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

#### 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?

#### 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?

#### crutschow

Joined Mar 14, 2008
26,062
You can use a low dropout regulator, such as the LT3080 (350mV dropout), in a constant current configuration (see data sheet).

#### Dodgydave

Joined Jun 22, 2012
9,874
You could use a Jfet constant current circuit, but your led will need 2.1v and the fet or regulator will need 1v.

#### Alec_t

Joined Sep 17, 2013
11,814
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.

#### crutschow

Joined Mar 14, 2008
26,062
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.

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#### 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
2,271
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

#### crutschow

Joined Mar 14, 2008
26,062
What is the voltage at the collector of the transistor when the input voltage is 2.9V?

#### dannyf

Joined Sep 13, 2015
2,197
from 2.9V to 10V.
The 2.9v can be hard to hit, depending on the leds used. A jfet CCS or a CCS-built off a LDO may be your best chance.

Or a boost converter.

#### 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
26,062
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.

#### 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
1,864
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.