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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Predictive failures
Date: Tue, 16 Apr 2024 13:39:07 -0400
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"Bill Sloman" <bill.sloman@ieee.org> wrote in message 
news:uvmao8$124q1$1@dont-email.me...
> 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.

Yes I've seen that a lot.
The power rails in the production product came up in a different order to 
those in the development lab.
This caused all kinds of previously unseen behaviour including an expensive 
flash a/d chip burning up.

I'd have it in the test spec that any missing power rail does not cause 
issues.
And any power rail can be turned on and off any time.
The equipment may not work properly with a missing power rail but it should 
not be damaged.

>
> 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
>
>
>