5 pcs 10W High-Power LED 20x20mm, 10 Watt (10V / 1A), warm white

LEDs are not driven by constant voltage! If you would connect straight 10V to a LED specified as 10V, the resulting current could vary for example from 800mA to 1.2A, resulting in 8W to 12W power consumption. As you can see, 8W wouldn’t deliver the maximum brightness, and 12W would burn out your LED pretty quickly.

So, how to drive a LED property?

You will need a current-limited source with a higher voltage than the LED specification, for example, 12V-15V. Then you limit the current until you reach 10W on the LED. This could be 9V and 1.1A, or 11V and 0.9A, for example.

Not all LEDs are identical, not even LEDs of the same brand and model. When you connect multiple LEDs in parallel and then rise the voltage beginning from 0, you will see that every LED will come off at a slightly different voltage.

Since high-power LEDs require a high current, you will not limit the current with a resistor. The resistor would burn a lot of energy and could overheat and burn out. You will have an active current limiter, or even better a PWM-controlled power source.

The good news is: A power supply with an adjustable current limit is cheap and available, and so are special electronic LED transformers (it’s a driver – don’t call it a ballast).

For example, you want to run 3 of these 10W LEDs together in a light fixture:
– Connect them in series
– Get a LED driver with adjustable current and >32VDC output
– Turn the current to the lowest, connect the LEDs
– Measure voltage and current on the LEDs
– Crank up the current until the product of voltage on the LEDs and current through the LEDs equals 30W

You could achieve a similar solution by using a 3W resistor with 2-3 Ohm.