For the life of me I am not able to find out what mistake I am doing.
I have built the same circuit as shown here :

Sorry for bad quality of image ! Its from a web cam.
Tx (26Vac)>> BR >> LM317 >> small indicator LED

I have tried R values from 10E to 22K. The red LED keeps blowing up always and the LM317 heats up. I have tried many LM317 chips, they all behave the same. So I think the chips are ok but there is some dumb mistake I am doing in the connections. Please help.

Thank you.

 

I calculated 62.5Ω is for a constant current of 20mA for the LED. The nearest value to use is 68 ohm.

I used 26V DC for the input and there was 22.5V drop across the LM317 which is 450mW of power dissipated on the 317. Not sure how you blew up the LED. Is your input voltage exceeded the 35V max voltage?


Allen

 

AHHHHHH!

Caught the mistake! Such a kiddish mistake. I was taking output from point no. 2 in your diagram. i.e BEFORE THE RESISTOR !

Now the LED glows but 1/4W resistor bellows smoke. But that I understand.

Thank you.

 

Allen

Thank you Allen for taking time out for simulating it.

 

Thank you Allen for taking time out for simulating it.

This has always come in handy for me. I don't know the author. Good luck.

 

This has always come in handy for me. I don't know the author. Good luck.

Were you using Excel to formulate the table?
So the value I calculated is correct!

Allen