Spectrophotometry very stable current light source circuit

[smilem]

There's no point in using a tight tolerance high stability resistor when the LED has very wide tolerance and a poor stability.

Where did you read that my particular LED has very bad stability over temperature? Did you read that it will be on for serveral seconds at a time?

Thanks.

However, the MAX device has it own stability rating over temperature which adds to the stability (or instability) of the resistor, so for adequate overall stability you subtract the stability of the MAX (5ppm/deg C) to get 15.8 ppm/deg C for the resistor: 20.8ppm-5ppm=15.8ppm

My resistor 330ohms was 15ppm like you say, so I just change it to 620ohms (because that is a standard value) or should you suggest to use two resistors to get exactly 625ohms?
I thought that if I use u resistors in parallel it would decrease stability no?

 

[ccurtis]

Smilem, connecting two resistors having equal ppm stability in parallel will result in an equivalent resistor having the same stability.

 

[Hero999]

This thread is getting too long to read every single reply.

LEDs dim as they age..

Their Vf changes with temperature, at higher temperatures it's lower. If the current is kept constant then the LED's power dissipation will decrease as its temperature increases. LEDs are dimmer and less efficient at higher temperatures.

If you want a stable light source then using a precision resistor will not help. You need to have a feedback loop to adjust the current to keep the brightness constant.

 

[ccurtis]

If you want a stable light source then using a precision resistor will not help. You need to have a feedback loop to adjust the current to keep the brightness constant.

The current is keep constant, but your claim was that it doesn't matter anyway because the light output varies with temperature, even if the current is constant. Are you now saying that the circuit does not keep the current through the LED constant, on top of that? I'm confused. The circuit has a feedback loop to adjust the current "to keep the bightness constant".

 

[Hero999]

The current is keep constant, but your claim was that it doesn't matter anyway because the light output varies with temperature, even if the current is constant.

Correct.

Are you now saying that the circuit does not keep the current through the LED constant, on top of that?

No, I didn't say that. Your circuit does keep the current constant regardless of the forward voltage of the LED.

I'm confused. The circuit has a feedback loop to adjust the current "to keep the bightness constant"

But it won't keep the brightness constant over the temperature range and lifetime of the LED. A constant current source just ensures that the brighness won't vary with the input voltage.

You need optical feedback to ensure a constant intensity regardless of the temperature and age of the LED. You need a photodiode or LDR to monitor the light level and increase the current if it's too dark and decrease the current if it's too bright.

 

[smilem]

You need optical feedback to ensure a constant intensity regardless of the temperature and age of the LED. You need a photodiode or LDR to monitor the light level and increase the current if it's too dark and decrease the current if it's too bright.

Hmm, interesting. Thanks for your input.

I have searched for "temperature compensated led circuit" and found tons of patents but no circuits I can use with ccurtis circuit that keeps the current very stable BTW

What photodiode I can use if my LED is UV LED 370nm? Are they very expensive?

Also the chamber that I put my sample for measurement contains the LED since it focuses light very well I did not use any lens on it. If I would use sensor photodiode then I must use some sort of tricky optics because the sample being illuminated give move light by fluorescence than my LED.

So if the photodiode is just placed at an angle to UV LED it will get false brightness increase. Or perhaps there are sensors with narrow field of vision say 370nm only?

 

[Hero999]

I've seen UV photodiodes before, I'm pretty sure PS components stock them.

This is the only circuit for constant brightness I've seen.
https://www.electro-tech-online.com/custompdfs/2009/06/LM138.pdf

It's for an incandescent but I'm sure a similar thing can be done with a BJT and a phototransistor to control the brightness of and LED.

I don't know how temperatuee stable it is though.