How do I keep a LED glowing constant from a variable DC voltage

Thread Starter

rigerman

Joined Apr 25, 2014
40
My problem is to keep a LED glowing constant as the output of my circuit varies from 2.9V to 10V. The LED now glows according to the output voltage. I would like to keep it glowing constant. I have tried using LM317T but, for the range 2.9V to 3.7V, the LED remains off as LM317T needs a min voltage of 3.7V at Vin.

Any suggestions?
 

Thread Starter

rigerman

Joined Apr 25, 2014
40
I think I need around 2.1v constant at LED pin no matter what the output voltage is. (2.9 to 10) Is there any sort of regulator for it?
 

Alec_t

Joined Sep 17, 2013
10,592
Here's a simple circuit which will give the appearance of a reasonably constant light. In this example, the LED current varies from about 9mA-12mA for the 2.9V-10V input range specified.
NearlyConstantCurrent.PNG
 

crutschow

Joined Mar 14, 2008
23,742
Here's a two transistor variation of Alec's circuit, which has the advantage of better regulation.
Its minimum voltage drop is <0.8V.
The current varies from 9.6mA to 10.7mA over a 2.9V-10V input range with a 2.2V LED, which likely will give very little change in the observed brightness.
The nominal current is ≈ 0.67/R1.

upload_2016-3-29_12-28-7.png
 

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Thread Starter

rigerman

Joined Apr 25, 2014
40
Here's a two transistor variation of Alec's circuit, which has the advantage of better regulation.
Its minimum voltage drop is <0.8V.
The current varies from 9.6mA to 10.7mA over a 2.9V-10V input range with a 2.2V LED, which likely will give very little change in the observed brightness.
The nominal current is ≈ 0.67/R1.
Thanks. It works!

I am having a slight problem with the V at 2.9v. The LED dims down significantly. I measured the voltage at it's pins. It is around 1.43V. My output voltage also drops at times to 2.3V which might be causing the issue.

I will provide some more feedback once I stable the output voltage.
 

RichardO

Joined May 4, 2013
2,268
You never said what kind of LED you are using. For instance, a red LED might only need 1.7 volts to light to maximum brightness. A blue or white LED might take as much as 4 volts to be at full brightness. Other colors of LED's require different voltages.
 

mcgyvr

Joined Oct 15, 2009
5,394
You never said what kind of LED you are using. For instance, a red LED might only need 1.7 volts to light to maximum brightness. A blue or white LED might take as much as 4 volts to be at full brightness. Other colors of LED's require different voltages.
aka see the "forward voltage" rating in the datasheet of the specific LED you are using
 

Thread Starter

rigerman

Joined Apr 25, 2014
40
My output voltage is dropping to a stable 1.9V. So if I can provide around 1.85V to the LED constant, I think the glow will be stable. The V range is 1.9V to 10.1 V.
 

crutschow

Joined Mar 14, 2008
23,742
My output voltage is dropping to a stable 1.9V. So if I can provide around 1.85V to the LED constant, I think the glow will be stable. The V range is 1.9V to 10.1 V.
Below is a CC circuit that requires a minimum operating voltage of only about 0.2V above the LED forward voltage to operate, which is likely about as low a dropout voltage as is practical.
It uses a low voltage op amp that will operate down to 1.2V, single supply, and has a built in 0.2V reference.
The two 10kΩ resistor dividers reduce the reference voltage to 100mV , giving a constant-current of 100mV/R1.

upload_2016-3-31_1-6-34.png
 

Thread Starter

rigerman

Joined Apr 25, 2014
40
I am sorry for the delayed reply. I will test the circuit (I do not have the opamp in my arsenal), and feedback.

Thanks a lot for your time...
 

John P

Joined Oct 14, 2008
1,780
It seems simplest to just make the circuit's output turn on a transistor, and use it as a switch to power the LED from some other source.
 
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