Electronics > Beginners
Triggering scope upon removal of periodic AC signal
Circlotron:
--- Quote from: iroc86 on December 05, 2019, 01:13:31 am ---Those are some cool ideas! There's always more than one way to skin a cat, and I appreciate learning some other techniques from you all.
--- Quote from: Circlotron on December 03, 2019, 04:38:58 am ---Maybe measure the peak AC current. Then put this much DC (only) through the winding, switch off and trigger on the spike and see what your snubber does.
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That's an interesting alternative approach to measuring the back-EMF. Could you elaborate a bit more on this, and how the test setup might be configured in practice? The motor is drawing about 1 A peak-to-peak at 120 VAC according to my Fluke clamp-on current probe. Since the motor is designed to operate at mains voltage, wouldn't I need a fairly high-voltage DC source to generate that sort of current?
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One thing I don't think you mentioned and that is, are you switching the motor with a relay contact or a triac? The relay might switch off at the max of 0.5 amps whereas the triac will switch off when the motor current dips below the triac's holding current - maybe tens of milliamps.
Anyway, for the method I suggested all you would need to do is apply enough DC voltage to the motor winding to make it conduct 0.5 amps (assuming you are using a relay). Motor will not run, of course. The needed DC voltage would depend only on the DC resistance of the motor winding. Upon turnoff the energy stored in the airgap of the magnetic circuit will try to arc the relay contacts, so your snubber will damp this of course. Just put the scope directly across the contacts and Bob's your uncle!
iroc86:
--- Quote from: Circlotron on December 05, 2019, 01:43:51 am ---One thing I don't think you mentioned and that is, are you switching the motor with a relay contact or a triac? The relay might switch off at the max of 0.5 amps whereas the triac will switch off when the motor current dips below the triac's holding current - maybe tens of milliamps.
Anyway, for the method I suggested all you would need to do is apply enough DC voltage to the motor winding to make it conduct 0.5 amps (assuming you are using a relay). Motor will not run, of course. The needed DC voltage would depend only on the DC resistance of the motor winding. Upon turnoff the energy stored in the airgap of the magnetic circuit will try to arc the relay contacts, so your snubber will damp this of course. Just put the scope directly across the contacts and Bob's your uncle!
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Gotcha! Silly me; I didn't realize that the DC voltage would be much lower due to the DC resistance of the winding :palm:. It's about 25 ohms, so that'd be just 25 VDC for 1 A.
The motor is being driven by a triac to chop the waveform and adjust the speed, but I'm toggling the circuit on/off through a regular switch to mains (just for convenience during testing; the final design may use a relay in the same location). In this case, I don't think the triac will play a role in the shutoff timing, but I see what you mean about the switch points. Ideally, I should probably be cutting the AC source when the current is at its peak during the half-cycle to generate the largest spike and test the snubber in its worst-case scenario. Then again, it sounds like your suggestion with the DC approach might accomplish the same thing.
iroc86:
I thought I'd reply back with some updates on the scope triggering options. I tried each of the suggestions mentioned in this thread and had pretty good luck. I admittedly need a proper 100X probe to properly measure and tune my snubber circuit, but this was a useful exercise for triggering purposes.
I didn't have a chance yet to test out Circlotron's method with DC current, but I will.
Mask Test Triggering (MarkL)
I created a waveform mask and set the acquisition to stop when the mask test fails, as well as automatically save the trace to memory. There are two disadvantages with this method, at least on my scope: 1) I need to use averaging mode because random peaking causes the test to fail, resulting in lost data due to the short duration of the back-EMF spike, and 2) the output is a trace capture, so I can't zoom in on the waveform after the fact. Even so, the technique was easy to set up and very repeatable.
Glitch Triggering (macboy)
This method required some fiddling, but it did work well. The glitch settings were negative polarity with a duration around 1/2 the period, or ~8.3 ms for 60 Hz. I started at 16.7 ms and slowly reduced the duration until the scope stopped triggering. Then, when I cut the power, it would trigger right in the center of the screen. In this situation, I think the duration is largely dependent on the shape of the waveform and its timing at the trigger voltage level. I'm using a triac to drive the circuit, so the output isn't a perfect sine wave.
Back-EMF Triggering (David Hess)
Turning on LF rejection and just triggering on the event itself worked reasonably well, although I'm definitely at the limit of my vertical range with only a 10X probe--the natural peaks and ringing from the LC input filter in my circuit interfered with the trigger, but I can see how this'd be a useful method.
Current Probe Triggering
A few posts up I mentioned about measuring the current with a clamp-on current probe... that'd work for triggering, too! I don't know why it didn't occur to me earlier. I just set the trigger threshold outside the steady state peak-to-peak value coming from the probe and triggered on the current spike at shutdown.
David Hess:
--- Quote from: iroc86 on December 18, 2019, 02:21:36 am ---Back-EMF Triggering (David Hess)
Turning on LF rejection and just triggering on the event itself worked reasonably well, although I'm definitely at the limit of my vertical range with only a 10X probe--the natural peaks and ringing from the LC input filter in my circuit interfered with the trigger, but I can see how this'd be a useful method.
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For oscilloscopes which support it, one trick is to use a delayed sweep and B trigger. The A trigger is used to trigger on an event which reliably occurs before the desired event and then the delayed sweep is set to trigger after delay with the delay set to just before the desired event.
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