Does the LM317 Have Output Short Circuit Protection

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Helsing

Well-known member
Joined
Jun 7, 2004
Messages
133
Location
San Diego, CA
I was building a power supply using an LM317 and accidently shorted the + output to ground, killing the 317 almost immediately. I had the typical datasheet proection diodes in place and the regulator was working fine before this event. I replaced the 317 and everything is back to normal.

I was under the assumption that the 317 was output short circuit protected. Perhaps I botched something in my design which is negating this protection. Any ideas?

Helsing
 
specsheet shows "internal short circuit current limiting" and thermal protection. I take that as IF it shorts it won't explode, but not much more than that. anything hooked up wrong will pop. protection diodes are usually there to keep current spikes from coming back into the outputs, not keeping it from shorting out though. I think your design will be ok if you followed the datasheet. i wouldn't worry much about protection unless you KNOW this device will be subjected to current/voltage spikes or any other kind of problem.
 
What is the output voltage? If it is above 37V you can be sure it's not short circuit protected.

Best regards,

Mikkel C. Simonsen
 
Mikkel's right, the LM317 is unhappy if the input to output voltage exceeds 40V. Even the "high voltage" version can only withstand 60V. You can use a clamp circuit like this to prevent blowing up your 317s inadvertently:

LM317_Clamp.jpg


X2 is a zener that sets the maximum voltage you want across the regulator. Q1 can be any NPN transistor with appropriate Ic and Vce specs, something like a TIP31C will work just about everywhere you'd stick an LM317. R4 will need to be a 2W or 3W part if you want it to withstand a short indefinitely.
 
The 317 is unhappy if the output is at a higher voltage than the input as well. This could happen in the case off switching a unit off and the resevoir caps staying charged after power off. This is why LM317 should have a diode from out to in to prevent this condition. This is also true for LM388K & probably the other adjustable regs in this range.

But not the 78xx & 79xx (I think)
 
My input to ouput is fine. 35v down to 18v. About a 100ma draw maximum. There are some space restrictions but Im adding a small heatsink to dissipate the extra heat.

I'll ask it this way. If I connected the +18v output to ground, would the Lm317 blow?

Helsing
 
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