# Constant Current LED driver for "Varying Input Voltage"

#### AshwinNambiar

Nov 11, 2014
31
I need to design a constant current LED Driver of 1A to drive an LED array (Green) 3 in parallel and 4 in series.
"The input voltage will vary from 16 to 32 and irrespective of this a constant current driver is to be designed.

1.I had tried using linear circuits-LM317,where there was too much heat dissipation (owing to the drop created from "16-32" input).
2.I used a TIP41C transistor and at the base provided a 60mA constant current device(AL5809-60P1-7),to obtain a constant collector current (Ic).However, the current was not constant because of the varying supply voltage,which caused a variation in Hfe cause of variation in Vce.

I have now decided to design an switching circuit.Avoided so far because wanted to avoid EMI issues.
**I'll be glad to receive help/guidance /suggestions as to how to go about with the design of constant current driver with varying input voltage.**

#### cjdelphi

Oct 26, 2011
1,166
A tip31 to offload the linear regulator, and then a resistor.... 60ma? I thought you want 1amp?

Maybe i misunderstood..

#### cjdelphi

Oct 26, 2011
1,166
Replace the constant voltage 12v (sim says 14v) source with an lm317, then vary the voltage

(So your 12 - 30v into the lm317, the 12v out to the tip31)

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#### BobK

Jan 5, 2010
7,682
You can buy a constant current driver with those specs for a few bucks. And it will be switching, so no power dissipation problem.

Bob

#### AshwinNambiar

Nov 11, 2014
31
Replace the constant voltage 12v (sim says 14v) source with an lm317, then vary the voltage

(So your 12 - 30v into the lm317, the 12v out to the tip31)
Thanks.But had already tried that.But power dissipation issues.The remaining drop that happens 20V creates a power dissipation and my LM317 is severely getting heated up.I'am struggling with the design because of the "varying input voltage"

#### AshwinNambiar

Nov 11, 2014
31
You can buy a constant current driver with those specs for a few bucks. And it will be switching, so no power dissipation problem.

Bob
I'am looking to design one myself.So if you could help me with that.

#### cjdelphi

Oct 26, 2011
1,166
No, the lm317 won't heat up, it's offloading the current onto the transistors..

Look at the circuit i designed carefully

#### cjdelphi

Oct 26, 2011
1,166
At 50v the Transistor dissipates 15w

Ay 15v each tip will disspate 5w

The lm317 will barely get warm.

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