Floating the 'scope
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From: John Larkin on 7 Mar 2010 20:44 On Sun, 07 Mar 2010 01:38:48 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sat, 06 Mar 2010 18:31:01 -0600, The Phantom wrote: > >> 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. > >How many screens will it window? > >7A13 will measure a 1mV portion of a 10V signal. > > >Does it have a calibrated comparison voltage? Useful for things like >measuring staircase steps. > >> >> 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. > >I've never needed 4 differential channels, but if I did, I'd use two >mainframes and four 7a13s (Yes, I do have four) > >I've used a 7a13 for vertical deflection on a couple of occasions. You can >put amplifiers in timebase slots. Full differential X-Y. A 7A13 is wonderful in lots of apps, but it won't do differential input and offset at the same time. If I have, say, a power amp with +-200 volt rails, and I want to measure the waveform across a small-value resistor flailing around with the output voltage, the TPS is perfect. And the traces are in color! John
From: George Herold on 8 Mar 2010 10:36 On Mar 7, 1:08 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> wrote: > George Herold wrote: > > > On Mar 6, 5:04 pm, "Michael A. Terrell" <mike.terr...(a)earthlink.net> > > wrote: > > > 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. > > > Yup, I'm just like most idiots! (But I put green tape on both ends of > > the cord.) The ground buster was before the universal AC input. > > Good for you. That would get you fired any place I've ever worked. > If you needed isolation, you plugged whatever you were working on into > an isolation transformer. If OSHA found a cord like that, the company > would be fined and it could run into thousands of dollars. > > -- > Greed is the root of all eBay.- Hide quoted text - > > - Show quoted text - Thanks Michael, The isolation transformer is on order. George H.
From: George Herold on 8 Mar 2010 10:53 On Mar 8, 4:44 am, Fred Abse <excretatau...(a)invalid.invalid> wrote: > On Sun, 07 Mar 2010 17:44:14 -0800, John Larkin wrote: > > 7A13 is wonderful in lots of apps, but it won't do differential input > > and offset at the same time. > > Two 7A13s, mainframe set to add, channel 1 Vc to -ve, channel 2 Vc to +ve.. > > Never done it, but I'll try when I get a chance. CMRR won't be as good, > admittedly. > > > > > If I have, say, a power amp with +-200 volt rails, and I want to measure > > the waveform across a small-value resistor flailing around with the > > output voltage, the TPS is perfect. And the traces are in color! > > Never could get along with digital 'scopes. Only use one when I want to > save or print a trace. > > I find different-colored traces confusing. I see colors as numbers. Can > you color the traces in number order - R, Or, Y, G, etc? That might work > for me. > > -- > "Electricity is of two kinds, positive and negative. The difference > is, I presume, that one comes a little more expensive, but is more > durable; the other is a cheaper thing, but the moths get into it." > (Stephen Leacock) Digital scopes use to have terrible user interfaces. But they are so much better now. The colored knobs are associated with the trace color. It's so easy to know which knob to grab... try it you may like it. George H.
From: John Larkin on 8 Mar 2010 11:29 On Mon, 08 Mar 2010 01:44:35 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sun, 07 Mar 2010 17:44:14 -0800, John Larkin wrote: > >> 7A13 is wonderful in lots of apps, but it won't do differential input >> and offset at the same time. > >Two 7A13s, mainframe set to add, channel 1 Vc to -ve, channel 2 Vc to +ve. That's like using two probes in A-B mode. The equivalentcommon-mode rrange is small, because the internal amps saturate at the equivalent of a couple of screens worth of offset. The TPS can show tou a 10 millivolt signal riding on the AC line, with 200 MHz bandwidth. The 7A13 is great for huge amounts of zooming, like looking at pulse tops or recovery to zero. It recovers from massive overloads instantly, which most scopes don't do very well. > >Never done it, but I'll try when I get a chance. CMRR won't be as good, >admittedly. > >> >> If I have, say, a power amp with +-200 volt rails, and I want to measure >> the waveform across a small-value resistor flailing around with the >> output voltage, the TPS is perfect. And the traces are in color! > >Never could get along with digital 'scopes. Only use one when I want to >save or print a trace. > >I find different-colored traces confusing. I see colors as numbers. Can >you color the traces in number order - R, Or, Y, G, etc? That might work >for me. Nowadays I find all-green scopes confusing. And I know that the purple probe is the purple trace. Nice. John
From: John Larkin on 8 Mar 2010 11:32
On Mon, 8 Mar 2010 07:53:52 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Mar 8, 4:44�am, Fred Abse <excretatau...(a)invalid.invalid> wrote: >> On Sun, 07 Mar 2010 17:44:14 -0800, John Larkin wrote: >> > �7A13 is wonderful in lots of apps, but it won't do differential input >> > and offset at the same time. >> >> Two 7A13s, mainframe set to add, channel 1 Vc to -ve, channel 2 Vc to +ve. >> >> Never done it, but I'll try when I get a chance. CMRR won't be as good, >> admittedly. >> >> >> >> > If I have, say, a power amp with +-200 volt rails, and I want to measure >> > the waveform across a small-value resistor flailing around with the >> > output voltage, the TPS is perfect. And the traces are in color! >> >> Never could get along with digital 'scopes. Only use one when I want to >> save or print a trace. >> >> I find different-colored traces confusing. I see colors as numbers. Can >> you color the traces in number order - R, Or, Y, G, etc? That might work >> for me. >> >> -- >> "Electricity is of two kinds, positive and negative. The difference >> is, I presume, that one comes a little more expensive, but is more >> durable; the other is a cheaper thing, but the moths get into it." >> � � � � � � � � � � � � � � � � � � � � � � �(Stephen Leacock) > >Digital scopes use to have terrible user interfaces. But they are so >much better now. The colored knobs are associated with the trace >color. It's so easy to know which knob to grab... try it you may >like it. > >George H. Yeah, some of the early digital scopes, especially the HPs, were nightmares to drive. It turns out that the classic arrangement of knobs is best, so most digital scopes now resemble old analog scopes. John |