Re: Re: Re: Question for O-scope people out there…

From: Altan Stalker <pinball_at_aaarfamily.com>
Date: Sat Sep 15 2012 - 12:10:07 EDT

iPhone...

On Sep 15, 2012, at 11:29 AM, Mark Shostak <shostakmark@gmail.com> wrote:

> Altan,
>
> Not to be a fire hose (possibly literally), and it's great to learn
> through experimentation, but not as much when there are knives in the
> sandbox. With all due respect, you're playing in an area where best
> case, you may destroy your scope, and worst case you could get hurt.
>
> If you really want to hook your scope to the "line" (when you find
> it), I suggest you buy a device specifically designed to make that
> happen. I've used one made by Oneac, and you can probably find them on
> ebay (I was using them in the '80s, so they're probably cheap). It has
> an IEC connector on one end and BNCs on the other, doing exactly what
> you're asking about.
>
> You'll also need a scope with a lot of storage, possibly more than
> exists. A lot of waves go by at 60Hz while waiting for an event. An
> alternative would be to see if you can find a recording line monitor,
> which will capture these events. I suspect this is more of what you're
> looking for (as opposed to what you're asking for). Again, in the '80s
> they just printed on register tape, and now probably usb stick or
> something. Look and you can probably find one cheap. This will take a
> lot of the time and effort out of the adventure, and provide
> information more useful to what you are trying to accomplish. Granted,
> it may not be as entertaining as connecting your scope ground lead to
> "the line". :-)
>
> My 2 credits*
>
> Thanks,
> -Mark
>
> * - w/ credit to whomever came up with that line
>
>
> On 9/13/12, Altan (GAPAS) <pinball@aaarfamily.com> wrote:
>>
>> 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 12:10:23 2012

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