"Fred McKenzie" wrote in message
...
In article
,
karotto wrote:

Trying to power a 1W Luxeon star LED. It says: Forward Voltage: 3.5V,
Forward Current 650ma. So I tried a 3.6V battery with a minimal
resistor. Very low current. Then I tried without the resistor but
still only about 100ma current. In order to get 650ma to flow through
the LED I have to raise the voltage to about 11V and of course I
already blew the LED. So... how can I get 650ma while only using 3.5V.
Seems like my battery has too much internal resistance. Thanks much
for your help.

Karotto-

Your use of 11 Volts suggests you may have connected the LED backwards!

There is something wrong with your interpretation of the specifications.
It is a one Watt device, but you seem to be trying to make it burn 2.275
Watts (3.5 Volts times 0.65 Ampere). I think the 3.5 Volts and 0.65
Amps are peak or maximum values, not for continuous use.

An LED is a diode. Voltage drop is just that, the Voltage measured
across the device when it is conducting in its forward direction. You
apply a current and measure the Voltage drop, not the other way around.
For a 1 Watt LED, I would expect around 3 Volts drop for 1/3 Ampere of
current (3 Volts times 1/3 Ampere equals 1 Watt).

To use a 3.6 Volt battery in theory, the series resistance would be 1.8
Ohms (0.6 Volts divided by 1/3 Ampere). With this approach, the light
output will dim fairly rapidly since the internal resistance of the
battery increases as it drains.

Use of a higher voltage with the LM317 constant current arrangement
suggested by Arfa, is probably the best approach. Just make sure you
don't set it for too high a current!

Fred

In general, it's not so much about applying a current and measuring the
voltage with LEDs, although your calculations are of course all correct. The
forward voltage drop on a LED is what it is - i.e. the voltage will be
pretty much constant irrespective of the current which that voltage causes
to flow. A bit like a zener diode. The actual voltage varies a great deal
between colours of LED, and even within colours, depending on LED power and
other factors. The key is that as long as you apply enough voltage to exceed
the forward voltage requirement for the LED in question, with a sensible
margin available, then that *actual* applied voltage is arbitrary, the key
requirement then being that the current is restricted in some way, to give
the desired light output / life expectancy, without exceeding the
manufacturers maximum figure for continuous (DC) operation, or pulsed drive.

It also has to be remembered that high power LEDs generate quite a bit of
heat, and this needs to be removed fairly efficiently. If the LED is allowed
to heat up exessively, its life will be considerably shortened. Its forward
voltage drop will also vary with temperature, so if you are feeding it with
a simple resistive current limiter, and running it close to max spec, you
might run into additional problems with the current increasing further than
you intend. Which is why it's an all-round better solution to drive with a
simple constant current source, as Fred agrees.

Remember also, that if you are intending to drive multiple examples of the
same LED in some kind of array, the normal way to do this is to put them
into series 'strings'. So, as an example, the LEDs you are using here have a
quoted forward voltage drop of around 3.5v. Lets say that you wanted to run
five of them at perhaps 150mA. So, multiply 3.5 by 5 to get 17.5v. Add a bit
of overhead to allow the LM317 to work, and call it 21v or so. A 15v
transformer with a bridge and decent sized filter cap on the end would be
just about right. Set the resistor values to give the 150mA, hook the five
LEDs in series, and away you go. You might need a small heatsink on the
LM317, but provided you don't go wild with the voltage that you're starting
out with, or the drive current that you're asking it for, the device should
not dissipate a lot of power.

Each LED in the string, will develop its own forward voltage drop, and this
may well be slightly different for each example, and may vary slightly
differently for each example as the temperature rises. No matter. The LM317
will adjust its output as required, to maintain 150mA through the string.
You don't have to worry about voltages, or matching currents.

If you wanted more than five LEDs, you just put more in series, and raise
the driving voltage appropriately, taking care of course, not to exceed the
maximum ratings of the LM317, and not to go to a level that could be
dangerous if touched. It is possible to parallel up strings to drive bigger
numbers of LEDs at lower voltages, but this requires some slightly more
complicated balancing arrangements, and protection against the current
through the remaining strings increasing, if one string fails by an LED
going open.

Arfa