Need help achieving constant current

[malfist]

I am trying to make a Grow Light, and I'm powering some expensive LED's so I want to protect them from being over driven.

The LED's consume 700mA @ 2.9 VDC, I want to know how to build a circuit to keep the current at the 700mA level, and I'd also like to keep it as cheap as possible.

I'm powering the circuit with a computer PSU, so I have access to 3.3V, 5V, and 12V, so I could possibly power more than one at a time. I'm looking to charge around 15-20.

I looked into using an NPC3063 (sample **broken link removed**). The theoretical upper limit on those are 1.5 A, so I could power two LED's from it. However, that example circuit won't work for my plans, and I don't understand the math behind it (I'm good at math, I just don't know the formula, and the datasheet isn't much help).

My boss at work suggested using a Atmel's PWM feature and driving them that way, but I have no idea how to do that.

Any suggestions?

 

[MikeMl]

Google for "Buck Pucks"

 

[malfist]

Buckpucks are ridiculously expensive to buy, and I'd kinda like to learn how to build these things so I can understand it.

 

[mneary]

Cheap and reliable: Three LEDs in series will be (approximately) 8.7V. (This will vary among supposedly identical LEDs.) To operate on 12V with 700mA, you need a resistor of (12V-8.7V)/(0.7A) = (3.3V/0.7A) = 4.7 ohms. The resistor will be dissipating (3.3*0.7) = 2.31 watts. I would use ten 47-ohm 1/2 watt resistors in parallel. Be sure the LEDs and resistors are well ventilated. Use as many "three-LED-and-10-resistor" arrangements as necessary.

Elegant: Three LEDs in series, with the NPC3063 circuit you've linked. Adjust (R2 = 39k) so that the max output is 9V. For the 700mA current limit you change R1 to (0.2V/0.7A) = 0.28 ohms. I would use two 0.56 ohm 1/4 watt resistors in parallel. Use as many NPC3063s as necessary, each will drive 3 LEDs. If you choose this route, L1 might need to be recalculated. I would not suggest trying to drive more than 3 LEDs from one NPC3063.

 

[malfist]

Thank you for showing the math, now I can do it!

That will work perfectly.

 

[superfrog]

If you want linear regulation, a simple LM317 would probably be better.

If I were you though , I would go for T5 tubes, much cheaper and not that much less efficient. Some do manage efficiencies close to what to LED do.

You can get 1150mm tubes for growing, at 50W each for £9, if you are fine with normal tube, you can get then for £3. A mix of cold and warm standard tubes works alright.

They have better frequency distribution that what you can achieve with high power led's (unless you found 650-660 nm red high power led, in wich case I d be interesed). They also have much more distributed spectrum, making non chlorophyl related process work better.

 

[superfrog]

Ohh, I forgot to add, for a 100W two tubes setup you need an extra £ 30 ballast and about £10 of cable and end fixings.

I d love to see a led solution approach this kind of price tag

 

[malfist]

Yes, Florescent lighting is cheaper on the off set, but typically an LED setup will pay for itself in electrical savings in under a year.

If you want the 660nm, you can go with the 67-1610-ND, it's cost is $0.65 per lumen @ 3.12 bulbs per lumen. This is a normal LED, not a power LED so it'd be easy to maintain and power (but you'd need a lot of them)

I however, am goind with the**broken link removed**. It's dominate wavelength is 626, however it costs $0.04 per lumens @ less than 0.01 bulbs.

The 626 wavelength is needed by chlorophyll b, and has a 28% absorption rate.
The 660 wavelength is needed by chlorophyll a, and has a 50% absorption rate.

While the 626 produces light that is less usable, it produces enough to over compensate for the high cost of 660nm bulbs. Plus, chlorophyll b is more important for plant growth than chlorophyll a.

 

[superfrog]

Kingbright | Optoelectronics and Displays | Optoelectronics | LED | 1206 Dome Lens Package |KPTD-3216SRC-PRV is probably more realistic for 660.

Last time I checked, you could expect about a 1/20th of a watt from those, which mean that to get an equivalent let's say 50 watt, you need a thousand of them. Not realistic really. PCB is going to be a pain, soldering mistakes are bound to happen. Checking polarity hand soldering 1000 led is boud to be a pain. On top of that, you probably would want a 3Kv, 20mA switching power supply to go with that. They also seem to scatter half of their light toward the pcb. Low power smd led is not really an option. Leaded ones are alright, but I do not feel any urge to drill 2k holes in FR4 without a machine to do it.

