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Spectrophotometry very stable current light source circuit

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smilem

Member
Hi, I need help with DC power stabilization to power UV LED at stable current 7 or 8mA.
This LED will be used as light source for spectrophotometry (light measurement) in an experiment.
The power should be clean and very stable. LED will be connected to the circuit using 50cm simple unshielded cable with switch. The power supply should have it’s own power switch and should be able to be always on. Temperature fluctuations should not interfere with power output.

This current stabilization circuit should run from 5V.
Current should be stable enough to reach stability at the second number after the decimal point or better.

I tried simple circuit LM317L with resistor but it will not run from 5V (has 3v drop itself, and my led will not work then) and it's not temperature stable either.

Thanks.
 
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birdman0_o

Active Member
Get an LM7805 or something similar. It will produce 5V very steadily, all you need is the right current limiting resistor (find the voltage drop of the diode) 5V - diode drop = X 7ma* resistor = x x/7ma = resistor value...should be close to 200 depending on the led voltage drop. Are you just trying to distinguish mixtures based on spectral data? You won't be able to change the wavelength of the led
 

ronsimpson

Well-Known Member
Most Helpful Member
BirdMan
I do not agree.
You need a current source not a voltage source with a reisitor.
 

smilem

Member
Thanks for suggestions I really appreciate your help.

I will use this as my light source for fluorescence tests of various substances. The LED I use is an UV type. PDF is here with all specifications: http://www.elfa.se/pdf/75/07500366.pdf

“Are you just trying to distinguish mixtures based on spectral data? You won't be able to change the wavelength of the led”

Yes. I will plot 2D X, Y line graph. See attachment. Then compare the results. If the current is not stable then LED light source will fluctuate and I will not be able to compare samples as the data will be not accurate.

I know I will not be able to change wavelength. This UV light source will be used only for fluorescence tests. For other tests the spectrophotometer has it’s own light source, but it’s UV-cut so I’m trying to find a solution.
 

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ronsimpson

Well-Known Member
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The picture is a common current source.
I would short out the load resistor.
Open the collector connection and add the LED from +5V to Collector.
Maybe open up the base and add a 1k resistor.
Sense resistor...about 70 ohms.
Replace zener with resistor to make a 10:1 devider to get 0.5 volts.
Use low voltage op-amp, common mode input range must include (-) supply.

Your LED needs about 4 volts so the sense resistor must use up less than 0.5 volts.
 

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smilem

Member
The LM134/234/334 is nice, but what about accuracy and thermal stability? I did not fully understand that part of PDF. Remember I nedd to reach stability at the second number after the decimal point or better.

To get two decimal place precision you need .005ma/8ma=0.06% precision.

Check out the current source circuit in the datasheet for the MAX6126: http://datasheets.maxim-ic.com/en/ds/MAX6126.pdf
Did you mean this schematic? The datasheet contains many packages and type of this chip which one would I need?

It shows one transistor and one resistor connected. Could you help me choose these components too? I have no knowledge how to do it myself.

I apologize if my question seems stupid or something and the answer seem obvious.
My simulator microcap9 does not have this part so I can't check anything.
 

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ericgibbs

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hi smilem,

A simple 2N3820 JFET will give a very constant low current source.

Drive the Gate wth the output of a OPA that uses a 6V2 zener as Vref input.

EDIT:
Found an example of its use.
 

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unclejed613

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btw, a "secret" about zeners and thermal stability. above 6V, zeners use an avalanche effect, and their thermal coefficient is negative (the voltage will drop with temperature). below 6v, they use the zener effect, and their thermal coefficient is positive. anybody who wants thermal stability in a zener, always uses 6 volt zeners (actually 6.2V), since their temperature coefficient is very close to zero. the reason for this is the zener is bordering on both modes of operation, and the temperature coefficients cancel.
 

Tesla23

Member
The LM134/234/334 is nice, but what about accuracy and thermal stability? I did not fully understand that part of PDF. Remember I nedd to reach stability at the second number after the decimal point or better.
You can null out the tempco depending on what you want. You haven't stated a temperature requirement.

If you do something radical like googling "precision current source" you get
things like the ADN8810, this precision current source is designed to drive diodes, will vary less than 0.05% over a 25C change, has a programmable output current to 300mA (12 bit resolution) and operates from 5V.
 

smilem

Member
hi smilem,

A simple 2N3820 JFET will give a very constant low current source.

Drive the Gate wth the output of a OPA that uses a 6V2 zener as Vref input.

EDIT:
Found an example of its use.
Whould this be better than using MAX6126 circuit?
 

ericgibbs

Well-Known Member
Most Helpful Member
Whould this be better than using MAX6126 circuit?
hi,
Quick look at the MAX, its an impressive spec, I would say it beats the 2N3820.!

My only concern would be that your LED is not a stable Vfwd, versus temperature.

So driving the LED from a precision voltage source via a resistor would mean the LED current would change as the Vfwd of the LED changed.

A constant current source would in theory give a constant current drive to the LED over a range of Vfwd versus Tempr.

You have to determine if the LED light output varies with changes with Vfwd v Temp! or Ifwd v Tempr.!
 

Tesla23

Member
As your diode needs 3.9V I'd try to scrap the 5V requirement, you're always going to be pushing muck uphill with a sharp stick to get precision current sensing and control within the spare 1.1V, given you want something like 50ppm/C stability. I'd get some voltage to play with then you should have multiple options, using a precision op-amp and precision voltage reference for example should make it pretty easy to make your current source.
 

smilem

Member
Since I transfer all data from specrophotometer to PC I wanted to use USB to power it and my external LED light source too :confused: And on a laptop there is not 12V plug like on desktop.
 
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smilem

Member
Also how do I calculate resistor for LM134?

I calculated like this: 1.059/8=0.132375
So I need 132Ohm rezistor to get 8mA
I can pair 2 rezistor values are 270 ohms parallelled with 270 Ohms to get 132ohms.

Is this correct. With LM134 I calculated that from 18C to 30C (ambient air temperature change) voltage instability woud be 8mA vs. 8.01mA

And if I connect it to +5V, LED needs 3.9V, LM needs 1V. so I have 0.1V spare left :D

Still waiting for somebody to suggest parts for MAX chip.

Thanks for all your input.
 

ericgibbs

Well-Known Member
Most Helpful Member
Also how do I calculate resistor for LM134?

I calculated like this: 1.059/8=0.132375
So I need 132Ohm rezistor to get 8mA
I can pair 2 rezistor values are 270 ohms parallelled with 270 Ohms to get 132ohms.

Is this correct. With LM134 I calculated that from 18C to 30C (ambient air temperature change) voltage instability woud be 8mA vs. 8.01mA

And if I connect it to +5V, LED needs 3.9V, LM needs 1V. so I have 0.1V spare left :D

Still waiting for somebody to suggest parts for MAX chip.

Thanks for all your input.
hi smile,
A constant voltage source will not give a constant current if the LED has a forward voltage change with temperature .
 

smilem

Member
But why then they are called The LM134/LM234/LM334 are 3-terminal adjustable current sources, not voltage sources then?

Also what is difference between 134 / 234 / 334?
I will turn on the LED only while taking a measurement (a few seconds) it will not be always ON. It's the current source that has to be stable (I think).
 
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