Question about parallel connection and constant current driver

[Erasmus]

Just a small question : if you connect 2 LEDs with different Vf's in parallel to a, let's say 700mA, constant current driver, how hard will both LEDs be driven?

For the Vf's, let's say 3.51V@350mA for LED A and 3.72V@350mA for LED B.

What I once learned in school is that with this parallel connection the current will split into 2 but the voltage will stay the same. What will be the voltage in this case where both LEDs have different Vf's?

Thanks a lot!

 

[krizoid]

If you connect them in parallel the voltage across each LED Will indeed be equal. The current will split in such a way that the 700mA will flow but not necessarily evenly to each LED.

The Vf of a LED is not actually a constant it is a function of current. If you increase the Vf the current will also increase.

The Vf of a LED is often specified at a specific current so the Low Vf LED requires less voltage to attain the same current as the High Vf LED.

To fully answer you question you need to know the Vf as a function of current for each LED when you have that information you can find the Voltage that will cause sum of the current in each LED to your constant regulated current (700mA).

If the forward voltages are not well matched one LED will get a much larger portion of the current and will be not only brighter, but also as risk for damage from overcurrent and high temperature conditions.

A good way to solve this problem is to connect the LEDs in series so that currents have to be the same.

 

[freedom2000]

Please have a look here to learn a lot on current versus voltage for leds :
http://candlepowerforums.com/vb/showthread.php?t=72528

and also here : http://www.candlepowerforums.com/vb/showthread.php?t=175615

JP

 

[Mr Happy]

Very short answer:

When you are regulating current you need to put things in series. When you are regulating voltage you need to put things in parallel.

Ergo, with a constant current driver you should put your LEDs in series so they all get the same current.