Very stable power supply from USB to UV LED

Thread Starter

smilem

Joined Jul 23, 2008
162
Thanks for quick reply.

I agree that voltage doubling chip makes voltage chokes that defeat the whole idea of stabilized power supply. Did you forget that my LED needs 3.9v and LM317 has 3v drop so I need at least 7volts for my USB port.

Another problem with changing the circuit is that I have mesured about 150 samples using it and even with digital multimeter I doubt it's possible to match resistors I have now so.

It could be possible with multimeter with 4 digit 3.000volts etc. but those are very expensive. The cheap ones as you know have 10% or 15% accuracy range - so having 3 digits like 3.00volts has no sense as it would mean that there can be 0.3volt precision lag.

I read that there is MAX 660 and MAX 665 chips:

MAX 660

Converts voltage from +1.5 to +5.5 V to −1.5 to −5.5 V without any chokes, only two capacitors are required. Can also be used as a voltage-doubler. The MAX 660 is a pin-compatible upgrade of ICL 7660


MAX 665

Converts voltages from +1.5 to +8 V to −1.5 to −8 V without any chokes, only two capacitors are required. Can also double the voltage.

Perhaps these would work?
 

SgtWookie

Joined Jul 17, 2007
22,230
PM from Smilem:
I posted a reply a day back but it's not showing as I get message "your post will be posted when moderator approves it"
This seems to be a relatively new board feature, and the Mods are so busy they may miss these approval requests.
Anyway, I posted that I can't change the circuit as you suggest because I made over 150 measurements and all of them would be lost as I doubt I can match the current precisely enough if I change the resistors.

Besides as you said LED needs 3.9V and there is about 3V drop from LM317L so I need at least 7 better 8 volts.
OK. The supply from the USB port is 5v. Your LED has a Vf of 3.9v @ 8ma.
Rlimit = (5v-3.9v)/8mA = 1.1/.008 = 137.5 Ohms.
You can use a 140 Ohm 1% resistor and be plenty close enough.

I found that there are some MAXIM chips

MAX 665

Converts voltages from +1.5 to +8 V to −1.5 to −8 V without any chokes, only two capacitors are required. Can also double the voltage

They are 10 times more expensive but they can convert without "chokes" I think it's what I need??
There's also the ICL7660, just requires a couple of caps; for a voltage doubler you'll need a couple of diodes too.

You don't need my help for that; suggested circuits are in the datasheet.

However, you are going to absurd lengths with this, and I'm not going to go another 10 replies helping you with this absurdity.
 
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SgtWookie

Joined Jul 17, 2007
22,230
By the way, I bought a couple of very cheap meters from Harbor Freight a couple of years back. They were $2.99 on sale.

Just out of curiosity, I tested one using a Fluke 5500 Calibrator that itself had just been calibrated. I was genuinely suprised how accurate my dirt cheap $2.99 meter was; within 1% of readings.

Accurate meters don't have to be expensive. Within 1% is good enough for general use - and even laboratory use.
 

Thread Starter

smilem

Joined Jul 23, 2008
162
It seems as you have misunderstood me from the beginning then. Sorry. My native language is not English so this is my fault.

As different power supplies have different level of stabilization and computer can have different load power fluctuates between +/-1volt.

I need a device to stabilize power for my LED on different power supplies (desktop, laptop, home computer etc.) computers I may have to connect it to.

Now I tried my current circuit board and got these readings:

In 12v - Out 3v - 10mA
In 7v - Out 3v - 9mA
In 5v - Out 3v - 3mA (I understand this particular measurement is wrong)

So It seems the device stabilizes volts and not Amps ?


I need it to work like this:

In 5v to 12v (ideal computer power output no load - it will never be like this)
In real life (4v-15v) input voltage boundaries.
Output always 8mA and always 3.7-3.9volts (don't care if it's 3.7 or 3.9 as long as its always the same) as stated in my LED spec sheet.

This way I can ensure always the same brightness of the LED. I'm not making UV torch here, I'm need precise illumination for measuring and plotting UV reflectance graphs with spectrophotometer.

See attached file how my graph looks like. If there is any increase or decrease in brightness my graph changes and I get different results. Now perhaps you can understand my paranoia to get stable output.
 

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SgtWookie

Joined Jul 17, 2007
22,230
OK, I see. It would have helped a great deal if you had said what you were doing in the beginning.

I don't know exactly how you are measuring the "voltage output", eg:
In 12v - Out 3v - 10mA
In 7v - Out 3v - 9mA
In 5v - Out 3v - 3mA (I understand this particular measurement is wrong)
but you SHOULD be measuring the voltage across the LED.

The LM317L when used as a current regulator has a minimum of a 3v "drop" across itself.
You say the LED you're using has a Vf of 3.7 to 3.9 at 8mA.
Therefore, you will need 3.9v + 3v = 6.9v minimum voltage supply to the LM317L, or the output current will not be regulated properly.

