Floating the 'scope
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From: The Phantom on 6 Mar 2010 15:49 On Sat, 06 Mar 2010 15:57:21 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: >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. What's the model number of that scope? > > 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: Paul E. Schoen on 6 Mar 2010 16:05 "Fred Abse" <excretatauris(a)invalid.invalid> wrote in message news:pan.2010.03.06.11.18.10.148083(a)invalid.invalid... > On Fri, 05 Mar 2010 19:05:54 -0800, George Herold 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. >> >> > > Why not invert one channel and switch to "add". > > Then use both inputs differentially. > > Most, if not all worthwhile instruments should do that. > > Probe compensation needs to be accurate for best CMRR. > > Or buy a differential probe? This will not work for reading a voltage of 1 or 2 volts or less, which is floating as much as 400 volts above ground. For instance, reading the gate voltage of an SCR on a 480 VAC mains system. You must float the scope, and I've done it safely and effectively, with a hand-held scope. There may be some differential probes that can handle this; I don't know. Paul
From: The Phantom on 6 Mar 2010 16:10 On Sat, 06 Mar 2010 07:17:49 -0500, Hammy <spam(a)spam.com> wrote: >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. > >Anyone use any of PICO's differntial probes? > > This ones only $350.00 Canadian. > >http://canada.newark.com/pico-technology/oscilloscope-x10-x100/active-differential-oscilloscope/dp/02M0836 Also available are these: http://www.caltestelectronics.com/www/Cat1GetSubCategory.asp?PN=General%20Purpose%20Differential%20Probes&ID=1.c.1&subcat=Differential%20Probes I think they may be made by the same folks that make the Yokogawa differential probes. I've used the 100MHz Yokogawa probes, and they are very handy.
From: Jamie on 6 Mar 2010 17:14 The Phantom wrote: > On Sat, 06 Mar 2010 15:57:21 -0500, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: > > >>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. > > > What's the model number of that scope? > > http://www.saelig.com/PSHA/PSSA017.htm HDS1022MN and the 100 Mhz version http://www.saelig.com/PSHA/PSSA023.htm
From: Michael A. Terrell on 6 Mar 2010 17:04
The Phantom wrote: > > 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? Most idiots just cut the ground pin off the power cord. -- Greed is the root of all eBay. |