Mysterious Self Destructing LM317s

Externet said:

A spike in current when the motor starts would kill the regulator on overcurrent before any raise in temperature can be sensed.
Your "about 2.5A inrush" is probably much, much higher.

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Has built in current limiting, should be able to handle that.

icedvolvo said:

2: yes there is a heat sink, its a TO220 package but shouldn't need one for two reasons:
- the avg current is only 0.5A with a 5V differential which is only 2.5W
???

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A TO-220 can not handle 2.5W without heatsink: it's about 60C/W which would put the junction temp at about 180C. The thermal protection should keep it alive, but that doesn't always work.

Icedvolvo:
Jump start your motor on with another supply say at 12.0V and while running, parallel feed the power from the LM317 output pre-adjusted at 12.0V

Remove the jump starter power and confirm the motor is capable of operating with the LM317 alone, check its temperature.
If it does, your starting current from stopped is way, way too high for the regulator.

Then if you insist in using the LM317; a choice is to hook up a 3 position switch (off - start - run). The LM317 unregulated input to start, the regulated output to run - if that is compatible with your gear-

It's just about impossible to kill a LM317 with a 24v dc supply running a 0.5A motor.

I agree with ohters that you probably have a massive over-voltage spike problem. Does the "24v DC supply" connect to anything else?

OK Short story is that the people who guessed a spike as responsible were correct! The back voltage was over 80V pp!!!!! And the second part of the story is the normal diodes are hopeless for killing this!

Long story is I went and bought a $30 PWM kit and in combo with a "fast" diode across the motor all is well!

Lesson 1: back EMF is a chip killer!
Lesson 2: dont be a cheapskate; do it properly!!!

Again my thanks to all who assisted, I hope the discussion helps some others as well.

icedvolvo said:

OK Short story is that the people who guessed a spike as responsible were correct! The back voltage was over 80V pp!!!!! And the second part of the story is the normal diodes are hopeless for killing this!

Long story is I went and bought a $30 PWM kit and in combo with a "fast" diode across the motor all is well!

Lesson 1: back EMF is a chip killer!
Lesson 2: dont be a cheapskate; do it properly!!!

Again my thanks to all who assisted, I hope the discussion helps some others as well.

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It has. Thank you for clarifying.....the LM317 would still do the job...you did not need the $30 PWM kit....

All that was really needed was a fast diode as in a FR104 (FR meaning Fast Rectifier)...problem sorted with back EMF suppression.
Even a BA159 would have done the job..

Now why did I not think of that?. Heck, I am dumb sometimes

Cheers
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