As a side remark, lumens are based on human perception, you do not really care about lumen figures at far end of the spectum, they do not mean much to the plant. Manufacturers provide lumens as an indication of how much perceived light you would get from one of those. Ie, similar lumen valued led will look the same all other things equal, it does not mean they emit the same power. ( luminous flux vs radiant flux )
Many led manufacturer do not even provide lumen figures for anything to hagh or too low in wavelength but give emitted mW instead.

As for High power ones you can find really good blue ones, red ones are discussable for plants if you ask me.

I am not trying to tell you it is a bad idea, I am trying to tell you that it is a cost inneficient idea, even when you consider electricity. And killing your efficiency with a 70% efficient PC PSU and adding resistor losses on top of that will kill any advantage the LED had to start with. I have been ruminating this project for the past 6 month, and I am now pretty convinced T5 are a better deal overall.

 

[malfist]

I still have a few more questions about that sample circuit. What is the purpose of the inductor between the chip, and the load, is the 47uH the exact amount it needs, or can it be more/less?

What is the purpose of the resistor between the CMP_IN, and ground? Does it's resistance matter?

Is C2, the filtering capacitor for TC necessary?

 

[malfist]

One more question, would it be cheaper to buy a big transformer, hook it into the mains, convert to DC and power more than just 3 LED's per circuit?

 

[MrAl]

Hi,


Believe it or not, a resistor that is sized correctly can work something like a constant current generator when it is coupled with a constant source voltage like that which you would get from a PC power supply like an ATX. That would mean for maybe three LEDs you would need one resistor and that's it.

The reason this can work is because the power supply voltage doesnt vary much at all, and the LED voltages dont vary too much, although they do vary a little with temperature so this has to be checked. The idea is to keep a decent amount of overhead voltage so that the resistor keeps the current somewhat constant even though the LED voltage changes a little.

For example, with three 3.2v LEDs @ 700ma in series that would total up to 9.6 volts at 700ma. To drive this string with a 12v supply it would require a resistor of 3.4 ohms (approximate) at 1.7 watts so lets try a common 3.3 ohm, 10 watt resistor.
At nominal voltage the string would draw 727ma but lets look at the high and low voltage too...
At 0.1v low, the string would draw 818ma, and at 0.1v high the string would draw 636ma.
818ma is a little high, so lets try a 3.9 ohm 10 watt resistor...
With this new value we get currents of 615ma nominal, 538ma high, and 692ma low.
This means at nominal voltage we will be running the LEDs at slightly reduced current, which means extended life.
At the low voltage point we would get almost 700ma which would still be ok for these LEDs.
At the high voltage point we would get a bit less current so somewhat less brightness.

 

[mneary]

I still have a few more questions about that sample circuit. What is the purpose of the inductor between the chip, and the load, is the 47uH the exact amount it needs, or can it be more/less?

The inductor between the chip and the load stores energy when the chip is 'off' and provides it during the dead times. It's an essential part of current regulation. It doesn't have to be exact. If you're using this circuit for three LEDs at 700mA, its value probably needs to be different. Let us know and we can help calculate.

What is the purpose of the resistor between the CMP_IN, and ground? Does it's resistance matter?

R3 is part of a divider that sets the limit of the voltage if there are no LEDs. The ratio of R2 and R3 is important.

Is C2, the filtering capacitor for TC necessary?

In the article, the capacitor on TC would be C3 (2.2n). It is necessary.

One more question, would it be cheaper to buy a big transformer, hook it into the mains, convert to DC and power more than just 3 LED's per circuit?

A big transformer is probably more costly than a PC power supply of equivalent wattage, especially if you have an old one ATX PS looking for a home.

 

[Hero999]

If you want linear regulation, a simple LM317 would probably be better.

If I were you though , I would go for T5 tubes, much cheaper and not that much less efficient. Some do manage efficiencies close to what to LED do.

You can get 1150mm tubes for growing, at 50W each for £9, if you are fine with normal tube, you can get then for £3. A mix of cold and warm standard tubes works alright.

They have better frequency distribution that what you can achieve with high power led's (unless you found 650-660 nm red high power led, in wich case I d be interesed). They also have much more distributed spectrum, making non chlorophyl related process work better.

Many florescent tubes are more efficient than most LEDs.

In this case a fluorescent tube run from an electronic ballast is going to be more efficient than LEDs run from an inefficient series resistor.

 

[malfist]

I am not using the resistor plan, slight variances in temperature can cause large fluctuations in current, which will easily destroy these LEDs. Resistors would be a good idea if I'm only powering the LEDs for a short while and they don't have time to heat up, but I'm going to be powering them for 16 hours at a time.

The henry value, how would I calculate it?

