# Driving LED's with Constant Current Source HELP!

Discussion in 'The Projects Forum' started by WonHungLow, Dec 17, 2010.

1. ### WonHungLow Thread Starter New Member

Jun 29, 2009
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i am desiging an aquarium hood for my fish tank and have bought a sheet of acrylic and drilled holes in it for the LED's (74 of them!) i am putting in 25 white, 21 UV, 16 blue, 6 green, and 6 red 5mm LED's. i am wanting to drive these without resistors so i was thinking about using a LM317 and wiring it in constant current mode to power them up. i was going to use 1 Vreg for each color so that i can turn off and on each one. i am going to use a 9VDCD wall adapter to power each regulator and wire the LED's in series to the regulator. does anyone see a problem or a more efficient way to acheive what i am trying to do. i will try to post a schematic of my work to help out. thanks for the help.

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
Each LED requires a set voltage to operate. By tying them all in series you will need a very high voltage to run them.

First figure out your voltage. (you said 9 volts)

Then get the data sheet on the LED's and see what Vf is. Red led's will have a Vf of 2 something volts usually. Put as many in series as you can WITHOUT going over your input voltage.

Example. red LED 2.2V Vf

Four in series would be 8.8 volts. This would come very close to the 9 volts and would need only a small resistance to current limit them.

For blue led's Vf might be as high as 4 volts, so, only use two of them in series.

Take the combined Vf voltage drops of the LED's and subtract it from the input voltage.

9 volts minus 8 volts(two blue LED's) would be 1 volt. a 50 ohm resistor would give you 20 mA through the series string with one volt dropped across it. The other 8 volts will be dropped across the LED's and since series current is the same for all components, only 20 mA would flow in the LED's Understand?

3. ### mik3 Senior Member

Feb 4, 2008
4,846
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It is possible to drive 25 LEDs in series with constant current but you will need a power supply of about 60V. This is not a very wise solution in terms of safety for an aquarium. You can make three strings of 8 LEDs each and drive them with a 24V power supply.

Apr 5, 2008
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5. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
5mm LEDs will be woefully inadequate for aquarium lighting. You'll need to use high-power LEDs, and you won't want to power them using linear regulators like LM317's - as you will have a room heater.

6. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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ok i can wire them however i need to
what i am trying to do is power them individually by color and keeping the current from my power supply as low as possible.
what combination of wiring them in series and parallel would help me with this. for the green and reds im not worried about but the UV, white, and blue i have quite a few to power. i can go up to 12VDC for the power to the regulators if i need to. just need help wiring the circuit with alot of LED's in them.

7. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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its only a 15gallon tank for small fish. im not worried about growing conditions for plants just to light up the colors of the fish and rocks

8. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
Use the math in the example shown above.

LED voltage drop(from datasheet) x number in series; subtracted from, voltage input equals voltage excess.

Voltage excess is placed in an Ohms law equation with the target mA level(you said 20 mA)

Then the resistance needed is found.

Arrange your LED's in series strings that have a LOWER voltage drop than your input voltage. Figure the excess voltage for each color string and calculate the series resistance for that string to give 20 mA(or a little less).

Each color string will end up with a different number of LED's and with a different value of current limit resistor. All will be driven from the 12 volt input.

Total number of strings x 20mA will be the total current draw on you power supply.

9. ### Audioguru Expert

Dec 20, 2007
10,668
1,190
An LM317 current regulator needs at least 1.5V for the regulator plus 1.25V for the current-sensing resistor so with a 9V input, the max output voltage is 9V - 2.75V= 6.25V which is not enough voltage for only two white or blue LEDs in series.

10. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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ok i can wire chains of led's in series then wire the chains in parallel to help with the voltage right?

11. ### Wendy Moderator

Mar 24, 2008
21,438
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Read Chapter 1 and 2 of the article LEDs, 555s, Flashers, and Light Chasers, it will explain this in great detail. In other words, that is where the answer to most of your questions are already answered.

12. ### #12 Expert

Nov 30, 2010
18,093
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All this paying attention to the LEDs when the resistor on the 317 is in the wrong leg. Be sure to double check that!

13. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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no the resistor is not #12 . it goes between the Adj and Out pin to set the current level. i have been building laser drivers for years with this simple setup. i just dont know as much about LED's and the most efficient way of powering them. i will read up on the link above

14. ### #12 Expert

Nov 30, 2010
18,093
9,683
Did you get any lasers working with the resistor in the sense leg of the circuit, or did you make the load current go through the resistor so it would have a voltage drop when current flows?

15. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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here is the driver schematic

16. ### WonHungLow Thread Starter New Member

Jun 29, 2009
9
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it is a constant current. as long as the power supply to the regulator is 1.25V above what the circuit being driven is needed it will work. if you put more V+ to the regulator that is needed it will dissipate as heat from the regulator

17. ### Wendy Moderator

Mar 24, 2008
21,438
2,958
This is explained in the article in great detail, you are not going to be able to use one constant current source and one 9V DC source. You are going to need a fixed voltage source and many resistors. The article will show the circuit, and explain how to calculate the resistors.

It will take less than 5 minutes for you to read, much less than it will take for me to retype the answer. It is the reason I wrote the thing.