Re: Voltage rating of LM317

pjdd@xxxxxxxxxxxxxx wrote:
pjdd@xxxxxxxxxxxxxx wrote:

The data*** of the LM317 adjustable regulator says that the
max voltage rating of 40V is for the difference between input
and output,


Can the LM317 be safely used with an
input of, say 45V and a filtered output of 12V ?


It's gratifying to see so many replies to my question
while I was out for the rest of the day. Too many to
reply to one by one, I'm afraid.

Okay. Accepted that it's not adviseable to use the LM317
with an input that may exceed 40V, no matter what the
input-output difference is. But for the sake of argument,
what about NatSem's statement that the device can be used
to regulate hundreds of volts as long as the input-output
differential does not exceed 40V ? This would then assume
that the load has 0 capacitance. Can such a load even
exist in practice, without even a pF or so of stray
capacitance ? If not, then that brings us back to the
matter of time-limited surge and rise time.

The regulator is to be used with a transformer-rectifier
input of 27 Vrms at normal line voltage, with load current
ranging from a very few mA to about 0.6A. The transformer
also has another load ranging from 0-3A. That's an input
of nearly 37V to the regulator at light load. That leaves
too little margin for mains fluctuations that may rise to
more than +10%.

Circumstances permit the use of only the most widely
available parts. I hadn't thought of the LM317HV. It's a
good choice. I don't have it in stock and will have to
send for it from another city - which is not as simple a
matter in certain parts of the world as it is in some
others. Otherwise I'll have to use another transformer for
the 12V supply, or take a different approach to the rest
of the design.


You can go for as much belt and suspenders as you want.
I would. But with a small cap on the output and a big
filter cap across the rectifier, your initial turn on
should not cause grief - it is a non-issue.

As to the belt and suspenders, the zener has been
recommended. In addition to that, is there any reason
you can't stuff some R - say 33 ohms - in the input to
drop the voltage a bit? It would help in the 317 dissipation,
too. Without the R you're looking at ~ 15 watts. With it
the 317 gets ~ 3 watts.

Ed
.

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