# This LED was sold to me as 5 Watt LED, what's its voltage rating?

I've go this LED

Polarity is marked but no rating is written on its body. It's claimed to be 5 watts.

Has anybody worked with this? What's the voltage rating?

• Read the data sheet if it has one. If it hasn't then use an ammeter and a voltmeter and gently raise the voltage on a power supply upwards (bit by bit from 0V) until volts x amps = 5 watts. If it burns before 5 watts is reached then it serves you right for buying LEDs without data sheets from crappy sources. If you have a data sheet then provide a link if you don't understand it. – Andy aka Nov 8 '15 at 14:36
• Before you apply watts of power to it, mount it to a heatsink (5C/W or thereabouts or it WILL burn up quickly. – Brian Drummond Nov 8 '15 at 14:58
• The seller should tell you. Otherwise google "5W led" and hope you find a visually similar one that happens to have the same electrical characteristics. It worked once before... but for 5W leds, there are lot more possibilities. Actually yours looks like a 3s3p one, which could narrow the search down a bit. – Fizz Nov 8 '15 at 15:04
• looking by the arrangement inside the LED, looks like 3 groups of 3 LEDs in series, so I'd guess 5W/3 = ~1.7W, 3.6Vforward*3 = 10.8V, Power = V.i, ~1.7 = 10.8*i, i = ~150mA per group, so ~450mA total at 10.8V (10.8*0.45 = 4.86W) – Wesley Lee Nov 8 '15 at 15:52
• You are asking the wrong question. An LED needs to be driven with a current source, not a voltage one. This will control the voltage to achieve a desired current through the LED. (When you use a resistor with a small LED you are approximating a current source) A starting point for figuring the current would be the wattage divided by the forward voltage measured in operation. You might read some data sheets for other similar parts, look at driving circuits, and experimentally put small currents through it with a current mode power supply or else a series resistor. – Chris Stratton Nov 8 '15 at 16:36