Floating the 'scope
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From: phantom on 7 Mar 2010 08:06 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? I'll have to check that on Monday, but I've never needed more. > >7A13 will measure a 1mV portion of a 10V signal. 10 volt offset is the max the 7A13 can do. The TPS2024 can do way more than that for large signals. > > >Does it have a calibrated comparison voltage? Useful for things like >measuring staircase steps. The value of the offset is displayed as a 3 digit number on screen. The deflection factor is also calibrated. > >> >> 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) When using two mainframes, how do you see all four traces on the same screen? > >I've used a 7a13 for vertical deflection on a couple of occasions. Isn't vertical deflection the usual mode? Maybe you mean that you used a 7A13 for horizontal deflection. Isn't that what you'd have to do for X-Y mode? >You can >put amplifiers in timebase slots. Full differential X-Y. Most modern scopes have an X-Y mode; so does the TPS2024. Since all the channels on the TPS2024 are effectively differential, its X-Y mode is a differential X-Y mode The TPS2024 can do math functions on differential signals, too.
From: George Herold on 7 Mar 2010 10:20 On Mar 5, 11:16 pm, John Tserkezis <j...(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. > > 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. Sorry I'm not trying to be stupid. I certainly wouldn't float the scope if I was looking at high voltages or AC line signals. Most often I float the scope because I've got some interference that looks like it might be a ground loop. I want to see if it's through the scope. I have a single AC line cord with the third wire clipped. It's got green tape on both ends of the plug. It stays in my wire drawer. If it somehow makes it's way onto the production floor I can easily retrieve it (The green tape helps.) George H.
From: George Herold on 7 Mar 2010 10:28 On Mar 6, 2:36 am, John Tserkezis <j...(a)techniciansyndrome.org.invalid> wrote: > On 6/03/2010 5:31 PM, Paul E. Schoen wrote: > > > Handheld and portable scopes often have floating inputs, and it is > > Even if a portable scope is double insulated, that's not always safe. > > If you clip the "ground" of the scope probe to a live line, that shield > and any metal bits all the way to the scope are also live. > > If the ground clip is exposed, it'll bite (I've frequently seen it, and > on some probes, that's the way it's built) , if the insulation is > damaged, it'll bite. > > Also note that none of the cable insulations I've seen are rated for > high voltage, only the centre conductor is good for the specified > voltage, not the outer to the outside world. > > Your only avenue is to completely isolate yourself from ground, which > is going to prove to be a bit difficult, considering the capacitance you > can't really avoid.. > Forget about wearing an antistatic strap. I've worked in some > environment where it's mandatory, so that's going to be a bit of a > challenge. > > Or use an earth leakage breaker, and have the thing cut out on you a > dozen times a day even under normal use. > > There are too many things that can go wrong. There are no procedures > to ensure all the avenues are covered and enforced to prevent shocks > from this method. Oh, connecting to line voltages is something I thankfully have to do rarely, and when I do it's with great care. (I do own a suicide plug that is a one piece AC to banana... scary...) George H.
From: George Herold on 7 Mar 2010 10:34 On Mar 6, 7:03 am, Hammy <s...(a)spam.com> wrote: > On 6 Mar 2010 03:09:01 -0600, The Phantom <phan...(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. Thanks Hammy, you're right we should buy an isolation transformer. Is it OK to power the 'scope with the isolation tranny? Or does this defeat the whole purpose again? "> Its not really a good idea to float your scope." What about floating it to get rid of ground loops? George H.
From: George Herold on 7 Mar 2010 10:47
On Mar 6, 3:18 pm, Fred Abse <excretatau...(a)invalid.invalid> wrote: > 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? > > -- > "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) Yup, that's what I should have done, But I'd have had to find the matching 'scope probe (They tend to wander around and become separated.) And the floating AC cable was lying there behind the equipment. (I'd just been using it.) George H. |