BK Precision 1665 - is this normal?

[naimis]

I've been trying to assemble a couple of switching power supplies based on the 34063 that Dave talks about in blog #110, and was seeing a really peculiar oscillating voltage out of the circuit.  After spending some time tracking it down, I found that the source was actually the bench power supply I was using to drive the switcher - my BK Precision 1665.  Let's see if I can figure out how to post an image from my scope.


Input 2 wasn't connected at the time and can be ignored, but as you can see I get these odd oscillating pulses on the output.  Also note that this was measured with DC-coupled probe.  I haven't yet figured out what the difference is between AC and DC coupled or when one should use one over the other.   

Anyway these crazy pulses of over 1V peak-to-peak are a bit alarming, even if they're not the source of the problem I'm having in my project.  This is my one and only bench power supply - should I be concerned about this?  I've used it for a number of projects, so I'm guessing it's just something I need to be aware of, like the slow ramp-up when the supply is switched on.  If this behavior is likely to cause problems, what sort of symptoms am I likely to see?

(FWIW, the power controller I'm building is for a camcorder, and I'm 90% sure the problem is insufficient power when the camera is switched on, and the "glitch" above is unrelated)

[jahonen]

I'd say that is harmless (but sometimes a nuisance) common mode noise. You can test it by connecting both the probe tip and ground to the same power supply output terminal (i.e. probe short-circuited), if you still see same noise, then it is common mode.

Regards,
Janne

[JoeyP]

That looks atrocious! 1.72Vpp on what looks like an 8V output is huge. If you are measuring that with your scope connected directly to the bench supply output terminals, then either the supply has inadequate output filtering caps, or it is out right breaking into oscillation, probably keyed by the high current pulses drawn by your switcher. If you are measuring it anywhere other than directly at the bench supply terminals, then it might just be ringing in the leads connecting your switcher. From my experience, "BK Precision" is an oxymoron. I wouldn't be at all surprised if it's the bench supply. In either case, it probably indicates that you need better filtering on the input of your switcher.

[alm]

I wouldn't discard Janne's suggestion that it might be common mode. Measuring with a shorted probe is a quick test and will tell you if it's in the circuit or if your probe is picking it up. The reason this works is that your 'shorted' probe is not a perfect short at higher frequencies due to inductance in the ground lead and capacitance in the probe.

[naimis]

I'd say that is harmless (but sometimes a nuisance) common mode noise. You can test it by connecting both the probe tip and ground to the same power supply output terminal (i.e. probe short-circuited), if you still see same noise, then it is common mode.

Regards,
Janne

Thanks to all who responded.  It does appear to be common noise, if this test is definitive.  I have to say I was a bit freaked when I tried to hook up the probe and it arced from the terminal to the probe tip.

The noise does appear to be regular and about 17us apart.