Help measuring voltage spike with oscilloscope

Thread Starter

Sprayman

Joined Mar 31, 2020
8
I am tasked with confirming a report that said we had a large voltage spike (~400v I believe) when turning some equipment on.

I went and put the oscilloscope on our main that is in the same box for the switch for the equipment.

I throw the switch and see my measurements react a little but I can't tell whats happening for the most part.

I am using a Tektronix TDS 3014C. Is there a way to set this guy up to show a history of spikes or show the max voltage spiked to?

Last time i used an oscilloscope this in depth was university so I am still getting my feet wet. Any thoughts would be greatly appreciated!
 

AlbertHall

Joined Jun 4, 2014
10,390
Acqisition type 'wave alert' which will find something different from normal or trigger based on rise/fall time - the spike can be expected to be faster than the supply wave form.
 

KeithWalker

Joined Jul 10, 2017
1,216
What is the supply to the equipment? If it is AC mains, you have a number of problems. The first is that this forum prohibits discussions which involve making measurements on live mains power. The second problem is that unless you have experience in making measurements on live AC voltages you are putting your life at risk.
My second question is how was the 400V spike detected in the first place? Their measurement technique should be tested as it may not be valid.
To capture the voltage trensient on an oscilloscope, you will need a scope with memory capabilities and a probe that will withstand the transient voltage without breaking down and blowing the front end off your scope (I've seen it happen!).
You need to set the triggering on the scope for fast AC (not auto) on the channel you are using and the timebase to around 1mS per div to start with. Set the vertical input set to a suitable high level, taking into account the attenuation of the probe.
It will probably take several tries with the trigger set to different levels before you capture the spike. Once you have it you can reproduce the measurement with more appropriate vertical and horizontal settings.
Good luck.
Keith
 
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danadak

Joined Mar 10, 2018
4,057
If you set trigger to something like 100V, and set scope up for one shot trigger, that will capture
the transient. So if scope is set for 50V / div, then set trigger to 2 boxes on screen. Then start
the equipment. Check sample rate is reasonably high, then after a trigger if you only have part
of the event raise the sampling rate.

remember, you trade off sampling rate. hence resolution, for total history in the memory. So if
transient is short you want high sample rate, but if it lasts a long time then you drop the sample
rate to get more history stored in scope memory for review.

https://www.tek.com/document/online/primer/xyzs-scopes/ch4/oscilloscope-systems-and-controls



Regards, Dana.
 
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Thread Starter

Sprayman

Joined Mar 31, 2020
8
AH, was unaware that AC was taboo, I am very experienced with AC however this is DC, They were using some sort of handheld device, (i'm assuming a scope?) I am in the process of checking with them but I would like to try it myself.

So from what I gathered, I can set my scope to lets say 300v and trigger it when the event spikes past 300. I assume this will freeze frame my scope at that point if it happens. If nothing happens, my trigger is too high, the sample rate is too slow, or there is no such event correct? The equipment supplies a pulsing voltage in the first place, this wont affect single trigger if it is set high enough if i understand correctly.

Thanks for all the feed back! First post on here, still learning!

(Also for you curious souls, I can't say what the equipment is due to an NDA, but if i were to print it out for you it would definitely be 3 dimensional as to what i am working on.)
 
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danadak

Joined Mar 10, 2018
4,057
I assume this will freeze frame my scope at that point if it happens.
Yes if in one shot mode.

If nothing happens, my trigger is too high, the sample rate is too slow, or there is no such event correct?
Yes, to a point. If sample rate too low, because scope is async sampling signal, eventually it will meet
conditions of sampling the transient and trigger. But display will be pretty unstable, unless doing
single shot trigger.

The equipment supplies a pulsing voltage in the first place, this wont affect single trigger if it is set high enough if i understand correctly.
Yes. You can also set a trigger for a pulse < some width or > some width. This is another way of getting at a transient.
Even pattern triggers can be done, or mask triggers. Depends on scopes capability.

1585673727966.png


Regards, Dana.
 
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