Electronics > Beginners
Triggering scope upon removal of periodic AC signal
iroc86:
I'd like to trigger my oscilloscope when a periodic AC signal is no longer present. In this particular application, I'm measuring the circuit response when 120 VAC mains is removed. The circuit is entirely AC, so there are no DC signals upon which to trigger.
If I play around with the trigger thresholds, I can sometimes hit things just right, but it's entirely luck of the draw... and I don't really want to cycle on/off repeatedly just to get a decent capture.
One solution might be to use a small isolation transformer with rectified DC output and trigger off of that. Is there an easier (or "best practices") method of triggering a scope in this situation?
Nerull:
Can you use normal (not auto) triggering mode set to trigger on the AC waveform, it should stop triggering when the AC vanishes leaving the transition on screen.
(assuming its a DSO)
KaneTW:
What triggers do you have available?
iroc86:
--- Quote from: Nerull on November 30, 2019, 10:35:43 pm ---Can you use normal (not auto) triggering mode set to trigger on the AC waveform, it should stop triggering when the AC vanishes leaving the transition on screen.
(assuming its a DSO)
--- End quote ---
That's a good idea, but unfortunately it doesn't work so well on my scope (HP 54645A). It's a DSO, but the screen refreshes after the trigger point--I'll see the transition for a split second before the trace flatlines. I can occasionally capture the trace in single shot mode, though.
Another option I have is setting the scope to "roll" mode (200 ms/div) and then just manually stopping the acquisition. I can zoom in after the fact. The 54645A has decent sampling memory for its age (and my purposes), but it'd be nice to be able to precisely trigger for capturing data at faster timebases.
--- Quote from: KaneTW on November 30, 2019, 10:49:44 pm ---What triggers do you have available?
--- End quote ---
I only have rising edge, falling edge, glitch, and TV. No fancy waveform triggers like the more modern scopes.
RoGeorge:
Not sure if I got the request right:
A very efficient way to filter out the AC influences out of a signal is to use averaging, if you have a digital oscilloscope. That way you'll implement a lock-in amplifier rejecting the mains (or other) frequency.
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