If the ratio between R2, and R3 is important, how would I calculate it too?

 

[malfist]

Many florescent tubes are more efficient than most LEDs.

In this case a fluorescent tube run from an electronic ballast is going to be more efficient than LEDs run from an inefficient series resistor.

Can you show me how you arrived at that conclusion?

 

[MrAl]

I am not using the resistor plan, slight variances in temperature can cause large fluctuations in current, which will easily destroy these LEDs. Resistors would be a good idea if I'm only powering the LEDs for a short while and they don't have time to heat up, but I'm going to be powering them for 16 hours at a time.

The henry value, how would I calculate it?

If the ratio between R2, and R3 is important, how would I calculate it too?

Hi again,


I would not have even suggested that method for a second if i didnt know it would work.
The calculations i was showing you take into consideration the variation of the nominal
voltage of the LED, that's why i mentioned the three different current levels.

Also, i have run and am running some Luxeon LEDs 24 hours a day 7 days a week using
this method and some of them have been running several years, like now going on five
years. That's quite a lot of hours of CONSTANT run time, not just 8 or 16 hours a day.

Of course i am not saying for one minute that a constant current source isnt a good idea,
it's a very good idea. The resistors are just cheaper, require less parts, and dont
break down over time.
There are simpler transistor circuits too if you want to look at those.

 

[malfist]

For example, with three 3.2v LEDs @ 700ma in series that would total up to 9.6 volts at 700ma. To drive this string with a 12v supply it would require a resistor of 3.4 ohms (approximate) at 1.7 watts so lets try a common 3.3 ohm, 10 watt resistor.
At nominal voltage the string would draw 727ma but lets look at the high and low voltage too...
At 0.1v low, the string would draw 818ma, and at 0.1v high the string would draw 636ma.
818ma is a little high, so lets try a 3.9 ohm 10 watt resistor...
With this new value we get currents of 615ma nominal, 538ma high, and 692ma low.

The calculation for this is (Source Voltage - LED(s) voltage) / Amperage = Resistance, correct? Basic Ohm's law. How did you calculate the wattage, and how do you now what the voltage change would be when they heat up?

 

[MrAl]

Hello again,


For the example i gave i used an estimate of 0.1v plus and minus.
For your actual setup, you should start with that and then *measure* the actual change in voltage
when you run the LEDs and adjust accordingly.

BTW, how many strings do you intend to build up?

 

[superfrog]

for the sake of simplicity, let's suppose your leds run 1A. 3W LED like that are resonably common.

You are getting 12V from your power supply, with resistor it is a series circuit everything has the same current flux going trough it, you are pulling 1Amp@12V form the psu. That is 12W in teletubbies world.

In real world that means pulling 12W/efficiency from your mains. A (very) good PSU might reach 85-90% overall at its sweet spot on all rails. Pulling just an amp from a single 12V rail, it will probably mean something in beetween 35% to 65% efficiency as the rail is both massively under the expected load and the load completely unbalanced (your 5, 3.3 and al the other one are unused). You will possibly get less that that on dodgy PSU, I'd be very surprised by more on anything.
In real world, at low drain, psu's tend to be massively over the target voltage, specially cheap ones. I have seen 12V rails at 13.5, it is not good, but it is usually not the end of the world, specially as under load the voltage sags to something more acceptable. So in reality you have 13W, that cost you beetween 20 and 37W to produce, and all that goes to actual light beeing produced using only using 9W. IT is not going to be 9 watts of light as the LED are going to heat a fair amount, and produce useless far infrared (unless you plan to run your LED at 900C)

Your LED are probably around 3V fw, with a range in beetween 2.8 an 3.4 or something in this ballpark. so you can only fit 3 in series if you want full power, giving you a range of 8.4 to 10.2 V . You need to dissipate the rest somehow, trough a resistor. Basically it is a badly inneficient PSU to power a not very efficient solution.

Any linear regulation will be inneficient for driving LED, unless your LED forward voltage can be guranteed to be very close to the PSU voltage.
As you cannot guarantee PSU output voltage and LED forward voltage spread is relatively high, you need to have quite a lot of safety margin, which equates quite a lot of dissipated energy.

A ballast that is designed just for the purpose of driving a 54W tube is likely to be more than 90% efficient. The tube itself is less good than led at producing useful energy for the plants, but with your 635nm led you are quite off anyway. That is how I reached the conclusion.

In my case I was planning on strings of 120 low power led with a linear regultor (mosfet + adjustable voltage reference) driven straight from rectified and filtered mains. The cost of putting together this system makes it usefulness dubious at best.

Hope I helped