Many advocate the use of an isolating transformer. The oscilloscope probe and ground are then used like multimeter leads. Tektronix and others maintain this is a dangerous practice and may result in cumulative damage to the oscilloscope. Differential probes are one answer, though quite expensive.
Comments please. Thanks
BobH
I can see no reason that using an isolation transformer properly can have any more detrimental effects on a probe or the 'scope itself than testing equipment that is self isolated [transformer operated]. When using an isolation transformer, the UUT [Unit Under Test] must have the circuit 'common' bonded to the 'scope common [and ground] usually by the probe 'pig tail' or even a separate test lead joining them. Indeed, if the probe tip is touched to an unbonded UUT, it can damage the 'scope or probe, regardless of wether it is powered through an external isolation transformer or through an internal transformer or SMPS. On the other hand, connecting an 'scope that has been 'floated' from ground by disconnecting the power cord ground pin CAN be severely damaged by connecting it to a non-isolated line operated UUT. The usual reason is excessive voltage applied between the 'scope common and the 'scope supply neutral or line. These usually turn out to be transient pulses that are unexpected.
Neil S.
What are you trying to measure?
Damage maybe if some voltage rating of a cap or something is exceeded but "cumulative " damage, to what?
It's not clear what you were trying to say.. "THIS? is a dangerous practice".
Floating a scope plugged into a utility line power source (and omitting the earth ground connection) is hazardous because the chassis (possibly the case and other exposed hardware) of the scope will be at the same potential as the probe ground clip while testing. Also, it's possible to exceed the scope input's safe limits.
When using a proper isolation transformer, the equipment/device being tested is commonly plugged into the I-T, not the scope (some exceptions apply, in-place industrial equipment which can't be powered by a servicing I-T, for example).
Some I-Ts have a pass-thru earth ground connection, and others may bond the line Neutral or one side of the line power (which isn't any type I've ever used, just read about here).
When servicing gear, I want the DUT device under test to have no paths at all, to the utility service/line power Hot(s) or Neutral connections. The DUT is then as safe as if it were powered by battery (except for only a miniscule uA leakage at the I-T transformer).
FWIW.. In the U.S. the AC line Neutral is often bonded to earth ground at the service panel).
Common Variacs or autotransformers do not provide line isolation, and shouldn't be considered safety equipment.
A 2 channel/input scope with a Differential Mode selection and proper isolation of the channel circuits can generally be used for safe testing, but only if this procedure is included in the scope's operating manual.
-- Cheers, WB .............
Thanks all for replies. Please google for 'Fundamentals of Floating Measurements and Isolated Input Oscilloscopes'. This is a Tektronix application note. The situation to which I refer is is at the bottom of page 4. I want to use an oscilloscope to check the PWM switching chip on the primary side of an SMPS.
BobH
...which will get you exactly where you want to be.
...to the bottom line of Tektronix, Inc.
...of which they couldn't provide a convincing example.
See "bottom line", above. The claim is bullshit, just like MICROS~1 telling you that software other than theirs will cause you problems.
you can do it, but there is a potential shock hazard if your scope chassis is at some strange voltage- but all depends on what you're measuring.
Floating your scope and probing around in a microwave oven so you don't don't smoke your ground leads is probably a bad idea for example.
It's pretty hard to completely float stuff anyways. I once got a memorable shock from touching one terminal of 16 kV capacitor that was just sitting on a bench. I'm still not sure why it was able to conduct to the ground I was standing on as effectively as it did, but it sure did.
heh... Gullibility, perhaps?
I have used differential probes and battery operated scopes. That's the only thing I would recommend.
Unless it's a specific service mans isolation transformer, most every commercial isolation transformer is NOT isolated from ground. Ground isolation is often what you after because neutral is tied to ground. Using complete isolation through a service mans isolation transformer still passes a low level capacitive ac line connection to the circuit, and measuring high impedance circuits can throw you off. Using a battery operated scope still connected to the charger will also pass capacitive ac line coupling.
Greg
You do NOT want to do this. Leave the scope grounded and put an isolation transformer on the device under test. But you still DON'T want to hook the ground of the probe onto the common node of the DUT. there's just too much common mode stuff floating around. You can get some information by using two probes and a dual-channel scope with one channel inverted and summed. Hook the probe grounds together, but to NOTHING else. You have to turn down the scope gain until EACH channel can handle all the common mode signals on the inputs. Often, that results in a sensitivity too low to get much info from the difference trace. And you'll see junk because the ground path is ill-defined.
The TEK A6902A isolated probe works well in this application. You can hook the ground lead of the probe almost anywhere you want.
In a perfect world, you can get by doing dangerous things. But it takes only one slip-up to smoke your probe, your scope, your DUT and YOURSELF!!!
I do this all the time - virtually every working day in fact. My current scope - a 4 channel 100MHz Hitachi that I've had from new - is probably 25 years old now, so if it has been suffering "cumulative damage", it's bloody slow to reach a point where it's a problem ...
My scope is normally grounded to the incoming line power ground connection. The SMPS that I'm working on is connected via a variac, that is plugged into a full 1 : 1 isolation transformer. To make measurements on the primary-side drive waveforms etc, I first solder a stiff wire to the "-" tag of the main filter cap. That is then used as the scope probe ground point. This arrangement has never caused me a problem on any type of switcher from simple single stage ones, right through to the multi-stage types that have a PFC supply at the front end.
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You're doing just what I was going to recommend to the OP,,,totally isolate the EUTor UUT or DUT, whatever you call the equipment being tested. Then you can call"Ground" whaatever you want to connect to the EUT that youwant to call ground, ie, the ground terminal of the scope probe. This is what I did for the 44 years I was at Bell Labs and never blew up anything, Tektronix or HP. Funny how HP never did make scopes as well as Tektronix, and Tek never made Spectrum Analyzers as well as HP.
Differential probes are one > answer, though quite expensive.
BobH
We used our Tek scopes in the A-B mode to do this all the time.
If you don't have the use of a dual channel scope then an isolation transformer is in order.
You do need to let us know what signals or what you are measuring.
Bob AZ
"pfm"
** Tek say no such thing - you bullshit artist.They do point out it can KILL the operator.
... Phil
"pfm"
** The warning is about the potential for a FATAL ELECTRIC SHOCK !... Phil
gregz wrote in news: snipped-for-privacy@news.eternal-september.o rg:
Proper practice is to put the Device Under Test on the isolation transformer,not the scope. leave the scope grounded.
TEK makes (or used to make) special probes/amps that isolate the scope from the device under test(DUT). they had one that used fiber optics,and another that actually disconnected the ground for up to 40V,and monitored the ground potential to reconnect the ground if the potential went above 40V.
A6902,IIRC.
TEK also makes differential amps that can be used with single ended scopes,but you need a separate probe power supply if it's not a TDS model scope.
-- Jim Yanik jyanik at localnet dot com
I don't know why this is so hard for people to understand.
Test equipment is grounded. Period.
If your probe can't handle the offset voltage, you're using the wrong probe.
If you're trying to measure something normally connected directly across the utility power line, then use an isolation transformer and you can ground any point in the device being tested.
Use a solid ground jumper. Do NOT rely on the ground path back through the scope probe.
Jeff-1.0 wa6fwi
-- "Everything from Crackers to Coffins"
Thanks. No question about the scope being grounded, no one suggested otherwise. Tektronix maintain that floating measurements, taken in a DUT powered by an isolating transformer, is bad practice.
Don't isolate the scope; instead power the unit under test through a isolation transformer.
PlainBill
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