Hello,
This may be a foolish question but I am going to ask anyways. So I am driving 4 - 3 watt LED's in series with a 20 volt power supply using a constant current supply see the schematic. It works great for the 4 LED's and my application requires that I also use a single 3 watt LED (forward voltage 4.1 Volts). If I were to use the same constant current circuit and the 20 volt supply, am I going to have a toasted LED? Will it last long?

Thank You,

Mark

 

You will not toast the LED. It still has the same current flowing through it so the power dissipated remains the same. What _will_ toast is the FET since it will have more voltage across it with the same current as before which is more power. You are setting the current at about 4 amps. The FET will have about 16 volts across it (20 volts - 4 volts). 20 volts at 4 amps is 80 watts! The FET will need a large heat sink or the power supply voltage will have to be lowered. A switching power supply set up as a current source will be much more efficient than the current source you are using but it is more complex.

 

Ah excellent! I'm running it in PWM mode and the FET is on a heatsink. Out of curiosity do you suggest any commercial product or circuit that would be suitable? (The fet i'm using is rated @ 50 amps, didn't have the right one in Eagle so I just used the IRF instead)

 

Are you putting the fifth LED in series with the four, or in parallel?

Depending on the forward drop of the four, it may work in series, but the current through the fifth one would be the same as the other four.

In parallel, the fifth will steal all of the current from the other four, so they will not light up. The FET will now be dropping most of 20V, so it is likely to get much hotter than before...

 

Out of curiosity do you suggest any commercial product or circuit that would be suitable?

For a commercial suggestion I like meanwell LDD drivers (they have a 700mA, 1000ma and a 1500ma version I believe.. you can get it with wires/pigtails already attached or a PCB mount version.

 

The fourth diode needs to be controlled independently, so it is not in series.

Ah, that is great, those meanwell drivers would have been a better choice, although it doesn't look like they go up to the suggested 2.8 amps that I need for a string of these diodes.

Thank You for all of your help.

 

The fourth diode needs to be controlled independently, so it is not in series.

Ah, that is great, those meanwell drivers would have been a better choice, although it doesn't look like they go up to the suggested 2.8 amps that I need for a string of these diodes.

Thank You for all of your help.

You don't need 2.8 amps.. If the leds are rated for 750ma then you simply supply 750mA to that series string. It doesn't add up for each LED.