How do I power 1 watt leds, can these not be hooked up straight to a power source with an appropriate resistor?

I have on ebay a number of cheap 1 watt leds, please see the pictures below.  Some look like large LEDs with perhaps thicker then normal anode and cathode leads coming out of them, others look like they are surface mounted onto a circular, flower shaped circuit board with possible multiple soldering point to apply power.  I am hoping to power 24 of the 1 watt LEDs off a modified computer power supply (I use these for powering everything that needs a bit more amperage).  LEDs are Forward Voltage: 3.2V~3.4V  Forward current: 350mA.  In some of these adds they refer to them also being able to supply the driver, is this a term for a power supply?  I was planning on running it off the power supplies 5v out with an appropriate resistor.  The all share the same forward current, while the forward voltage varies from 300mA to 350 mA.  Can I treat these as regular LEDs, mind you they will get quite hot I assume, or am I missing something?

Picture of How do I power 1 watt leds, can these not be hooked up straight to a power source with an appropriate resistor?
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I think the secret to understanding LEDs is the realization that they want constant current, not constant voltage.

You can buy LED driver circuits in module form, and essentially what this "driver" circuit does is regulate current. There is a current sensing resistor, and the circuit watches the voltage on this resistor, and through the magic of negative feedback, the current (or time averaged current) is kept constant.

The trouble with high powered LEDs is that if you try to supply them with constant current using a linear current regulator, even or just a resistor, you wind up wasting a lot of power, specifically Pwasted = ILED * (Vsupply - n*VLED) , for a string of n of the same color LEDs.  As you can see this power loss gets worse with increasing LED current. 

Also looking at that equation you might be tempted to somehow make Vsupply exactly match the voltage on your series string of LEDs, so that the difference is zero.  But I must warn you this is folly.  It is basically the same as assuming you can run your LEDs from a constant voltage supply, which is wrong, wrong, wrong!  Remember LEDs want constant current, not constant voltage.

Besides there is a better way, and that is by using a switchmode regulator.  The way that works is your feedback signal, from the current sensing resistor, controls the duty cycle of a transistor that is being switched on and off.  Then the time-averaged current flowing through this transistor can be controlled, and made constant.

That's usually what you get when you buy an LED driver module, some sort of switchmode, constant current, regulator.

I actually built a switchmode constant current regulator for driving high powered LEDs, and I have the circuit diagram around here somewhere. Although it may turn out to be cheaper/easier to buy these regulators in module form.

Anyway you'll need one such regulator, or "driver", for each series string of LEDs.  Also there will be some upper limit on the supply voltage for that regulator, and that upper limit determines the maximum number of LEDs per string.
+10 + one free internets.

For the not electronically inclined: http://www.dealextreme.com/search.dx/search.led%20driver has LOTS of battery source and line voltage source drivers for cheap. There's no other good way to drive the mammoth 100Watt leds they sell.
iminthebathroom (author) 7 years ago
Thanks everyone!
seandogue7 years ago
Technically, they should used used with a control circuit, rather than a passive element like a resistor. The control circuit is basically a constant current source, but they often also have temperature feedback as well, since high power LEDs are susceptible to thermal failures .
iminthebathroom (author) 7 years ago
I really should first search instructables before asking certain questions, found a couple right after hitting the publish button. Maybe someone has some good tips though. The LED's/drivers will be mounted on a sheet of aluminum so that could act as the heat sink, anything else other then perhaps mica that will allow heat transference yet stop current from flowing into the aluminum? And I guess the biggest question, would I need more then 1 driver for all 24? This does seem a little much for 1 driver but...