LM317 robustness in thermal & short circuit limiting?

Thread Starter


Joined Jun 10, 2015
I am curious if the protection features of an LM317 (or perhaps any regulator with those features)
will, in a circuit that utilizes them as a feature, something that is repeatedly aggrieved, will it eventually
fail? For instance, can I short an LM317 over and over again, forever, without fail? Same for
thermal protection? Or will I kill it?

I've been teasing a pass-transistor circuit on the workbench, and when overloaded, the lamps
I'm using for a load dim, then slightly flicker, endlessly, until I remove the short/overload.

This one is from Bill:

(I think I can spot an error: the protection diode should reach all the way back to the input at 18v...)

Thread Starter


Joined Jun 10, 2015
From National's datasheet:

"In addition to higher performance than fixed regulators, the
LM117 series offers full overload protection available only in
IC’s. Included on the chip are current limit, thermal overload
protection and safe area protection. All overload protection
circuitry remains fully functional even if the adjustment ter-
minal is disconnected"

And Texas Instruments datasheet:

"The LMx17HV offers overload protection like current
limit, thermal overload protection and safe area
protection, which make the device blowout proof."

Should have seen for myself first. :confused:
Now I'm off to go prove it. Wish me luck.


Joined Mar 14, 2008
Continual thermal overload (but not a short) will thermal cycle the regulator die to near its maximum temperature, and that could shorten the theoretical life of the device, or slightly change some of its characteristics, such as its internal reference voltage.
But it still should likely last as long as the practical life of your circuit.

Thread Starter


Joined Jun 10, 2015

Using a bypass transistor will disable the current limiting of the LM317.
Have a look at pic 36 on this page to implement current limiting:

Yes, I tried Bill's circuit (above) with many different resistor combinations, but eventually returned to the original schema because it allows the regulator to hit the wall first, keeping the pass transistor out of danger. Thanks for the awesome link!