Re: Voltage rating of LM317


ehsjr wrote:
pjdd@xxxxxxxxxxxxxx wrote:
pjdd@xxxxxxxxxxxxxx wrote:


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 a non-native user of English, I'm not familiar with the
term "belt and suspenders". But the way you use it, it
seems to mean things that are not strictly essential, but
are added anyway to ensure satisfactory performance and/or
extra security and safety. Correct ?

If that's correct, then I'm also a belt and suspenders man,
but not in the way some designers like to keep on adding
more and more stuff to compensate for a problem that could
be solved more elegantly by revising a flawed approach.

Back to the topic. The load imposed on the transformer can
be divided into two main groups, the one requiring a +12V
supply being the lighter load. Instead of using separate
rectifiers for the two sections, I'm thinking of using a
common FW rectifier, heavily filtered, and insert a 1A
diode in series with the lighter load. That section will
have another, lighter, filter cap and the diode will act
as a buffer against the effects of fluctuations caused by
the heavier load.

The main filter will be 10000uF, and the secondary one 470
or 1000uF. The filter at the output of the regulator will
be about 47uF Al electrolytic, paralelled by a .22uF
ceramic disc. Other filters of the same order will be
placed at entry points to various sub-sections. The total
capacitance seen by the regulator output will be no more
than 200uF. I think that will take care of the rise times.

The belt and suspenders would be to use the HV version,
while the slow input rise time would allow using a plain
317 should the need ever arise.

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

You might have missed the part where I said that the
regulator load varies from a few mA to about 0.6A. That
pretty much rules out using a series resistor. A value
high enough for a significant voltage drop at light load
will be much too high when the load rises. One possible
alternative would be to place a dozen or so diodes in
series with the regulator input.

So the options are : multiple series diodes, a different
transformer, switched-mode regulation, or a heavy heatsink.
Comments please ?

.

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