On 8/22/2017 2:32 AM, Phil Allison wrote:
Bob Engelhardt wrote:

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... If the 5V rail goes out of tolerance, an SCR is triggered to ...
... the 5V rail would be full of HF switching noise which could trigger
the SCR. ...


How would the monitoring circuit work, in general terms?


** Essentially, just a 400mW Zener diode rated at 4.7 or 5.6 volts.

The SCR has a resistor of say 100ohms from gate to ground and the Zener connects from that to the 5V rail. Takes about 0.8V to trigger the SCR so about 8mA of Zener current.

I have added such to several linear & SMPSs to protect digital boards from faulty PSUs. In one case, I had to replace about 100 digital ICs after a linear 5V supply failed to circa 10V DC.

Luckily they were all in sockets.



.... Phil


OK ... I was thinking that the AC switching noise itself would have to
be detected, but it sounds like you just need to look for the noise
peaks, which would exceed the zener htresho0ld.

Bob