Floating the 'scope
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From: Paul E. Schoen on 6 Mar 2010 17:48 "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote in message news:4B92D17E.A66A9652(a)earthlink.net... > > 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. Sometimes when I have seen this, I complete the job by cutting off the plug. Makes it safe again, but not very useful. Paul
From: The Phantom on 6 Mar 2010 18:37 On Sat, 06 Mar 2010 15:25:03 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 14:26:01 -0600, The Phantom 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. >> >> 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. > >7904, two 7A13s, job done. TPS2024. No plugins. 4 channels. Job done, and battery operation for ultimate portability.
From: The Phantom on 6 Mar 2010 19:03 On Sat, 06 Mar 2010 15:25:02 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 16:05:28 -0500, Paul E. Schoen wrote: > >> >> "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. >> > >The OP was talking about 15 volts common mode, where the technique will >work perfectly well. > > >56 dB CMR with a common mode voltage of 480 RMS is beyond the >capability of even a 7A13. There are differential probes that will do it, >but they have 4 figure price tags. The CT2593-1 described here: http://www.caltestelectronics.com/www/Cat1GetSubCategory.asp?PN=General%20Purpose%20Differential%20Probes&ID=1.c.1&subcat=Differential%20Probes is specified for 86 dB CMRR at 50 Hz, 66 dB at 20 kHz, and 1400 volt common mode rated for only $331 > >I've never needed to go to that extreme. Most of what I do is IGBTs with a >DC bus of around 280V, and the (now occasional) SCR bridge at 200V RMS. A >7A13 will handle those, as will a fairly cheap differential probe (few >hundred bucks). > >I hope the handheld you used was battery powered. I wouldn't like to stick >670-something volts peak across the PSU isolation of a floating >line-powered 'scope.
From: The Phantom on 6 Mar 2010 19:08 On Sat, 06 Mar 2010 15:25:01 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 14:39:01 -0600, 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 people that do electrical measurments and are in the habit of breaking the ground would have ground busters on hand. They might have to go to the hardware the first time to get one (or more), but never again. To disconnect the ground inside an instrument like a scope or signal generator would involve voiding the calibration. Just have a ground buster on hand. > >The one can be done in five minutes for free, the other involves going out >to the hardware store and costs money?
From: The Phantom on 6 Mar 2010 19:18
On Sat, 06 Mar 2010 17:14:50 -0500, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> wrote: >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 That last link has a better picture. Where are the scope inputs? I see 4 multimeter-like inputs along the bottom, colored red, red, black, red from left to right. Are these also the inputs for the scope? I don't see any other inputs. Are they on the side? |