each string. That's 63 LEDs but since the current is slightly less we still end up with 54 watts worth of LEDs.
Dropping 3v at 0.2857 amps means we need a resistor value of 10.5 ohms, and power dissipation is still close to 1 watt so we should use a 2 watt resistor or better. A 10 watt resistor here would stay cooler though and those sizes are quite common. More than likely though we could get away with a 10 ohm resistor but if we have to we increase it slightly with another smaller value resistor of 0.47 ohms at 1/4 watt.
But things are not always this perfect either, so the 10 ohm resistors might have to be adjusted in value a little unless they provide less current than we wanted, and then we can simply put up with the slightly dimmer light levels.

Obviously if you want to use an LM series three terminal regulator to regulate current you'll have to use one device per string. Theoretically you can use one LM device total for all strings, but if one LED in one string blows open, that will force more current through the remaining good strings which would then stress them too much and probably overheat them. You also might have to drop the number of LEDs in each string so that the LM device has sufficient overhead voltage to work with. If your power supply is well regulated you might get away with 9 in each string.

So that's one way to go about it.

You also want to check to make sure that your power supply can put out 30v at 2 amps continuously.

Also, under driven LEDs have much longer life so you may want to drive them at somewhat lower current levels like 250ma or even 200ma perhaps. The life goes up quickly as the drive current drops it's not even close to proportional. You might have to check for noticeable color shift however.

Carl,

I think i covered that scenario :) Also, he has 30v right?
 
Thanks Guys :).
My Led's are suppose to be rated at 350ma (is there any way to find this out) but i will be running them at 300 ma. I can use 6 strings of 5 led's each =30 leds. That is great. Since my LMxxx can deliver 3 amps then can i put the stings in parallel. Here is my rough layout
**broken link removed**
forgot to delete 2 led.

the formula for the R = 1.25/ (total led amps)

so now if i want to use led's in strings then do i need 1 resistor for all the strings or 1 for each string.

Thanks and Regards.
 
The LM350 should be set for 300mA. You need one LM350 for each string of LEDs.
If your LEDs are actually 3V each then you can connect 9 in series and in series with an LM350.
If you connect only 5 LEDs in series then the LM350 has 13.75V across it and it will dissipate 13.75V x 300mA= 4.1W.
 
The LM350 should be set for 300mA. You need one LM350 for each string of LEDs.
If your LEDs are actually 3V each then you can connect 9 in series and in series with an LM350.
If you connect only 5 LEDs in series then the LM350 has 13.75V across it and it will dissipate 13.75V x 300mA= 4.1W.
is there anyway to use the led's in parallel. lets say 9leds in 3 strings etc.. and use only 1 regulator
 
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Carl,

I think i covered that scenario :) Also, he has 30v right?
Yup you did. I missed that. Also somehow converted 30V to 20V. Guess it wasn't my day. :eek:
 
is there anyway to use the led's in parallel. lets say 9leds in 3 strings etc.. and use only 1 regulator

As i said before in this thread, if you use only one and you connect the strings in parallel you run the risk of one string drawing more current than another, and if one string blows open that might burn out all the rest too.

If you already have a DC output that is regulated you can use resistors, you dont need a LM 3 terminal regulator.
 
As i said before in this thread, if you use only one and you connect the strings in parallel you run the risk of one string drawing more current than another, and if one string blows open that might burn out all the rest too.

If you already have a DC output that is regulated you can use resistors, you dont need a LM 3 terminal regulator.

Ok Sir got you loud and clear. I guess the LM regulator and led cost almost the same so why take the risk. 7 or 8 leds cost a lot more than a LM regulator. Better stay safe and cool.

Thanks a lot.

Regards
Masud.

I will post pictures of my project which is a Aquarium hood light. soon.
 
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Hi,

OH yeah it would be nice to see some pics of the finished product.

What kind of fish, fresh water or salt water, or other types of animals?
 
is there anyway to use the led's in parallel. lets say 9leds in 3 strings etc.. and use only 1 regulator
That is what cheap Chinese flashlight manufacturers do. But the strings do not equally share the current. The LEDs last only for a couple of hours before they begin to burn out. When one or two strings burn out then the remaining strings get too much current and they also burn out.

If you buy thousands of LEDs, measure the forward voltage of each one and sort them so that they are in groups having exactly the same forward voltage then one group can be connected in parallel or in parallel strings.
 
Hi,

OH yeah it would be nice to see some pics of the finished product.

What kind of fish, fresh water or salt water, or other types of animals?

Its for freshwater planted tank. performance is good. i am running it at 300ma They are rated at 350ma.


**broken link removed**
I am using 7 leds for testing. I plan to use 15 leds in the final design


**broken link removed**


**broken link removed**
I have also attached a 12v cpu fan to keep the heat sink temperature low. It seems to be working very well. The heat sink is cold to touch. With 15 leds it may heat up a bit more.

Installed in an empty tank which has a dark 3d background. The rocky background further decreases the light.

**broken link removed**
with only 7 leds the light looks very bright. with 15 leds it will be more than enough for a planted tank of this size.


**broken link removed**
Top view.
 
Hi,


Oh ok nice pics. So you're going to put fish in it then? What species of fish?
 
Hi,


Oh ok nice pics. So you're going to put fish in it then? What species of fish?

Hi.
My aim is to make it a planted tank. I will put soil and put some foreground plants and then put some small tetra fish like neon's etc.

Do you have aquariums or fishes in ponds etc?

Regards.
 
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