Floating the 'scope
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From: The Phantom on 6 Mar 2010 19:31 On Sat, 06 Mar 2010 15:59:17 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 17:37:01 -0600, The Phantom wrote: > >> 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. > >No comparator / voltage slideback function. That's what the vertical offset knob is for. You only get one differential channel per 7A13. Each isolated channel on the TPS2024 is a differential input when the ground clip can be connected anywhere, so there are 4 differential channels.
From: Hammy on 6 Mar 2010 20:08 On 6 Mar 2010 14:26:01 -0600, The Phantom <phantom(a)aol.com> wrote: > >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. Point taken. >You need to use floating differential probes, or a scope with true isolated >inputs. Yep a scope with true isolated inputs is the best solution.
From: Jamie on 6 Mar 2010 20:32 The Phantom wrote: > 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? > yes, they are on the right bottom side. I don't know why they don't show the picture of them. Power jack on the top left, along with the serial port/USB and back light switch on the left side.
From: Hammy on 6 Mar 2010 20:13 On 6 Mar 2010 15:10:02 -0600, The Phantom <phantom(a)aol.com> wrote: >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. The last time I went looking for one they were all in the thousands or at least over a grand. That was a couple of years ago though. There seems to be a lot more out there now for just a couple of hundred. Maybe its time to look around again.
From: Michael A. Terrell on 6 Mar 2010 21:44
"Paul E. Schoen" wrote: > > "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. I've seen a lot of IEC power cords with the ground pin cut off. Those were cut up for scrap. -- Greed is the root of all eBay. |