dimming LED's!!

[mybuickskill6979]

hi, i'm new here.

i'm an aquarist. i'm trying to set up a LED array running of 26.4 volts at 1.34A (at the PS)
the leds are 62(which should be no prob being run) k2 1w emmiters. of 50% white and 50% blue.

heres what i would like to be able to do.

i would like to be able to dim the white led's and the blue LEDs seperately.

i am wondering if the use of two potentiometers(one for the white and one for the blues). and one transistor per series(current activated).

i'm wondering if this would work. i've looked up tranisitors and how they work until i'm blue in the face. but can't find out anything about what amount of current activates them.

i am by now means an electronic genius or even educated in electronics. so could somebody help me understand how these little critters work? and will my "plan" work? and what sizes would i need.

i read about potentiometers as well i undestand they have a voltage divider prong on them. but didn't understand the explanation given. could some one educate me on this as well?

i've been messing with cheap radioshack LED's and a PC PSU for a while and have blown 6 fuses in my multimeter cause i kept forgetting to switch the terminals to measure the amperage. so i know a little bit but would like more control over the final outcome. Thank you!! _Chris

 

[jrz126]

62 1-W leds, that must be pretty bright and quite expensive too.

I dont think you can do this with a single transistor though. You'll probably have to use pulse width modulation. You basically switch the LEDs on and off at a fixed frequency and then adjust the % of the time the leds are on. (thats a horrible explaination, google should be able to do a better job at it)

What kind of aquariums do you have? My dad is aquarist as well. He has a 75 gal saltwater tank with a 20-30 gal sump.

 

[mybuickskill6979]

i'm setting up a mini reef (10g) and i wanna have MH type lighting, without the heat. which is why i want so many.

The prices won't be so bad cause i got a buddy that works in the office of an electronics place, but apparently has no knowledge of electronics.
did the math again it come to 63 same but okay.
9 series of 7 running in parallel. so running nine tranistors is a no go huh? okay, i will look that up on google and wikipedia see how i can expand my knowledge. Thank you. _Chris

 

[mybuickskill6979]

would a PWM something like this work?
**broken link removed** i'm looking up the pieces(and how they work) or if not what type of modifications would i have to make to it?

 

[audioguru]

You forgot to attach the datasheet for your LEDs. It probably shows a range of forward voltages and each LED will be different. Maybe you are unlucky and got 3.5V LEDs that need a supply voltage much higher than you have. Or you can use only 6 in series instead of 9.
Maybe your LEDs are only 3.1V.

LEDs use a current reduction circuit for dimming. The PWM circuit for motors uses full power pulses with their duty-cycle reduced. It would blow up LEDs.

You could use a transistor in an adjustable current source circuit for dimming each series string of LEDs. 9 strings is no problem and they can be drivin by a transistor that is controlled by the brightness pot.

 

[mybuickskill6979]

thats what i was thinking. how does the sizing on transistors and pots work? i'm trying to look that up so i can kind of draw up a schematic to see if you all think it will work!!

 

[audioguru]

The size of a transistor is determined by how much power it dissipates as heat. You can design the circuit so they don't get too hot.

You have a 26.4V supply at 1.34A. If your LEDs are 3.5V then you can use strings with 6 in series for 21V, then 26.4V - 21V= 5.4V will be for the current sink transistors. You can use 10 strings then the total number of LEDs is 60. The max current in each string is 1.34A/10= 134mA.

At max brightness, the current sink transistor is fully turned on so its saturation voltage is about 0.5V. Then the emitter resistor has 5.4V - 0.5V= 4.9V across it and 134mA through it so its value is 36.6 ohms. Use 39 ohms/1W. Each transistor will dissipate a max amount of heat of 164mW so any little transistor is fine that can pass 134mA well, such as the 2N4401. Its max base current is about 2.2mA. 10 of them totals 22mA.

A driver transistor drives the LED drivers. Use one for white LEDs and another for blue LEDs. Its base is the brightness pot with about 6V across it.

See if you can sketch the circuit I described.

 

[mybuickskill6979]

i think some one asked for the datasheet on the k2's!
Here's the link (Its a PDF) K2 emmiter datasheet

isn't a milliamp 1/1000 of one amp?

okay i'll read this one again and give the drawing i try.

i don't have paint on this PC cause i deleted it but i'll draw it and take a pic of it with my cam and a pic of my old plans.

