Help with lm317 ?

Thread Starter

mike69

Joined Dec 22, 2012
83
Hello,
I built on a test pcb the attached power supply. It worked until I made a short circuit on the output and the LM317 passed away.
More exactly, now I have the input voltage on the output of the power supply (33Vdc).
I used 4k7 instead of 5k resistors, BC556 transistor, 1k pot instead of 1.5k. I used those values because were the single available in my workshop.
I used a 10A rectifier and 4700uF/63V filter capacitor.
Why the short circuit protection didn't worked ?
 

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danadak

Joined Mar 10, 2018
3,701
Not sure but your short circuit powered off the LM358. What happened to
adj pins during the transient.......

You can power the LM358 off the input to the 317's because it has decent PSRR.


Regards, Dana.
 

mvas

Joined Jun 19, 2017
538
Hello,
I built on a test pcb the attached power supply. It worked until I made a short circuit on the output and the LM317 passed away.
More exactly, now I have the input voltage on the output of the power supply (33Vdc).
I used 4k7 instead of 5k resistors, BC556 transistor, 1k pot instead of 1.5k. I used those values because were the single available in my workshop.
I used a 10A rectifier and 4700uF/63V filter capacitor.
Why the short circuit protection didn't worked ?
I believe that Short Circuit Protection did work.
But, very quickly you had a voltage difference of 33 volts AND a current of 1.8 amps flowing.
The device then went into thermal runaway faster than the Thermal Protection Circuit could detect & shutdown.
The LM317 was generating 60 Watts of Heat = 33 Volts x 1.8 Amps !
60 Watts of heat ( 33 Volts x 1.8 amps) is way outside the Normal Operating Parameters for the LM317.

The LM317 can handle either a 33 Volt Drop or 1.8 Amps, but never both simultaneously.
Given your huge 33 Volt Drop, you must add an External Circuit to limit each LM317 to < 1 amp short circuit current.
A 1 amp fast blow fuse, or smaller, per LM317 may add some protection against short-circuit melt down
33 Watts of Heat will require large heat sinks.
 
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