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Voltage regulator problem

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Muhammad Nasar:
I am trying to built  a AC to DC converter, using LM317 as seen in the image but the problem is that when i ever build this circuit the Potentiometer and reference resistor burns. I don't know what is the problem plz help me out.
Plz see the image down here.

LM317 are designed for up to 40V DC input

If you start with 36V AC then fullwave rectify it, you will charge up your capacitors to 50V DC

36VAC * sqrt(2) = 50VDC (peak)
This is because the 36V AC is RMS.   It will be around 50V peak to peak and that will charge up the capacitors.

The LM317 is probably dieing from too much input voltage and once it's burned out the full 50V gets onto the output which burns your 120ohm resistor and pot (if the pot is set to a low value)

you will probably have to use a SMPS chip. A lot of them have up to 60V input voltage.
Which is a better choice anyway because it won't generate as much heat as LM317 :)

Muhammad Nasar:
Thanks people for helping me out

Use the LM317 HV which works up to 60V.

Your schematic is confusing because you've drawn the transformer s centre tapped so it appears to be running at 18V.

By the way, the LM317 only sees the difference between the input and output voltage so you can convert 50V down to any voltage above 10V. Unfortunately you loose the short circuit protection with input voltages >40V because as soon as the output is connected to 0V, the LM317 shuts down, sees the full power supply voltage and smokes.


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