[sdiy] LM 317 regulators

[Magnus Danielson]

From: Tim Ressel <madhun2001 at yahoo.com>
Subject: Re: [sdiy] LM 317 regulators
Date: Fri, 22 Mar 2002 13:48:19 -0800 (PST)

> No direct experience with that part, but I can give
> you some general knowledge about reliability.
> 
> HP (my former employer) did a 10-year study on
> reliability and how to improve it. their goal was a
> 10x improvement in reliability.
> 
> They found that the most important factor was the
> derating of semi ratings. For example: If a 16V cap is
> used in a 15V circuit, it will not have very good
> reliability. If a 35V cap is used, it will have way
> better reliability.

A small warning: This might not be generically true! Some caps require
a fairly high DC potential for them to be capable in the long run to
handle their maximum rating. In the end you have to balance different
fault mechanisms. In the end, it is the knowledge of different fault
mechanisms which enables you to do good design choises.

But as the general statement, yes, derating is a good practice, and
not only for reliability purposes.

All to often designers choose components so they run at the rimb of
their specs. This can have catastrophic results. You must let
components have what I call headroom, DC regulators for instance must
have both voltage, current and power headroom.

> HP's overall spec is to derate values by 50% whenever
> possible.

That is just 3 dB (or 6 dB, depending on the parameter) added margin.

> Now in your case, keep in mind the #1 enemy
> of semis is heat. So if your regulators are running at
> the top of their specs and/or have marginal heat
> sinks, then that could be the whole problem right
> there.

Indeed.

> Also keep in mind semis fail. 20 years for a 1A
> regulator doesn't seem bad to me.

No.

Also, one must also accept that certain components degrade over time,
and that replacement of for instance electrolytic capacitors may be
seen as a standard precaution service before they start causing
trouble. By not letting too old components age out and burn of, you
may reduce the load on other parts of the system. Many age related
faults have secondary faults which was caused by a long term
stress. Not only wise design, but also wise maintenance will ensure
long lifetime and good specs on things.

Cheers,
Magnus