# Very stable power supply from USB to UV LED

Discussion in 'The Projects Forum' started by smilem, Jul 23, 2008.

1. ### smilem Thread Starter Senior Member

Jul 23, 2008
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0
I'm I'm trying to find out how to make a stable power supply for UV LED.

It should be able to feed the LED from destops and laptops and remove any fluctutations that can influence LED brightness.

The UV LED will be used with spectrophotometer to measure various papers for UV brighteners etc.

The LED I'm going to use is:

OA-260019 LED UV 5mm (781)
http://www.elfa.se/pdf/75/07500366.pdf

I know the basic electronic parts etc but I'm not very experienced so please bear with me. Thanks with for your time.

2. ### SgtWookie Expert

Jul 17, 2007
22,183
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The easiest way to make a constant current source is using an LM317 or LM317L with a resistor from the output to the adjust terminal, and take the current output from the adjust terminal.

Iout = 1.25v/R
Alternatively, R = 1.25v/Iout
Since you need 15mA, R = 83.333... Ohms.
You could get close to that resistance by using a 75 Ohm and 8.2 Ohm resistor in series, or 75 Ohm and 9.1. It depends upon how close the tolerance of your resistors are.

You'll need to determine the wattage required of your resistors.
P = E^2/R, or Power in Watts = Voltage Squared / Resistance in Ohms.
Voltage = 1.25
In this case, since the value of resistance is so large, even 1/10 Watt resistors will be adequate.

One problem is that your uV LED's have a typical Vf of 3.9. The LM317/LM317L will have around a 3v drop across them, which means you'll need at least 7v to supply your LEDs. If you're planning on using a USB port to power your device, this isn't going to work - unless you have a DC-DC boost converter.

3. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0
Thanks for your quick reply, I think I could use 12v from PSU then.

Apr 5, 2008
15,806
2,389
Hello,

When you want to use the USB port for the led, there is a led-driver with buildin dc-dc converter.
See attached datasheet.

Greetings,
Bertus

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5. ### SgtWookie Expert

Jul 17, 2007
22,183
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The driver IC that Bertus found is great, but it's a QFN mount. Unless you're pretty experienced with making SMT circuits, it'll be pretty difficult to work with.

A Zener diode is for setting a voltage level, not necessarily for current control. As the voltage across the Zener device approaches the breakdown voltage, the junction begins conducting current. The voltage remains fairly stable for a reasonably wide range of currents.

However, that can change with temperature, so it really won't be stable enough for your purposes.

Jul 23, 2008
133
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7. ### SgtWookie Expert

Jul 17, 2007
22,183
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Yes.
It's actually on page 15 (shown at the bottom of the page) but Adobe Acrobat Reader shows the page to be 20.

Vref is normally 1.25 on the LM317 series.

A fixed resistor will be more robust than a potentometer.

Apr 5, 2008
15,806
2,389
Hello,

You should calculate a resistor for the LM317 so that the current is between 10 mA (recommeded in the datasheet) and 15 mA (maximum allowed continuis current).

Greetings,
Bertus

9. ### SgtWookie Expert

Jul 17, 2007
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I showed him how to calculate it in my first reply, and suggested ways to get the 83.333... Ohms he needs.

10. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0
Thanks for help.

I can get LM317T not LM317 will it work?

Apr 5, 2008
15,806
2,389
Hello,

It will work with the LM317T. The only differrence is the maximum current of 0.4 A in sted of 1.5 A.
Since you will only use 10-15 mA there will be no problem.

Greetings,
Bertus

12. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0
Thanks for your reply, I'll go building the thing, will post some photos

13. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0
Here is the photo,

I'll get my LED in a week will post an update then.

Apr 5, 2008
15,806
2,389
Hello,
You can check the current with a multimeter in the current measurement mode.
Put the multimeter in the place of the led.
In this way you are sure the led will not be damaged.

Greetings,
Bertus

15. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
It is difficult to tell by the picture which pins you have connected the resistors to.

You have a 25 Ohm resistor connected in series with an 8.2 Ohm resistor, for a total of 33.2 Ohms. If you have them connected between the ADJUST (top pin) and OUTPUT (center pin) then you will get 37.65mA current output. This will very likely destroy your LED within moments of connecting it, as that is far more than twice it's maximum rated current.

[eta] I misread the colors; what I thought was red is actually violet. In that case, you should have roughly 83.2 Ohms in those two resistors, which should result in 15mA current output.

You can check it's output with a multimeter on the milliamp scale, but I suggest instead to use an accurate 100 Ohm resistor between the ADJ terminal and ground, and measure the voltage across the resistor.
Since I=E/R, if your current regulator is accurate you should read exactly 1.5v across the resistor.
1.5/100 = 15mA

The advantage of measuring voltage across a resistor instead of current is that you are much less likely to blow the fuse in your meter.

Last edited: Jul 27, 2008
16. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0

INPUT: Computer PSU 12V

OUTPUT: 12V 17mA. (measured with 100ohm resistor as load)

The PDF spec sheet for my led specifies that I need from 10mA to 15mA so I need help

The output voltage is 12V without load and 1.3v with 100hhm reisistor.

Last edited: Jul 28, 2008

Apr 5, 2008
15,806
2,389
Hello,

There is current regulation. The voltage will adjust according to the current.
To make the current lower increase the resistance between the lm317 pins.
With 100 Ohms you will get 1.25/100 = 12.5 mA.

Greetings,
Bertus

18. ### smilem Thread Starter Senior Member

Jul 23, 2008
133
0
I donėt understand why nobody specifies LED resistance. I could connect a resistor instead (while I wait for my led and to avoid damage).

Then measure current and voltage- adjust if needed. But without specified resistance I can't do this. Or do I need to calculate if spec sheet. If so I don't know how.

19. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Well, either your reference voltage in that LM317 is quite high (1.4V) or the resistors you used are a good bit out of tolerance, or if you were measuring V across a 100 Ohm resistor, it may be well over 100 Ohms, or a combination of the aforementioned. In any case, you will need to increase the resistance between the OUT and ADJ pins by at least another 11 Ohms.

The easiest way to do that would be to remove the 8.2 Ohm resistor and replace it with a 20 Ohm resistor.

Last edited: Jul 28, 2008
20. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
The LED does not have resistance; it has conductance, which is the inverse of resistance.

The LM317, when set up as a current supply, will furnish regulated current at whatever voltage is required to establish the current (up to the Vlimit of the input supply, less the 3v dropout of the LM317 itself.)