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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Predictive failures
Date: Wed, 17 Apr 2024 02:58:41 +1000
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On 17/04/2024 1:22 am, John Larkin wrote:
> On Tue, 16 Apr 2024 09:45:34 +0100, Martin Brown <'''newspam'''@nonad.co.uk> wrote: 
>> On 15/04/2024 18:13, Don Y wrote:

<snip>

> Sometimes BIST can help ensure that small failures won't become
> board-burning failures, but an RMA will happen anyhow.

Built-in self test is mostly auto-calibration. You can use temperature 
sensitive components for precise measurements if you calibrate out the 
temperature shift and re-calibrate if the measured temperature shifts 
appreciably (or every few minutes).

It might also take out the effects of dopant drift in a hot device, but 
it wouldn't take it out forever.

> I just added a soft-start feature to a couple of boards. Apply a
> current-limited 48 volts to the power stages before the real thing is
> switched on hard.

Soft-start has been around forever. If you don't pay attention to what 
happens to your circuit at start-up and turn-off you can have some real 
disasters.

At Cambridge Instruments I once replaced all the tail resistors in a 
bunch of class-B long-tailed-pair-based scan amplifiers with constant 
current diodes. With the resistors tails, the scan amps drew a lot of 
current when the 24V rail was being ramped up and that threw the 24V 
supply into current limit, so it didn't ramp up. The constant current 
diodes stopped this (not that I can remember how).

This was a follow-up after I'd brought in to stop the 24V power supply 
from blowing up (because it hadn't had a properly designed current limit).

The problem had shown up in production - where it was known as the three 
back problem because when things did go wrong the excursions on the 24V 
rail destroyed three bags of components.

-- 
Bill Sloman, Sydney