Products > Test Equipment
Floating Scopes
dicky96:
I've googled this and yes it had been discussed but I still don't understand why this is a big no no
I do a fair amount of repair work on plasma/LCD/TVs and also some switch mode PSU repair
I understand the whole reason of using an isolation transformer on the DUT but currently I don't have my isolation transformer here where I am working in Gran Cararia (it's heavy and still in UK) and I need to connect scope ground lead to hot ground and probe the SMPS 'half bridge' controller on a Plasma TV I am trying to repair. I have an analog scope in my workshop here and a Rigol 1052
Yes I know if I disconnect the mains ground the scope ground then floats at up towards whatever potential is on the probe gound - but the Rigol internally has a SMPS so the probe circuitry and 'ground' isolates from the mains live/neutral there, and anyway it is made from plastic apart from the BNC ground and a couple connectors on the rear.
Everywhere you google says this is really dangerous, yet in the early 80's when I did my City&Guilds in TV repair we were taught by our instructors to float the scope when working on TVs. Unless you were lucky enough to work in a repair shop with isolation transformers which were not that commonplace.
Also no TVs had an earth connection then anyway, they were all captive leads with two conductors.
In the Valve sets and earlier transistor sets the chassis was connected to neutral (but you always taught to check the mains plug wiring first just in case the owner had it the wrong way round ::) )
And If the TV was one of those 'new fangled' 1970s designs that used a full wave bridge rectifier followed by a 100Hz Thyristor 'chopper' circuit to regulate the supply voltage by chopping a chunk out of each mains half cycle, then the whole chassis was connected to the bridge rectifier negative terminal and floated at what they called 'half mains voltage' - which actually meant 'full mains voltage for half of the time'
Depending which half cycle of the mains was coming out of the bridge rectifier negative at that instant the chassis was either at Neutral or Live potential!! This is exactly the same as what is called 'hot ground' these days
So your floating scope (of which you were taught to be careful not to touch the metalwork) was only at the same potential as the entire TV chassis you were working on anyway - which you were equally careful not to touch! So between floating scope and floating chassis I honestly don't understand the extra danger
So what I don't get is why it is more dangerous and a huge NO NO to disconnect the ground on a modern plastic scope in 2017 to do what was normal practice with a metal chassis scope back in the day???
Surely if you understand and respect what you are doing this is as safe (or even more so due to almost no exposed metal on a Rigol) for you, your test equipment and the DUT as it was in the 80s??
By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves. Doesn't that say something about the perceived 'danger' of floating scopes?
On googling, some folks on this forum say there is no reason to ever ever float a scope - I would ask those posters if they ever worked on SMPS primary side controllers?
Rich
Rich
grumpydoc:
Just because a dangerous practice was acceptable in the 1980's did not make it safe back then, nor acceptable now - perception and acceptance of risk has changed in the intervening 30 years (for the better IMO).
savagemadman:
I've been wondering about this myself. My scope doesn't even have a grounded plug on it! The manual says to just ground to the device under test.
alm:
If you read documentation from scope manufacturers back then, they also recommended against it. And yes, back then people were killed by their floating scope. Tektronix made a special isolation transformer that would ground the instrument as soon as its ground floated to far away from earth. As for isolation transformers being heavy, differential probes are much lighter and can do the majority of the measurements you would float a scope for. As are scopes with isolated channels.
--- Quote from: savagemadman on June 21, 2017, 10:09:15 pm ---I've been wondering about this myself. My scope doesn't even have a grounded plug on it! The manual says to just ground to the device under test.
--- End quote ---
That must either be a special kind of isolated scopes or a very crappy/old one (WW2 era). Tektronix made a couple of battery-powered scopes that were isolated. Kind of hard to guarantee an earth connection in a battery powered scope. But they took special care in the mechanical design and used special probe connectors.
--- Quote from: dicky96 on June 21, 2017, 09:38:30 pm ---By the way out of 16 trainees (some of them complete beginners) on the full time TV repair course, non of us electrocuted ourselves. Doesn't that say something about the perceived 'danger' of floating scopes?
--- End quote ---
If one in sixteen would kill themselves over the course of a repair course of a couple of months (approximately 25% change to die each year), there would be pretty much no repairman over 30 years old. I am sure the chance to die is much lower than 25% per year. But still, does TV repair really pay enough to take such unnecessary risks? Just get the proper tools, especially if you are doing it professionally.
Do you also rip seat belts and airbags out of your car, because people did without them in in the seventies?
savagemadman:
--- Quote ---That must either be a special kind of isolated scopes or a very crappy/old one (WW2 era). Tektronix made a couple of battery-powered scopes that were isolated. Kind of hard to guarantee an earth connection in a battery powered scope. But they took special care in the mechanical design and used special probe connectors.
--- End quote ---
Its a ~17 year old portable Fluke, probably made by Phillips. AC or battery powered. The AC has no ground connection. As far as I know the newer units do not have a ground either. I'm assuming it's a similar situation to the Tektronix models you mention. The probes look like normal BNC to me though, just shrouded.
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