Floating the 'scope
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From: George Herold on 7 Mar 2010 10:54 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. George H.
From: Michael A. Terrell on 7 Mar 2010 13:08 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.
From: whit3rd on 7 Mar 2010 14:20 On Mar 6, 3:51 am, John Tserkezis <j...(a)techniciansyndrome.org.invalid> wrote: > Though it still needs to be said, if someone is of the mentality that > they can freely dismantle a piece of equipment that says "do not > dismantle" to intentionally remove a safety mechanism ... Lotsa luck selling the 'don't tamper' argument on this newsgroup. Better to point out the option of making a CLEARLY VISIBLE indication of the floating case, like a hulking isolation transformer next to the 'scope. Floating the 'scope is one of the great useful techniques, it must be implemented somehow on a good test workbench.
From: default on 7 Mar 2010 18:46 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. Think about it. The scope has a very high input impedance. With no ground, common mode unbalanced hash (inductive or electrostatic) will always give one a wave form - but not necessarily any useful one. That's one of the reasons you have two channels - one for the signal, one for the reference - and both shielded from common mode noise, A+B or dual/add etc. Learn to use your scope. A B&K scope made for TV servicing might have a floating ground - but unless your' re servicing TV's and only need a high frequency of 20 KHZ, you probably don't want some specialized, idiot proof, scope. The scope is part of the "circuit under test," your body (room, power distribution network, etc.) is part of the circuit too. Think about it and learn to deal with it. Those little pigtails from the scope probe with the ground alligator? aren't for convenience - they reduce noise pickup. Likewise a really serious switching circuit could require a very short wire to the ground slip ring at the tip of the scope and circuit under test. Every circuit is an RF circuit if it has switching transients. --
From: phantom on 7 Mar 2010 20:27
On Sun, 07 Mar 2010 11:30:16 -0800, Fred Abse <excretatauris(a)invalid.invalid> wrote: >On Sun, 07 Mar 2010 07:06:08 -0600, phantom wrote: > >>>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. > >That's at 1mV per division. >+/-100V at 10mV to 50mV per division >+/-500V at 0.1 to 0.5V per division >+/-500V at 1V to 5V per division. >Displayed voltage multiplied by 10 when x10 probe >sensed. >IOW, the +/- offset voltage can be applied to either input without >attenuation. It's a true slideback measurement using a variable >voltage and built-in DVM. Sufficient offset range achieves the same measurement result. > >> The TPS2024 can do way more >> than that for large signals. > >I guess you mean at greater input attenuator settings? > >As can the 7A13 > >> >>> >>>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. > >The 7A13 offset is actually a "real" DC voltage, indicated by a "real" DVM. >Also available at a front panel socket for external measurement. I see no advantage to being able to measure it at a front panel socket compared to having the scope tell you what the offset voltage is. > >There aren't many instruments whose deflection factor isn't calibrated. >They wouldn't be of much use without. You mentioned "calibrated comparison voltage" and "measuring staircase steps". The calibrated deflection factor serves the function just as well. > >> >> >>>> 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? > >You don't. You use two screens. Hardly the same as having four color traces on one screen, is it? > >> >>>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're right, I meant horizontal (X). I tend to think "X and Y". My mistake. >Peccavi et mea freakin' culpa! > >> >>>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 > >I don't think I've ever had a 'scope that wouldn't do X-Y. going back 40 years. >Some had restricted X bandwidth. Indeed. So since I had already told you that the TPS2024 had 4 effectively differential inputs, why did you mention "You can put amplifiers in timebase slots. Full differential X-Y." as though that were an advantage that the 7A13 has that the TPS2024 doesn't? This whole back and forth seems to consist of you telling me things that the 7A13 can do that the TPS2024 can't. So for, I see that the TPS2024 substantially outshines the 7A13. Others can draw their own conclusions. |