Once I purchased a 10 watts LED Flood light for Rs. 450. The Ad was so attractive that it is capable of giving light similar to 40 watts Tube light with low current consumption. But the result was disappointing. So hacked it to see the power, current and voltage consumption. It is a 12Volt 10 Watts LED Flood light as mentioned in the pack. The result was very interesting. Nothing matches with theory, specification and real measurements. Generally, the 10 Watt White LED works on 12 V DC and requires 500 mA to 1 Amps current to get full brightness and Flood light effect. It has around 10V Forward voltage. It is usually powered by a high current LED driver that delivers 500 mA to 1 Amps current. Here comes the reality. Many LED flood lights use low current LED drivers or Capacitor power supply. LED Driver is a small SMPS with an output current rating. It must match with the current requirement of the LED. Cheap Capacitor power supply can give only 75 mA current per uF. So we purchase the lights by looking the specifications on the package and not realizing the actual condition of the light. To experiment with that Flood light, opened it to see what is happening inside.Secret revealed.
Connected the Flood light in a Power Meter. It showed that the load (10W Flood light) is 3.07 Watts and consuming 36 mA current. Is it the Power and Current of the Driver or the complete load? Needed more checks.
Opened the case of the Flood Light. There is a 10 watt LED directly connected to an LED Driver. LED driver is powered by 220V AC. The specification of the LED driver is
Input voltage – 170- 250 V AC
Output voltage – 2.5V – 12V DC
Output current – 250 mA (0.25 Amps)
So that driver can give maximum 0.25 Amps current for the 12 V LED. So what about its power?
P= V x I = 12 V x 0.25A = 3 W.
That was the Power Meter reading took initially. Power meter is not lying. It shown as 3.07 W.
Ampere meter was connected in series with the Driver and the LED. Ampere meter showed that, the LED is using 0.22 Amps (220 mA) current. Good. The driver specification is 250 mA maximum. Then what about the power of that LED as per theory?
P=Vx I = 12V x 0.22 A = 2.64W. Again that is similar to the earlier Power meter reading of 3.07 W.
So confirmed. The Power meter reading is for the complete load of Driver + LED. The Driver will also consume current. So 3.07W – 2.64 (LED Power) = 0.43 watts is that of the Driver.
So how can we say that LED Flood light is 10 Watts? Power meter shows that it is 3.07 watts. Measurement of current and theory says that it is 2.6 watts. Let us go further to find out the reason.
Removed the LED from the Driver and connected to a 12 volt 1 Ampere power supply without any current limiting resistor. Input voltage was 11.4 V at that time . The result is ….
The LED is consuming 0.6 Amps (600 mA) current. Good, nothing is between the power supply and LED to change the power. We can apply the power formula
P=V x I = 11.4V x 0.6 A = 6.84 Watts.
So if the LED is directly connected to 12 volt 1 Amps power supply, it is 6.84 W or simply 7 watts. We should consider some power loss also because the LED is too hot while connecting to high current. Let us consider that 3 watts as power loss.
So 7 watt (actual LED power) + 3 watts (power loss) = 10 watts.
So we can come to a conclusion. If the 10 watts LED is directly connected to 12 V , 1 Ampere DC power supply, its power rating will be almost 10 watts. If it is connected to a Driver that is giving only maximum 250 mA, the power will be 3 watts.
Hacking revealed the Secret! LED is a real 10 watts one but the Driver is for 3 watts LED. Got it. The 10 watt LED is giving the brightness of 3 watt LED.
Now let us see what is happening in the cheap LED Flood lights using Capacitor power supply. See the image below.
We know that the 10 Watts LED requires 500 mA to 1 Amps current to give maximum brightness. But even if the Capacitor power supply is an efficient one, it can give only 50 mA to 100 mA current per 1 uF capacitance. So the power supply using 1 uF capacitor delivers around 75 mA (taking the mid value). The output DC from the power supply will be around 20 V which drops when the LED is connected. Then the power at 20 V is
P= V x I = 20V x 0.075A = 1.5W
That means, theoretically, the 10 watts LED connected to 220V AC, with a dropping capacitor and a bridge rectifier, its power is 1.5 watts because the current is too low. But what about the brightness? Very low because the LED is getting only 75 mA in the place of its normal 750 mA current requirement to give the “Flood light “. See, 10 times lesser current than the required value.
So learned a lesson from this. To get a “Flood light” from 10 watts LED, either use 12V, 1 Ampere High Power LED driver or 12V, 1 Ampere Adapter using a Transformer. In this case, a series resistor is must. Fix the LED current as 600 mA. The forward voltage of the 10watt LED will be around 10 volts. So the value of the resistor will be
12 V– 10V / 0.6A = 3.33 Ohms
So use a resistor between 3.3 ohms and 5.6 ohms to get good brightness. Remember, the wattage of the resistor must be 10 watts. Use a good heat sink for the LED, otherwise the LED will damage soon.
Arrangement should be like this
See the LED with 5 Ohms 10 Watt series resistor is consuming 300 mA current. Input voltage is 11.5V and the Forward voltage drop of LED is 10.
11.5 – 10 / 5 = 0.3 Amps or 300 mA.
Use other resistors also for more or less brightness. Lower the value, higher the brightness.
3.3 Ohms – 600 mA
3.9 Ohms -512 mA
4.7 Ohms -425 mA
5.6 Ohms -357 mA
6.8 Ohms -294 mA
10 Ohms -200 mA