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Yet another oscilloscope thread

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dknguyen

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I'm getting to the point where my projects are involving lots of switching power circuits and I probably need to get my own oscilloscope. Prior to this I mainly needed an oscilloscope as a logic analyzer but that got old really fast and went out and bought one.

The main issue is there are used analog scopes out there with really good bandwidths and 4-channels, perfect for hunting down waveform problems and transients in 3-phase switching circuits. The only problem is that they can't catch single shot events, so no observing startup waveforms or startup transients. But for the same money, getting this single-shot capability comes at the cost of reducing the scope down to 2 channels with almost 7x less bandwidth.

I'm curious as to the experience of those who have worked with switching power circuits a lot and how often they had to turn to single-shot capture modes.
 

MrAl

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Hello there,


Many times a cheap two channel analog scope can work out pretty well for the basic switching circuits at 100kHz or less, but probably even higher.
You can often see the start up waves anyway if you look at the scope while turning on the circuit and have the trigger level set right. Granted a storage scope is going to be easier to use, but probably not necessary.
Even a bandwidth of 20MHz isnt too bad for this, but i dont think i would go any lower than that. Two channels is a mandatory feature really though because input/output sync'ed signals are something we very often have to check out.
 
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dknguyen

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Won't the startup waveform not last long enough to be seen on a non-storage scope? I can sort of cook up rules of thumbs on what is needed to properly view a switching waveform, but to view troublesome transients...I have no knowledge of what type of BW Is required there except for pulling really conservative numbers out of thin air.
 
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MrAl

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Hello again,


You can see many transients by turning the room lights down and setting the scope trace on the brightest level, setting the trigger level, then initiating the startup sequence (as in turning on the power). Again, 20MHz min bandwidth and fairly new scope (old one might have lower level phosphor).

I only needed a storage scope once when trying to find out the current during a line tie operation with a converter running off of a large solar panel and supplying power back into the line. True though it would have helped other times too, but it wasnt that often when it would have made a difference.

The choice is really up to you.
 
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