 

[mybuickskill6979]

okay one last thing before i up and draw the schematics.

if at full current (on a BJT) its wide open. would it help, with not overdriving the LED's to crazy, to limit the current going to the potentiometers. what is the max current that should go to the second leg on the BJT?

 

[audioguru]

Your power supply can't supply enough current for these high power LEDs to be anywhere near as bright as their rating. So consider using fewer strings of LEDs and operate them at higher current.

Which K2 are you using? The 350mA, the 700mA or the 1000mA ones? When your power supply is operating at its max current rating then for 10 strings of 6 LEDs each LED will have a max current of only 134mA.

The forward voltage has a range from 2.79V to 4.23V at 350mA. Then the graph showing how much the forward voltage drops when the current is less shows about 2.63V to 3.98V at 134mA. If you have 3.98V ones then 6 in series is 23.88V but the voltage from your 26.4V power supply is much too low.

You must measure the forward voltage of each of your LEDs if you want as many in series as is possible.

The transistor's emitter resistor determines the max LED current when the transistor is turned on hard.

The transistor that drives the LED driving transistors has a max emitter current of only 22mA, so its base current is only about 0.22mA. Then the current in the brightness pots is only about 2mA. Since the voltage across the brightness pot is about 6V max then its power dissipation is only 6V x 22mA= 132mW. A 1/2W pot is fine.

 

[ljcox]

It seems to me that you are trying to walk before you can crawl.

Draw the circuit so we can help you further. You said that you don't have Paint, do you have PowerPoint?

There are free drawing packages available, I use PowerPoint so I don't know of any off hand, but someone else may.

 

[mybuickskill6979]

**broken link removed**

is what i was gonna try but have sense found that the transistors are in the wrong area.i can't find the PS i was gonna use any where so its back to a 12v, higher current to be able to drive 60-62 LED's i will re-draw something i probably am getting ahead of myself. i will give more info when i bring the other drawing!!

 

[ljcox]

isn't a milliamp 1/1000 of one amp?

Yes

The diagram you posted won't work. The configuration is wrong and there are too many LEDs in each string if you want to power it from 12 Volt.

As Audio wrote, each LED needs 3.42 Volt (typical). So a string of 7 needs 7 * 3.42 = 23.9 Volt. So you need a power supply of at least 26 Volt.

You wrote that your power supply is 12 Volt. So you could only have no more than 3 in a string.

And if you are intending to control the current using a pot and transistors, you are going to need transistors that are capable of dissipating a lot of power.

So a PWM solution would be the better option.

 

[Hero999]

Why use LEDs?

Fluorescent tubes are far more efficient and cheaper to buy too.

If you don't have an AC power supply available then you can by decent quality DC electronic ballasts for the tubes or use an inverter.

LEDs aren't really suited to this application, the only reason I can see for using them if they are going to be in a position where they are very hard to replace other than that fluorescent tubes are definitely the way to go.

 

[audioguru]

The max forward voltage of K2 LEDs at 350mA is 4.23V. If you don't measure them all then you might be unlucky and make a few strings of 3 of them that need at least 12.7V. They won't light if you use a current sink transistor to dim them.

 

[mybuickskill6979]

true yeah i know that drawing was WAYYYYYYY off. i had found a 24v supply i was going to use. but couldn't find it. i found a 12v 16A i might use. i have another simpler drawing that i know will work as far as lighting the LEDs go but with no control over the dimming part. using a 12v 16A source and one resistor but its 20 rows of 3 leds
the 750mA ones i think the max for the whites was like 4v. which is why i only went with three in series.

i think some one gave me a thing to draw i'm gonna look for that and draw it up then i'll post it up here a little later!!

 

[3v0]

Why use LEDs?

LEDs aren't really suited to this application, the only reason I can see for using them if they are going to be in a position where they are very hard to replace other than that fluorescent tubes are definitely the way to go.

Using them makes sense if you can afford them. It is much about what is wrong with fluorescent tubes in the reef environment.

 

[Hero999]

I disagree.

Fluorescents are even more efficient than LEDs aquariums because they're so much better at producing the blue, Violet and near-UV light that corrals rely on for healthy development. LEDs are more expensive, use more power and produce less light, the only advantage is they are maintenance free.