led power help pls

[paulcobra 1975]

I have just got a 10w 900mha led with forward voltage of 9 - 10v, I am looking to use this as an overtank light for a small fish tank. Do I have to run this of a proper led driver or can I use a 12v supply and a resistor? (Would it need to be 900mha? ) also if I needed to dim this in case of it being too bright how would I go about this? Or can anyone point me in the direction of a correct dimable driver? I have looked at drivers and seem to loose myself as I can find different mha but right volts or right volts but different mha??? And when I looked at dimmable that made it even worse.

 

[DIWdiver]

With a fixed input voltage, it's not unreasonable to use a resistor. You may have to experiment a little to get exactly what you want, but once you get there, it'll stay there a good while, if you got a good LED.

I'm going to guess that 10W is way too much for a small fish tank, and you'll want to use less than full power, but let's start there and work down. With 12V in and 10V on the LED, that leaves 2V for the LED. If you want the full 900mA (not mha) which is also 0.9Amps, you'd want 2V/0.9A = 2.2 ohms. Power dissipated in the resistor would be about 2V*0.9A = 1.8W. As luck would have it, 2.2 ohm 5W resistors are cheap and easy to come by.

But remember I said you might have to experiment? If the LED turns out to be only 9V instead of 10V, then you want 3V/0.9A = 3.3 ohms, and the power would be 3.3V*0.9A = 2.97W. As luck would have it, 3.3 ohm resistors are also common.

But I have a different suggestion. Get 10 pieces of 22 ohm, 0.5W resistors. You can buy these at Radio Shack for $1.49 for a pack of 5, if you are in the US. Any local place that sells electronic parts should have them. Or you can order them for a few bucks. Try just one, and see what the light looks like. If you want more light, tie another one in parallel (that means connect the resistors together at both ends). If your LED turns out to be 9V, it's safe to use up to 6, if it's 10V it's safe to use all 10.

Also, you'll need to put a good heatsink on the LED. At 10W it would DEFINITELY burn up in very short time with no heatsink.

 

[paulcobra 1975]

Brilliant, thankyou for the help.

With a fixed input voltage, it's not unreasonable to use a resistor. You may have to experiment a little to get exactly what you want, but once you get there, it'll stay there a good while, if you got a good LED.

I'm going to guess that 10W is way too much for a small fish tank, and you'll want to use less than full power, but let's start there and work down. With 12V in and 10V on the LED, that leaves 2V for the LED. If you want the full 900mA (not mha) which is also 0.9Amps, you'd want 2V/0.9A = 2.2 ohms. Power dissipated in the resistor would be about 2V*0.9A = 1.8W. As luck would have it, 2.2 ohm 5W resistors are cheap and easy to come by.

But remember I said you might have to experiment? If the LED turns out to be only 9V instead of 10V, then you want 3V/0.9A = 3.3 ohms, and the power would be 3.3V*0.9A = 2.97W. As luck would have it, 3.3 ohm resistors are also common.

But I have a different suggestion. Get 10 pieces of 22 ohm, 0.5W resistors. You can buy these at Radio Shack for $1.49 for a pack of 5, if you are in the US. Any local place that sells electronic parts should have them. Or you can order them for a few bucks. Try just one, and see what the light looks like. If you want more light, tie another one in parallel (that means connect the resistors together at both ends). If your LED turns out to be 9V, it's safe to use up to 6, if it's 10V it's safe to use all 10.

Also, you'll need to put a good heatsink on the LED. At 10W it would DEFINITELY burn up in very short time with no heatsink.