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Triggering scope upon removal of periodic AC signal

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not1xor1:

--- Quote from: iroc86 on November 30, 2019, 05:49:34 pm ---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?

--- End quote ---

I might have misunderstood what' the problem, but if that is a dual channel scope why do not  you just use a channel for monitoring a permanent AC signal (triggering on that) and another for the periodically removed signal?

Daixiwen:
How about cutting the mains with a relay and connect the relay control voltage to the scope's trigger input?
Then to run the test you cut the relay control voltage instead of the mains itself

iroc86:
Just for context, I'm tuning an RC snubber on a small shaded-pole AC motor, so I'm using the scope to measure the back-EMF at shutoff. That's why I'm looking for a nice, defined event upon which to trigger. When the motor is running, triggering off the AC source is just fine to see the whole 60 Hz waveform (10 ms/div), but I'm looking to capture the spike over a very short period of time (10 µs/div or faster).

Comparing the primary AC source against a reference signal at similar frequency/phase is an interesting idea (vk6zko's reply). I can generate another signal, but comparison would be tough since the primary source is at 120 V RMS. It'd need to be stepped down to a more manageable voltage.

Based on the other replies, it sounds like rectified AC might be the easiest option, but perhaps through a step-down transformer for safety (or a plugpack as gcewing suggested). The relay idea is cool, too.

I'll think about this some more and see what parts I can scrounge up. I can post back later with the results.

David Hess:
If you are using the 120 VAC mains, why not use the line trigger source which is built into the oscilloscope?  That is what it is for.

vk6zgo:

--- Quote from: iroc86 on December 03, 2019, 02:35:39 am ---Just for context, I'm tuning an RC snubber on a small shaded-pole AC motor, so I'm using the scope to measure the back-EMF at shutoff. That's why I'm looking for a nice, defined event upon which to trigger. When the motor is running, triggering off the AC source is just fine to see the whole 60 Hz waveform (10 ms/div), but I'm looking to capture the spike over a very short period of time (10 µs/div or faster).

--- End quote ---
A dual timebase analog 'scope could see 10usecs in 10ms, but might not trigger reliably, & couldn't store the result, in any case.
A DSO may have too low sample rate at 10ms/div to see the spike, so it looks like the 'scope needs to be set to an appropriate time/cm setting for it, & be set to only run when triggered by the loss of AC source.

As I don't have a modern, or indeed, any DSO, I may be missing some facility available on such instruments.

--- Quote ---Comparing the primary AC source against a reference signal at similar frequency/phase is an interesting idea (vk6zko's reply). I can generate another signal, but comparison would be tough since the primary source is at 120 V RMS. It'd need to be stepped down to a more manageable voltage.

Based on the other replies, it sounds like rectified AC might be the easiest option, but perhaps through a step-down transformer for safety (or a plugpack as gcewing suggested). The relay idea is cool, too.

I'll think about this some more and see what parts I can scrounge up. I can post back later with the results.

--- End quote ---

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