When you were supplying the LM317L with 12v, the current output increased, because the LM317L was getting heated up by the extra power dissipation. When the regulator becomes heated, the Vref will change, which causes a corresponding change in the output current. This is documented in the datasheet. You can help stabilize the temperature of the regulator by attaching it to a heat sink, and keeping the voltage supplied to the LM317 between 7v and 9v to reduce the power dissipation.
 

Thread Starter

smilem

Joined Jul 23, 2008
162
OK, I see. It would have helped a great deal if you had said what you were doing in the beginning.

I don't know exactly how you are measuring the "voltage output", eg:

but you SHOULD be measuring the voltage across the LED.

The LM317L when used as a current regulator has a minimum of a 3v "drop" across itself.
You say the LED you're using has a Vf of 3.7 to 3.9 at 8mA.
Therefore, you will need 3.9v + 3v = 6.9v minimum voltage supply to the LM317L, or the output current will not be regulated properly.

When you were supplying the LM317L with 12v, the current output increased, because the LM317L was getting heated up by the extra power dissipation. When the regulator becomes heated, the Vref will change, which causes a corresponding change in the output current. This is documented in the datasheet. You can help stabilize the temperature of the regulator by attaching it to a heat sink, and keeping the voltage supplied to the LM317 between 7v and 9v to reduce the power dissipation.
Yes I measure current and voltage like shown here:
http://www.sciencebuddies.com/science-fair-projects/project_ideas/Elec_HowToMeasure.shtml

As you say I can't power it using USB because it gives only 5v and I need "you will need 3.9v + 3v = 6.9v minimum voltage supply"

How can it be heated if I connect if for a few seconds to take the measurement? The LM317L is dead cold, and I'm using ultra low current how can it heat up?

The LM317T was made to attach to a heat sink but LM317L does not.

So what would you recommend in my case? I do need to connect the LED to laptop to make it mobile. And I need it to be stable repeatable as much as possible.

Since every computer PSU or USB port has different voltage based on load and other factors the LED power stabilizations should take care of this and provide only desired current as it directly influences LED brightness and if brightness is not stable the results will be flawed.

So is it possible to use different chip or something?
Thanks for your time and effort to help.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

I found this in my PM

Hi, could you help my with power stabilizer Hi, please see my old thread here:

http://forum.allaboutcircuits.com/sh...ad.php?t=13191

Since SgtWookie has refused to help me and I do need a way to make the led work from USB port to be able to use it on different computers I'm willing to try and make the stabilizer like you posted. Using SMD IC chip TPS61042DRBRG4

Could you help me to design the circuit to drive my UV LED. Could you help me pick other parts to make it work, I would like to use non SMD parts if possible.

I need to to get output of 3V at 8mA for my LED. My datasheet is here http://www.elfa.se/pdf/75/07500366.pdf

And this is how I'm going to use it:
http://forum.allaboutcircuits.com/sh...2&postcount=44

Thanks for any help.
I do not answer on PM questions.
They are behind closed doors.
I like to play it open in the FORUM.

Here is a smal idea for you.




The current will be round 15 mA, depending on the FET characteristics.
The voltage for the circuit may be between 4 and 30 Volts.

Greetings,
Bertus
 

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Audioguru

Joined Dec 20, 2007
11,248
LEDs get dimmer as they are used. It might affect the accuracy of your project.
Maybe the circuit will need calibration every so often.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

If you want the LED to keep the same intensity, you could try to make a controlling circuit that measures back the intensity.

Greetings,
Bertus
 

Thread Starter

smilem

Joined Jul 23, 2008
162
Hello,

If you want the LED to keep the same intensity, you could try to make a controlling circuit that measures back the intensity.

Greetings,
Bertus
I think this idea that I need controlling circuit that measures back the intensity is very good.

You say " The current will be round 15 mA, depending on the FET characteristics."

I need no more than 8mA because my measurements are currently made in that intensity.

"The voltage for the circuit may be between 4 and 30 Volts."

So that means it would run from USB port?

What about stability.
 

Thread Starter

smilem

Joined Jul 23, 2008
162
LEDs get dimmer as they are used. It might affect the accuracy of your project.
Maybe the circuit will need calibration every so often.
The led has a pust button that makes it ON only when taking measurements (few seconds) it can't get dimmer maybe after 10 years or so :)

I'm also thinking to put a switch in from of LM317L to be able turn off the whole circuit and use 9Volt battery instead.
 

Thread Starter

smilem

Joined Jul 23, 2008
162
Got some free time to put this thing together, I still have my question unanswered. How to modify this circuit to work from 5V (USB) power supply. It seems to be designed to work with 12V.

Also schematic shows 5V output "5v to circuit" do I need to connect it to multiple 5V "inputs" shown as empty circles shown as "5V"?

Also the LM324 is 14pin chip. What pins I need to connect to "+", "-", "R7"?
"1IN +" "pin3" connect to "+"
"1IN" "pin2" connect to "-"
"1out" "pin1" connect to "R7"
"GND" "pin11" connecto to ground.
 
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