OK, so I have access to an oscilliscope on loan. :)
I need to watch the 120 VAC line, and also monitor the other side of an
optoisolator, which is, of course, supplied from a +5 VDC circuit that
shares no common ground with the AC side.
My question is, how do I do this on a single, dual channel instrument?
I seem to remember from college that I need to isolate the scope using a
2 to three prong adapter, so I don't just short one leg of the AC line
to ground through the test lead. that would take care of the AC
measurement. The problem is, how do I also measure the +5 VDC side
without making a common ground? Can I (safely) tie the AC neutral line
to the circuit's ground? I see sparks and singed eyebrows in my future
if I do that...
my other option is to trigger only on the zero-cross from the A/C line,
so I could possibly get enough "noise" by using the +5 VDC and ground
lines for the measurement, and trigger on the A/C line. I'd get more
useful data, however, by seeing both traces at once.
> OK, so I have access to an oscilliscope on loan. :)
Answer 1. oscilloscope
^
> I need to watch the 120 VAC line, and also monitor the other side of an
> optoisolator, which is, of course, supplied from a +5 VDC circuit that
> shares no common ground with the AC side.
Answer 2.
Use a small transformer from 120 VAC to 6 or 12 volts. Monitor the output
of that instead of the 120VAC line.
Do Not connect your 'scope to the power line. Do Not float your 'scope by
lifing the ground prong.
Bob Blick wrote:
> Use a small transformer from 120 VAC to 6 or 12 volts. Monitor the output
> of that instead of the 120VAC line.
The output of the transformer that supplies the +5 VDC regulator should
work, then.
Will there be any phase difference between the primary and secondary
side? That is, will both cross zero at exactly the same moment? I'd
sort of assume yes, as, well, that's what transformers do... This is a
dual secondary, with the outputs connected in parallel for higher current.
> Do Not connect your 'scope to the power line. Do Not float your 'scope by
> lifing the ground prong.
Thanks for the advice. I didn't want to fry myself, my project, or the
equipment.
At 04:13 PM 12/6/2005, M Graff wrote:
>OK, so I have access to an oscilliscope on loan. :)
>
>I need to watch the 120 VAC line, and also monitor the other side of
>an optoisolator, which is, of course, supplied from a +5 VDC circuit
>that shares no common ground with the AC side.
>
>My question is, how do I do this on a single, dual channel instrument?
Recognize that the AC Hot is just that: hot with respect to
ground. Ground as in earth ground.
Assuming that your scope is grounded by a good ground in the AC plug,
simply touch the tip of the scope probe to the Hot side of the AC
line. Make sure that the probe is in the 10X position if it has one
and ensure that the combination of the scope probe and input setting
of the vertical attenuator for that channel allows for safe
measurement of that voltage (about 150V peak).
Note that the ground lead for that scope probe is NOT connected anywhere.
You should now be able to see a good sine wave on that channel of the scope.
If that is the case, simply use the other channel of the scope as per
normal and measure the output side of the opto.
Note that this technique is fairly limited - noise on the protective
ground can give the impression that the incoming AC sine wave is
dirtier than it really is. But the alternative method of floating
the scope AC ground and connecting the ground lead of the scope probe
to the incoming neutral (and hoping that you don't get it wrong) is
DANGEROUS and should not be done.
Celebrating 21 years of Engineering Innovation (1984 - 2005)
.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Do NOT send unsolicited commercial email to this email address.
This message neither grants consent to receive unsolicited
commercial email nor is intended to solicit commercial email.
>> Use a small transformer from 120 VAC to 6 or 12 volts. Monitor the
>> output
>> of that instead of the 120VAC line.
>
> The output of the transformer that supplies the +5 VDC regulator should
> work, then.
Not unless one of the transformer terminals is at circuit ground. That
would be if you have either a center tapped transformer, or are doing only
half wave rectification.
> Will there be any phase difference between the primary and secondary
> side? That is, will both cross zero at exactly the same moment? I'd
> sort of assume yes, as, well, that's what transformers do... This is a
> dual secondary, with the outputs connected in parallel for higher current.
I would not use a transformer that is driving a rectifier load if you need
high accuracy. Use a separate transformer, with a small load resistor (a
percent or two of rated current) for most accuracy.
Good advice! Ballpark of 20 years ago, one of the other engineers where
I was working desperately "needed" to do just that. He roped off the
area with "caution tape" and set things up. Setup was fired up and all
was going well. I happened along and was warned about what was going
on. Another engineer walked up, and was told what was going on. He
immediately crossed into the taped off area and attempted to adjust the
scope controls "for a better view". I got quite a lesson in colorful
language and the third engineer got a lesson in caution.
Bob Blick wrote:
>Do Not connect your 'scope to the power line. Do Not float your 'scope by
>lifing the ground prong.
>
planet.eon.net>
