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Old 08-14-2010, 09:45 PM #1
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Question LM317 Always outputting 168mA

I have a LM317 based DDL driver set up with two 1/2 ohm resistors in parallel to set the current to 500mA. Problem is, no matter what resistor(s) I use, the output is always ~160-170mA. Even if I take off all resistors it still outputs at 168mA... which is really weird. I have double and triple checked that I set the circuit up right, including checking the resistors for the correct resistance with my DMM and making sure I have the LM317 pinout correct.

Any ideas what's going on? My only guess is that my LM317 chip is not working correctly and I need a new one, but if anyone has experienced this or has insight into what might be going on, I would love the information.

In case this helps:
Using a test load of 3 1N400X diodes with a 1 ohm resistor.
Powered using 4 AA alkaline batteries. (4x1.5v = 6 v in) (lm317 takes 2.25 + .7 for the reverse polarity protection diode which leaves plenty for the 1.82 forward voltage my the diode.)
Ultimately going to drive a 100mW green module.


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Old 08-14-2010, 11:37 PM #2
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Default Re: LM317 Always outputting 168mA

Oh no! You have set your driver to work at 5 Amps! It MIGHT be dead. You need 2.5 Ohms to set it to 500mA!
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Old 08-15-2010, 12:47 AM #3
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Default Re: LM317 Always outputting 168mA

Quote:
Originally Posted by Meatball View Post
Oh no! You have set your driver to work at 5 Amps! It MIGHT be dead. You need 2.5 Ohms to set it to 500mA!

OH NO is right! Thanks for catching that! I forgot about that the mA to Amp conversion. Maybe that will fix things.
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PHR running 135mA in a matching custom host.
CUSTOM ADJUSTABLE RGV - PHR @ 37-125mA, green module @ 0-300mA (about 20mW output), and a LOC @ 42-300mA

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Old 09-03-2010, 10:01 PM #4
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Default Re: LM317 Always outputting 168mA

Quote:
Originally Posted by awlego View Post

In case this helps:
Using a test load of 3 1N400X diodes with a 1 ohm resistor.
Powered using 4 AA alkaline batteries. (4x1.5v = 6 v in) (lm317 takes 2.25 + .7 for the reverse polarity protection diode which leaves plenty for the 1.82 forward voltage my the diode.)
Ultimately going to drive a 100mW green module.
Dont forget to include Vref =1.25v in your calculation : Vin required = 2.25 + 1.25 + 0.7 = 4.2
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