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From: Tim on 4 May 2010 19:22
In article <84bo4oFv1cU1(a)mid.individual.net>, invalid(a)invalid.invalid
says...
> Tim wrote:
> > In article <Le1En.91519$3k7.8275(a)en-nntp-13.dc1.easynews.com>,
> > zapwireDASHgroups(a)yahoo.com says...
> >> "Tim" <tim(a)tim.tim> wrote in message
> >> news:MPG.264a75e4d2dc173b98974d(a)news.aliant.net...
> >>> I have been trying to get a simple ac waveform to measure correctly on
> >>> this and it just won't display it properly.
> >>>
> >>> Basically if I attach the 10:1 probe to an AC transformer with say 6 VAC
> >>> output, the displays says it is 7.3 VAC, but the waveform will occupy 4
> >>> divisions on the scale at 5V setting, indicating over 20 volts.
> >>>
> >>> Have I missed something here?
> >> Probably. A "6VAC" transformer is spec'd as putting out ~6V RMS at its rated
> >> load, typically, and under *no* load will put out rather more... such as 7.3V
> >> RMS.
> >>
> >> A 7.3V RMS sine wave have a peak-peak amplitude of 2*sqrt(2)*7.3=20.6V, which
> >> if what you're getting by counting divisions, right?
> >>
> >> This gets even more fun when you go from dBm to volts or divisions...
> >>
> >>
> >
> > OK, so if I work the waveform equation backwards, I can work out the
> > voltage of the transient strikes that are killing the circuitry on the
> > other side.
> >
>
> If it's really transients, chances are that a scopemeter is too slow for
> that, not enough bandwidth. Depends on what sorts of spikes you suspect.
> You might need some TVS there.
>
>
> > I take it the scope is reading the peak and the meter side is is reading
> > RMS values.
> >
>
> That's the customary way. Meters read RMS and scopes show the waveform,
> they cannot "show" RMS because they display the whole sinewave. What
> some scopes can do is calculate the RMS and display it in a corner
> somewhere as a numerical value.
>
> [...]
>
>
I believe that is what this one has been doing, and I was too dumb to
see it.

I check the machine, and it was peaking at around 142 VAC, with a 50VAC
mechanical meter counting the pulses. Now I know the values to put in to
stop the problem. The voltage was exceeded by about 50% of the max for
the components. No question as to why they failed.

Thanks to all for your concise replies.

- Tim -
From: Jamie on 4 May 2010 20:07
Tim wrote:
> Does anyone have any experience with one of these?
>
> I have been trying to get a simple ac waveform to measure correctly on
> this and it just won't display it properly.
>
> Basically if I attach the 10:1 probe to an AC transformer with say 6 VAC
> output, the displays says it is 7.3 VAC, but the waveform will occupy 4
> divisions on the scale at 5V setting, indicating over 20 volts.
>
> Have I missed something here?
Lets see here at 6 V RMS
6*1.414=8.484*2=16.968 P-P
Now at 7.3 RMS
7.2*1.414=10.3222*2=20.64444

Rup.
Looks like I know Kentucky math!

it appears that your scope is working fine because what you're seeing
is a Peak To Peak reading when accounting for those 4 divisions vertically.
20VPP is what you're seeing..

But your scope is giving you the base line to RMS reading from what I
can tell. Which is most likely what you're after.

Here, this may help you!

http://www.youtube.com/watch?v=88E0TYijc5I

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