Electrical – Effect on LED driven at a higher Voltage than maximum forward Voltage


My current PCB configurations are
(10 series 4 parallel)x3 in parallel making 10 series and 12 parallel I am using
Meanwell ELG-75-42 driver giving a voltage of 42V at 1800mA, and a power of 75W. It is a constant current + constant voltage driver.

While each LED is driven at 150mA dropping a voltage of 3.0V so a total of 1800mA and 30V,

LED specs
typical current-150mA
typical voltage-3.0v
maximum voltage-3.6v
As the voltage is going higher than maximum will it affect my LED's?

Best Answer

It is important to understand that an LED is a current driven device. This means that when designing an LED circuit, we are concerned with setting the right current through the LED. It then happens that the LED has a voltage across it when driven at its rated current. This voltage can vary from LED to LED and that is what you are seeing in your LED datasheet. It is saying that when you drive the LED with 150 mA, it will have a voltage maximum of 3.6 volts across it but more typically it will be 3 volts.

While we drive the LED based on current, the voltage across the LED is an important number because it tells us that the supply for the LED must be capable of developing that voltage in order to supply the required current.

When LEDs are placed in series, the same current flows through each LED. Since they are in series, the voltage drop of each LED adds. So if you have 10 in series, the current through them should be set to 150 mA. The voltage across the string will then be 30 to 36 volts. But it is still the current that must be controlled.

The supply you are using will only limit its output current to 1.8 amps. If the current is less than this, it simply acts as a 42 volt supply. This means that you will need to add series resistors in each series string to limit the current to 150 mA in each. Since the series string will drop 30 to 36 volts at 150 mA, the series resistor must drop 6 to 12 volts at 150 mA. A 3 watt 82 ohm resistor would do the job.