Need help achieving constant current

MrAl said:

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?

Click to expand...

3-6, possibly more.

How did you calculate the wattage dissipated by the resistor?

Hi again,


The wattage (power P) is calculated knowing the current and the value of the resistor as:

P=I*I*R

where
I is the current in amps, and
R is the resistance of the resistor in ohms


I also agree that a switching regulator is a better choice if you can put up with the
extra complexity of the design.

malfist said:

Can you show me how you arrived at that conclusion?

Click to expand...

The only time an LED is more efficient than a fluorescent is when the LED is taken in isolation and the power loss in the power supply is ignored.

Luminous efficacy - Wikipedia, the free encyclopedia

Most efficient LED lamp:
8.7 W LED screw base lamp (120 V): 10.1–13.6%

Most efficient fluorescent lamp:
T8 tube with electronic ballast: 12–15%

Also a professionally designed LED power supply is likely to be much more efficient than one designed by you. The chances are the power supply which powers your LED supply will be as efficient as an off the shelf LED driver, anything you add to it is only going to increase the power loss.

Sorry but I think you're wasting your time and you'll get better results with a traditional fluorescent grow lamp.

Hero, in this case the issue is a bit different. If you can emit discrete spectrum light that matches the absorbtion peaks of the plants you want to grow than you can be more efficient with led. The phosphors of the fluorescent tube makes it impossible to have sharp spectrum bands.

There is just a whole bunch of ifs attached to this statement.

- You need to get 660 and 460 band covered. 460 is possible to find in high power, 660 is not. All the red leds are far past the peaks.

- With a 80-90% efficient custom made smps, led would be a nice setup, specialy if it is a complement to natural light or a source of continuous spectrum light to cover lesser light related phenomenon in plants. You are not interested in lumens but by sum over the spectrum of the product of emitted power spectrum by the absorbtion function of plants.

- a good led based system is going to be much much more expensive. For 100W output, even supposing efficiency equal, the tube setup is going to cost less than £50, a led setup is going to cost 150£ at least.

You make some valid points but I have to pull you up on one thing:

The phosphors of the fluorescent tube makes it impossible to have sharp spectrum bands.

Click to expand...

I don't believe that to be true.

Phosphors do have sharp emission bands, that's the problem with fluorescent tubes: the sharp peaks at certain wavelengths give a poorer colour rendering index than an incandescent which emits a continuous spectrum.

Modern fluorescents are designed to emit as continuous spectrum as possible, as are white LEDs.

Specialist fluorescent lamps such as blacklights and growing lamps have specialist phosphors that emit sharp peaks.

Now I'd probably agree that LEDs are more efficient at long wavelengths because in a fluorescent lots of energy is wasted in converting UVC all the way down to red light but at short wavelengths they're certainly as efficient or more efficient than LEDs.

All this is moot unless he buys an efficient SMPs to power his LEDs rather than lashing together an inefficient power supply.

What about using something like this: Digi-Key - 102-1902-ND (Manufacturer - VLD24-700)?

What voltage should I supply this with? Will it step up/down the voltage for me?

It looks like the same idea as a buck-puck, yet a fraction of the cost.

I would supply it with about 30-34V and use it for about 8 LEDs in series. I would get the 30-34V from a 24VAC transformer with a full wave bridge and about 1000uF per ampere (IMO ripple isn't critical here).

It will step the voltage down, not up. Data sheet excerpt: "voltage range Vin is at least 1.5~4V higher than Vout"

What about using an LM317T as a current regulator?

malfist said:

What about using an LM317T as a current regulator?

Click to expand...

Suggested in post #6.

Since it's a linear regulator, its efficiency is the same as resistors (it'll get hot, and it's costly to operate). It does have the advantage of being insensitive to supply variation.

ah, I missed it. It would be more reliable than simple resistors though. Right?

A regulator is easier to attach to a heat sink, so I would tend to prefer it. A regulator also will provide a consistent current for variations in LEDs and when input voltage changes.

lm317 is going to need 1.25 in the sense resistor 2V for the regulator itself. It is quite a bit when you start from 12V.