Re: Question for O-scope people out there…

Great info, William. Thanks for replying.

Could you clarify a couple of things for me?

>>> "… you have to be careful that you never connect the scope probe ground lead to the Line because that's a short circuit."

What is the "Line"? Does this mean the home's ground or neutral line?

What is a reasonable place to connect the probe's ground when checking an AC signal?

>>> "The ground of the two sources gets connected by the scope probe ground leads to the Earth ground of the mains AC system but as long as the supply of the DC system is isolated from the AC mains then there's no problem"

If the power board uses the typical 4 diode approach (bridge rectifier) to convert from AC to DC, does this count as isolated? I believe every pin/vid I've seen uses this approach. Although there is a transformer more upstream. Is the transformer performing the isolation?

>>> "A solution is to plug the scope into its own isolation transformer. This is what I always do"

Could you provide me a link to a reasonable isolation transfer, just so I'm sure I'm looking at the right thing?

Thanks again!

… Altan

On Sep 11, 2012, at 9:06 PM, William Boucher <wboucher6@cogeco.ca> wrote:

> Typical scopes can certainly handle the voltage of 110/220 VAC (aka mains power), but as always when dealing with AC power you have to bear in mind that the standard scope probes are not "isolated" meaning that the probe ground leads are common to the scope chassis and so are also common to the AC mains Earth ground (which ultimately is tied to Neutral at some point in the system).
>
> So to answer your question, you can certainly measure 110VAC with a scope but you have to be careful that you never connect the scope probe ground lead to the Line because that's a short circuit. Also, the short path is through the scope probe lead so it might in fact "light up" quite literally unless something else inside the scope blows out first.
>
> You can measure DC voltage on one channel and AC voltage on the other channel, but again, keep in mind that the ground (or 0V point) of each channel must already be common or you will short them together as soon as you connect the scope probe ground leads to the 0V point of each circuit that you are trying to measure.
>
> This is the same situation as using two probes to measure two different DC voltages. The two voltage sources must have a common ground. The ground of the two sources gets connected by the scope probe ground leads to the Earth ground of the mains AC system but as long as the supply of the DC system is isolated from the AC mains then there's no problem. This said, some DC systems must remain isolated from the AC mains. For instance, most, if not all, raster monitors have internal circuitry that must remain isolated from AC mains. Connecting a scope (that's plugged directly into the wall) probe ground to the video input connector ground will likely blow the monitor (for example the HOT horizontal output transistor may instantly die). A solution is to plug the scope into its own isolation transformer. This is what I always do.
>
> Whenever you want to measure the voltage across something like a "current sensing resistor", you should use two probes connected one on each side of the resistor and both of the probe grounds to the circuit ground. The difference between the two signals provides the desired waveform. If you feel that you must connect a single probe directly across the sense resistor, you can under certain conditions. For instance, if one side of the sense resistor is already connected at the 0V point (common ground) then you can use the 2'nd probe to measure something elsewhere. Bear in mind that id neither side of the sense resistor is at 0V (ground) then when you connect a probe directly across it, you cannot use the other probe for anything because it's ground lead would short one side of the sense resistor to whatever point to which you connect the 2'nd probe ground lead.
>
> The best solutions:
> 1/ Think carefully about what you are doing before connecting anything.
> 2/ Use isolated probes (cost more and must be purchased separately)
> 3/ Use an isolation transformer where appropriate
> 4/ Use a battery powered scope so that it remains isolated from Earth ground.
>
> DO NOT cut the ground lead off of the scope power cord. Doing this will not isolate it. Remember that the output ground terminal is connected to the Neutral terminal back at the breaker box. Keep in mind that the Ground return is a redundancy/backup path for the Neutral path.
>
>
> William Boucher
> http://www.biltronix.com
>
>
> ----- Original Message ----- From: "Altan (GAPAS)" <pinball@aaarfamily.com>
> To: <vectorlist@vectorlist.org>
> Sent: Tuesday, September 11, 2012 8:34 PM
> Subject: VECTOR: Question for O-scope people out there…
>
>
>
> Is it safe to use an entry level o-scope (like the Rigol DS1102E or Owon xxxx) with AC voltage? I suspect the answer is an obvious "Yes", but wanted to verify. It doesn't seem to be specifically stated (again, probably because it's obvious).
>
> Being able to look at AC as well as DC would be helpful when trying to trigger on AC voltage drops and viewing results on regulated DC.
>
> The follow up: is one able to have an AC source on CH1 and a DC source on CH2?
>
> Thanks!
>
> … Altan
> www.aaarpinball.com---------------------------------------------------------------------------
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Received on Thu Sep 13 21:07:29 2012