# Electronic Circuits and Projects Forum

## Confused with watts & amps calculation

1. Originally Posted by audioguru
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

2. Originally Posted by MrAl
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.

3. Originally Posted by skmdmasud
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.

4. Originally Posted by MrAl
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.

5. 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?

6. Originally Posted by skmdmasud
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.

7. Originally Posted by MrAl
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.

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

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.

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.

Top view.

8. Hi,

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

9. Originally Posted by MrAl
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.

10. Are the fish blinded by the high power LEDs?

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