>What is the "Line"? Does this mean the home's ground or neutral line?
"Line" is another way of saying "Live". This is the polar opposite of
'Ground' and 'Neutral'.
>What is a reasonable place to connect the probe's ground when checking an
>AC signal?
'Ground' would be the safest. In theory, the 'Neutral' should be safe but
someone might have wired the outlet incorrectly so you are better off
checking it using a 3-light plug-in outlet checker first to make sure.
>If the power board uses the typical 4 diode approach (bridge rectifier) to
>convert from AC to DC, does this count as isolated?
Absolutely not. The rectifiers definitely do not provide any isolation.
Connecting a scope ground to the 0VDC side of a bridge or rectifier will
make a short across the rectifier. Never do that.
>I believe every pin/vid I've seen uses this approach.
No they don't. You have to be misinterpreting something because it would
for sure damage something.
>Although there is a transformer more upstream. Is the transformer
>performing the isolation?
Transformers themselves do provide isolation from the main AC line, however
that doesn't mean that there are no windings tied to AC or Earth ground so
you have to be sure that there are not. Most arcade games have an isolation
transformer specifically for safety and because most monitors require one.
If you take a scope that is plugged into the wall and connect its probe
ground to the 0VDC point within a monitor that is required to operate with
an isolation transformer, YOU WILL BLOW the monitor and possible damage your
scope. To use the scope safely, you MUST use either isolated probes or plug
the scope into its own isolation transformer.
An important word about using an isolation transformer with a scope. While
the L and N wires will be isolated, the Earth/Chassis ground typically is a
direct path. This is important to note because this is the path that needs
the isolation! To break this path, you must remove the ground prong from
the scope power cord. Some isolation transformers come without an enclosure
so you would install it inside a project box and wire through only the 2
primary coil wires from the AC supply side (through a fuse and power switch
is recommended) and wire only the 2 secondary coil wires out to an outlet
that you then plug the scope into. You do not want to connect the ground
wire of the output side outlet to anything. Some isolation transformers
come with their own 3-prong outlet on them. The ones that I have had the
ground prong on the output side connected to their chassis/Earth ground. I
thought about disconnecting that wire but instead I use a power cord with
the ground prong removed. The trick is to remember that removing the ground
prong from the scope power cord only isolates the scope if you have it
plugged into an isolation transformer, not the wall !
>Could you provide me a link to a reasonable isolation transfer, just so I'm
>sure I'm looking at the right thing?
I'm sure that you can Google this as easily as I can. An isolation
transformer is a 1:1 with single coil primary and single coil secondary.
Most arcade supply online stores sell them. I believe Happ Controls also
sells them. I did a quick search on eBay and tons of different ones came
up. All you have to do is consider the number of amps that it must handle.
Also, I read Mark's suggestions and agree 100% with everything that he said.
There's some excellent advice there. While standard oscilloscopes can
measure AC line power and certain DC stuff at the same time, it's really not
something that I'd recommend anyone doing. There is specialized equipment
available that works safely with AC line power. All it takes is one little
mistake and bang, you or your equipment can be fried toast.
William Boucher
http://www.biltronix.com
----- Original Message -----
From: "Altan (GAPAS)" <pinball@aaarfamily.com>
To: <vectorlist@vectorlist.org>
Sent: Thursday, September 13, 2012 9:07 PM
Subject: VECTOR: Re: VECTOR: 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 Sat Sep 15 15:03:53 2012
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