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Topic: LM317T Driving me nuts (Read 5 times) previous topic - next topic

SgtOneill

LM317T is driving me nuts.

Power Supply 5v 2Amps
LED pulls 3.6v and 1Amp (works fine with 1.5amps)

I want a simple voltage drop, from Whatever voltage input (from 1.2v to 35v according ot the datasheet) to 3.5 / 3.6v output.


This doesn't work. Should be vOuting 3.5volts or so. Its fixed at 1.25v no matter what.



This works but it seems insane to leave the Adj pin disconnected from everything...




Any idea what might be wrong here?

dshay

#1
Jun 30, 2011, 05:56 am Last Edit: Jun 30, 2011, 05:58 am by dshay Reason: 1
A) Please provide values for R1 and R2

B) Please provide the datasheet's recommendation for nominal current on that LED.

Your problem lies with B. You need a current limiting resistor.

If you need help choosing one, kindly visit the following website:

http://led.linear1.org/1led.wiz

You'll need to plug in values for Voltage Source, LED Forward Voltage and LED Nominal Current, latter two you will get from the LEDs datasheet.

SgtOneill

#2
Jun 30, 2011, 06:32 am Last Edit: Jun 30, 2011, 06:50 am by SgtOneill Reason: 1
R1 = 220ohm
R2 = 470ohm

(this should make vOut 3.7v)
I used this circuit and calculator: http://www.whatcircuits.com/lm317-calculator-v2/

LED Datasheet http://www.seoulsemicon.com/en/product/prd/zpowerLEDp7.asp
3.6v 2.8amps
3.6v voltage drop


The weird thing is that, the LED lights up if i dont ground the ADJ pin, making the 470ohm useless. So the ADJ and vOUT pins are connected with teh 220 resistor, the LED is being fed by the vOut pin with 3.x volts (not sure how) and grounded.
I tried 2 LED's in series and the vOut pin cranks to 6volts wich seems correct but... should I be the one choosing the voltages?

im confused =)

MarkT

That regulator has about 2.2V drop-out at 1.5A, so the greatest output voltage you can get from 5V input is 2.8V

Also you are missing decoupling capacitors - regulators in general need them to prevent oscillation.
[ I won't respond to messages, use the forum please ]

dshay

#4
Jun 30, 2011, 06:59 am Last Edit: Jun 30, 2011, 07:02 am by dshay Reason: 1

That regulator has about 2.2V drop-out at 1.5A, so the greatest output voltage you can get from 5V input is 2.8V

Also you are missing decoupling capacitors - regulators in general need them to prevent oscillation.


He's good on drop-out, his source is 9VDC.

Decoupling caps would be highly recommended but the problem here is likely thermal shut down, no current limiting resistor is going to put this voltage regulator into shut down mode immediately as it's going to peg at 1.5 Amps.

Reconsider your power source. I recommend an LED driver made for this.




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