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

Discussion in 'The Projects Forum' started by rigerman, Mar 29, 2016.

1. ### rigerman Thread Starter Member

Apr 25, 2014
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0
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?

2. ### rigerman Thread Starter Member

Apr 25, 2014
40
0
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?

3. ### crutschow Expert

Mar 14, 2008
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3,244
You can use a low dropout regulator, such as the LT3080 (350mV dropout), in a constant current configuration (see data sheet).

4. ### Dodgydave Distinguished Member

Jun 22, 2012
4,999
745
You could use a Jfet constant current circuit, but your led will need 2.1v and the fet or regulator will need 1v.

5. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,804
1,105
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.

6. ### crutschow Expert

Mar 14, 2008
13,042
3,244
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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7. ### rigerman Thread Starter Member

Apr 25, 2014
40
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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.

8. ### RichardO Well-Known Member

May 4, 2013
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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.

9. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,771
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aka see the "forward voltage" rating in the datasheet of the specific LED you are using

10. ### crutschow Expert

Mar 14, 2008
13,042
3,244
What is the voltage at the collector of the transistor when the input voltage is 2.9V?

11. ### dannyf Well-Known Member

Sep 13, 2015
1,825
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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.

12. ### rigerman Thread Starter Member

Apr 25, 2014
40
0
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.

13. ### crutschow Expert

Mar 14, 2008
13,042
3,244
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.

14. ### rigerman Thread Starter Member

Apr 25, 2014
40
0
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...

15. ### John P AAC Fanatic!

Oct 14, 2008
1,634
224
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.