Re: Voltage rating of LM317


ehsjr wrote:
pjdd@xxxxxxxxxxxxxx wrote:
ehsjr wrote:


It is time that you do make some detailed calculations.
You started out asking a question with an unreasonable
change in the DC at Vin to 45 volts. First of all,
that won't kill the 317 based on Vin-Vout. Nor will the
concept of the uncharged cap on Vout be a concern as
has been shown and particularly with the resistor in there.
But a line voltage variation that would yield Vin of 45
volts would be way more than normal worst case design,
and would strongly suggest a different approach. Your
calculations will show that. You'd need the line voltage
to rise from 120 to ~146 to get 45 volts at Vin. That's
over a 21 % variation in line voltage, and if your equipment
must function in that kind of environment, a basic linear
regulator is not your best choice.


I appreciate all the feedback and enlightening discussion
that expanded from my original question. Please remember
that I - and the equipment - have to work under primitive
conditions. Surges of +20% do happen, though not an everyday
occurence. (We use 230V mains here).

I'm including a mains window circuit and VDRs at strategic
points, but I want everything to survive a momentary spike
in mid-operation before the protection circuit has time to
kick in. OTOH, sustained dips of more than -20% are common,
and the equipment has to keep on working.

Yes, a linear regulator is not the best choice from a
technical point of view, and I may ultimately go for a
switched-mode supply. But I was exploring the feasibility
of using the much simpler linear version.

Now take the other extreme case where the DC input rises
to 45V at light load and abnormal line voltage. If I use
a 39V zener, the resistor will briefly pump 0.4A into it
at startup. I wonder if that's advisable even with, say
a 5W zener diode. And I'm not sure I can get a 39V 5W
zener.

You misunderstand. The zener goes across the 317.

I do understand that the zener goes across the 317. When
the output is momentarilry shorted to ground, the only
thing that limits the current is the series resistor.
I = (Vin - zener V)/R. What I didn't think of was that a
part of that current will be shunted by the regulator.

.

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