Help with a simple lm317 circuit

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Help with a simple lm317 circuit

Postby PCBindex » Sun Oct 21, 2018 9:41 pm

I have lots of interests, and play around with many different things. Mechanics, sewing, art, music, plumbing, you name it. I have long had an interest in electronics, and have fiddled about some. I have a basic understanding of what I am doing, but it is indeed basic.
I have a Christmas display which includes a number of pieces that are illuminated by LED's, and each piece has a pair of AA batteries, which means there is an attached battery box, and switch. The display sits on a shelf above a window in my home, so getting up there to turn all those switches on and off is a pain. I figured I would make a simple little power supply to run all of the LED's. My idea was to use a wal wart which I already owned. It supplies 12V at 500 ma. Since the LED's are powered by 2 AA batteries, I expect that they are getting just a bit over 3 volts, so I figured I would use an LM317 voltage regulator to set my voltage at 3 volts, and life would be good.
As is often the case, I apparently know just enough to get myself into trouble lol. So, I used an online calculator to get my resistor values, and set up my circuit on a breadboard to test. I started by powering it with a 9V battery, and got 3.1 volts, which I figured would be fine. I then hooked up the 12V wal wart, expecting to see that same 3.1 volts, but that was not the case. When I power the circuit from the wal wart, I am getting 2.69 volts from the LM317.
Can anyone explain to me why I am seeing the difference in voltage between the battery power, and the wall adapter? Thanks in advance!
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Re: Help with a simple lm317 circuit

Postby tayken » Thu Oct 25, 2018 5:31 am

How hot is LM317? Also how much is the current draw of the LEDs? My guess is due to the difference in voltage LM317 is getting too hot and over temp protection circuit is kicking in.
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