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 <[email protected]> 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>_______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
