[time-nuts] Re: Death of a Capacitor

[Bob kb8tq]

Hi

If you start dumping major current spikes into a common ground, it’s amazingly difficult to 
get rid of the results.

Bob

> On Sep 26, 2021, at 6:21 PM, John Ackermann N8UR <jra at febo.com> wrote:
> 
> I got some interesting and unintended data today. I was measuring low phase noise oscillators using a set of power supplies I just finished putting together.
> 
> The configuration is ~24 VDC into a TPS-53400 switching regulator that outputs 19.2 volts at up to 3 amps.  That output is fed to separate regulator boards for each oscillator.  Those boards each have an LT-1086 linear pre-regulator that drops the input to about 17 volts, which then goes into an ultra-low-noise LT3045A outputting 15 volt to drive the oscillator.  So there are two linear regulators and lots of caps, inductors, and ferrite beads to isolate the oscillators from the switching supply.
> 
> Due to an error by an assembly tech who will remain nameless, the wrong electrolytic was installed on the output side of the switching regulator.  It should have been 33uF at 50 volts, but what got installed was 330 uF at 16 volts, so it was rated below the operating voltage. (I was building two boards at the same time, one for 5V and one for 19.2V. Apart from the voltage setting resistor, the only difference between the two was the output cap.  I managed to swap them.)
> 
> I tested the system on the bench for 24 hours and everything worked fine, so I buttoned up the enclosure and started a 4 hour data capture. About 70 minutes in, the electrolytic became very unhappy and whatever it turned into caused the switcher to start spewing all sorts of crud. The regulator kept working (sort of) through the end of the run, but when I came into the lab the next morning it had shut down completely and troubleshooting showed that the cap had shorted at some point after the run completed, and the regulator chip went into shutdown.
> 
> Attached are a plot of frequency showing the whole run with the very obvious change when the cap failed, and another zoomed view of the critical moment.  The failure was very abrupt with no visible lead-in.
> 
> What I find interesting is that all that crud got through not one, but two linear regulators, one of which is touted for its extremely high PSRR (and I did my best to follow the recommended PCB layout for that chip).  That must have been one ugly 19V line when the cap went...
> 
> John
> <cap_failure_freq_2.png><cap_failure_freq_1.png>_______________________________________________
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