Floating the 'scope
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From: The Phantom on 6 Mar 2010 15:26 On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam(a)spam.com> wrote: >On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom(a)aol.com> wrote: > > >> >>Tektronix has probes designed to solve these problems: >> >>http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN >> >>They are specifically intended for floating use with the TPS2000 series scopes, >>which, as John mentioned, have true isolated inputs. >> >>Or, you can use these probes with an ordinary scope: >> >>http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN >> >Those are nice expensive but nice. > >You should use an isolation transformer on any line powered device >your testing. You can usually find a 500VA one for under 100 bucks. I >got a TEMA 500VA one for 45 bucks from Newark. Well worth the money. > > Its not really a good idea to float your scope. A problem arises when you need to make measurements with a 2 or 4 channel scope at locations in the circuit which don't have a common reference. For example, suppose you need to look at the gate-source voltages of the two top MOSFETs in an H-bridge configuration. Isolating the equipment doesn't allow you to connect the ground clips of the two scope probes to different points in the circuit that have a substantial voltage difference. You need to use floating differential probes, or a scope with true isolated inputs.
From: The Phantom on 6 Mar 2010 15:32 On Sat, 06 Mar 2010 07:03:56 -0500, Hammy <spam(a)spam.com> wrote: >On 6 Mar 2010 03:09:01 -0600, The Phantom <phantom(a)aol.com> wrote: > > >> >>Tektronix has probes designed to solve these problems: >> >>http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13471&lc=EN >> >>They are specifically intended for floating use with the TPS2000 series scopes, >>which, as John mentioned, have true isolated inputs. >> >>Or, you can use these probes with an ordinary scope: >> >>http://www2.tek.com/cmswpt/psdetails.lotr?ct=PS&cs=psu&ci=13415&lc=EN >> >Those are nice expensive but nice. > >You should use an isolation transformer on any line powered device >your testing. In days gone by, cheap consumer devices like televisions and table radios often had a "hot" chassis; it was connected to one side of the line. And, in those days, the line cord wasn't polarized; there wasn't even a third ground prong on the outlet. Using an isolation transformer when repairing such a device was a good idea. >You can usually find a 500VA one for under 100 bucks. I >got a TEMA 500VA one for 45 bucks from Newark. Well worth the money. > > Its not really a good idea to float your scope.
From: Jamie on 6 Mar 2010 15:57 The Phantom wrote: > On Sat, 06 Mar 2010 12:26:16 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: > > >>John Larkin wrote: >> >> >>>On Fri, 5 Mar 2010 19:05:54 -0800 (PST), George Herold >>><ggherold(a)gmail.com> wrote: >>> >>> >>> >>>>Why do they make 'scope inputs ground referenced to the third wire of >>>>the AC plug? Today I smoked the 10 ohm resistor that was separating >>>>The AC ground from the 'ground' of my circuit. I'd forgotten to float >>>>the 'scope and connected -15V to the ground clip of the scope probe. >>>> >>>>George H. >>> >>> >>>That's a safety thing. >>> >>>Our TPS2024 has four truly floating channels and floating trigger >>>input. You can hook a scope ground clip anywhere. >>> >>>John >>> >> >>So does my cheap crappie OWEN 20 mhz hand held. That also includes >>isolation between the DMM inputs. > > > Is this a two channel scope? If so, what does an ohmmeter measure between the > shells of the two BNC inputs of the two channels? > > Aren't the two BNC input grounds connected to the same internal ground > reference? > >> > The 2 BNC's are isolated. I have no idea how they are doing it how ever, I can say this, I don't like the BNC receptacles. It's a hard plastic shell instead of metal. I guess that's part of the isolation. It does work.. I've used it to measure signals around a resistor that is far from any common node with one probe while the other has it's common attached to the device common. It seems to work .. He DMM inputs are standard DMM probes.. and are also isolated. I don't think its that hard to fab 20 mhz isolated analog amps.
From: The Phantom on 6 Mar 2010 15:37 On Sat, 06 Mar 2010 15:16:07 +1100, John Tserkezis <jt(a)techniciansyndrome.org.invalid> wrote: >On 6/03/2010 2:49 PM, Phil Allison wrote: > >>> "George Hairoil" >>> I've been floating 'scopes for.... say since the early '80's. I >>> haven't been biten by it yet. (Though I've made plenty of other >>> mistakes.) What are the obvious safety reasons? > >> ** My god you are stupid. > > Welcome to an entirely new class of stupid, Phil. > > At a place I used to work at, the cro on my bench was floated by the >previous "tech", he just forgot to tell anyone about it. And forgot to >label it too. No clues at all unless you measured it, or took it apart >to actually look. Why would anyone take a scope apart and disconnect the line cord safety ground from the chassis inside the scope? Whenever I've seen anyone do this, they just use a ground buster, easily obtainable from the hardware store, to disconnect the safety ground at the outlet. > > Made worse by the fact it was a dual trace cro, and could have done the >job without floating it. > > I can only presume these idiots have been doing for so long, they stop >caring about the fact you hadn't had to do it since two trace (or more) >CROs appeared on the market. > Hey, I don't care if they want to kill themselves, but their idiocy >will continue to kill others well into the future. > > I'm starting to think that being dropped as a baby isn't enough to >explain this level of stupid.
From: The Phantom on 6 Mar 2010 15:39
On Sat, 06 Mar 2010 12:18:17 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 15:16:07 +1100, John Tserkezis wrote: > >> On 6/03/2010 2:49 PM, Phil Allison wrote: >> >>>> "George Hairoil" >>>> I've been floating 'scopes for.... say since the early '80's. I >>>> haven't been biten by it yet. (Though I've made plenty of other >>>> mistakes.) What are the obvious safety reasons? >> >>> ** My god you are stupid. >> >> Welcome to an entirely new class of stupid, Phil. >> >> At a place I used to work at, the cro on my bench was floated by the >> previous "tech", he just forgot to tell anyone about it. And forgot to >> label it too. No clues at all unless you measured it, or took it apart >> to actually look. >> >> Made worse by the fact it was a dual trace cro, and could have done the >> job without floating it. >> >> I can only presume these idiots have been doing for so long, they stop >> caring about the fact you hadn't had to do it since two trace (or more) >> CROs appeared on the market. >> Hey, I don't care if they want to kill themselves, but their idiocy >> will continue to kill others well into the future. >> >> I'm starting to think that being dropped as a baby isn't enough to >> explain this level of stupid. > >Not such a new class. > >I used to have a technician who had worked at a large equipment hire >outfit. He told me that checking for disconnected ground was SOP when >checking returned instruments. As I asked John, why would anyone go to the trouble to get inside a piece of equipment to disconnect the safety ground, when they can just use a ground buster at the wall socket? > >They found several each week. |