linear regulator crapping out

Yup. And Robert Obemayer posted the schematic on

4/20. 'Course the op might need the extra nudge from your post - he's resoundingly silent since 4/20.

Ed

You might try reading ALL of the thread before responding. .

It's a poor craftsman who blames his tools. . . On a related note: Google Groups, USENET, "context", and your posting technique

't-see-*-*-*-*-Google+You-don't-*-*-*-*-*-*-*-previous-post+click-*-show-options-*+zz-zz+*-leave-enough+click-THAT-Reply-link+If-you-don't-*-*-*-*-your-reply+faqs http://66.102.9.104/search?q=cache:SXIajvWUVHAJ:groups.google.com/support/bin/answer.py=answer=12348+Summarize-what-*-*-*-*-*+remove-*-*-*-irrelevant+show-options+zz-zz+click-Reply+BOTTOM+*-*-reply-appears-*-*-*-before-the-original-*-*

You'll chime in with another incorrect "answer."

Nah- the problem is an inductor in the 7805 collector destabilizes the regulator. It has nothing to do with any reverse discharge.

That reverse diode is a holdover from the days of e-cap "sputtering" and has little relevance to modern components.

AARL educated I see....

Try it, then get back to us. If I were in the habit of saving blown parts from 15 years ago, I'd be glad to mail you a few samples.

The real world doesn't run in SPICE.

-- john, KE5FX

You do know that the input rectifiers block substantial reverse current flow through the regulator? When power is removed from the input, the input capacitor will simply discharge to approximately 2x(internal NPN)Vbc below the output at which point the output capacitor discharges into it as well as the internal Vref and error amplifier circuitry. The whole circuit then drifts down to zero at something like a 4-8mA rate. You might try reading some of the earlier app notes by George Cleveland. The idea for the reverse protection OUT/IN diode originated in a Robert Widlar app note where he was taking care of every possible failure mode and every possible applications abuse of the part including high temperature, hot plug with late GND connection, and the rectifier input filter capacitor sputtering problem. The sputtering problem was an instantaneous input capacitor short causing high reverse current through the regulator and permanent damage. This is categorically different from a weak current limited discharge taking place over the order of milliseconds. There must be literally billions of cumulative device-hour time on existing circuits that are doing just fine without the unnecessary reverse protection diode. As with everything else, this is conditional. I have seen clueless application circuits where the unregulated input is loaded by circuitry in addition to the regulator. In this case, the reverse diode may be required, and possibly this is what you are recalling about blowing parts.

Sure, but I've always been under the (entirely unjustified) impression that the damage is done before that point.

That's an interesting point, and not one I've ever considered. I don't know that I'd call it cluelessness on the user's part, though... these regulators have always been marketed on the basis of their supposed indestructibility, and for most users (as you note) they live up to the hype. I've certainly never seen anything in a data sheet warning against that scenario.

It's possible that really was what was happening in my case. But, in any event, the fix is the same: don't store a lot of electrons on the output side of your regulator unless there's somewhere else for them to go at turn-off time.

I apologize for the SPICE troll, by the way. I wasn't expecting such a civil reply to it. :-)

-- john, KE5FX