using an isolation tranformer with my oscilloscope

[52613 sfw356645eh5ah6j75]

hi there, i'm having some problems with my osciloscope....the thing is that i wanted to do some floating messurements and unluckly blew my rs232-usb adaptor....
the circuit was isolated from ground but forgot that the pc wasn't and when i connected the serial's ground to the circuit, it took that potential and well....the rest is obvious. even my speakers which are connected to an external sound system made some noise ....
i've been reading, though, that using an isolation tranformer isnt a good idea, that it isnt safe, that it stresses the oscilloscope's power supply and that that could lead to failure in the future even if grounded again (from a tektroniks' pdf), that it's better to use a dom and some other stuff...

i'm not measurring the mains, just 12v from the universal ac adapter whichi is isolated and maybe from the power supply from my pc which is earth grounded. At must some inductive spikes.

any advise? should i buy one or use the ch1-ch2 method?

 

[ronsimpson]

I have floated my scope many times.... not safe.
If you are only 12v from ground that seems safe, up to 40 volts.
There are RS232 isolators and (slow speed) USB isolators.

 

[Boncuk]

Also check if the power supply connects to protective earth.

It's not a good idea to make mains measurements with PE and circuit ground connected to the chassis of the oscilloscope.

I removed PE after the HV-probe arced.

Boncuk

 

[JimB]

Use the two channels and set the scope to Ch1+Ch2 and Invert Ch2 (usually on all the scopes I have used).
What is displayed on the screen is then the difference between the two channels.

JimB

 

[Nigel Goodwin]

I have floated my scope many times.... not safe.

Perfectly safe - as long as you know what you are doing, and why - in many cases safer than using an isolation transformer.

But if you don't know what you're doing, an isolation transformer is the best bet.

Personally I wouldn't entertain using an earthed scope - far too dangerous for the way I (and most of the service industries) work.

 

[JimB]

For more infomation on this topic, have a look here:



A Tektronix application note.
Rumour has it that Tektronics knows a thing or two about oscilloscopes. Thats what I heard anyway.

JimB

 

[52613 sfw356645eh5ah6j75]

(so after reading the tektroniks' pdf, sorry for the delay, i'm pretty busy with a proyect )
so the only danger of floating a oscilloscope is that the chasis takes the voltage reference of wherever you plug the oscilloscope's gnd clip and the added parasitic capasitance and indutance, and the ringing caused by those two.
luckly, my dso has an all plastic case and handle with ruber buttoms, so no problem with the earth reference there, except for th usb, the bncs, and serial conectors in the back, there's commonly no metal at fingers' reach (not meassng high voltage anyways)

so summarising...the only diference, in practice, between the transformer and the ch1-ch2 methods is the amount of probes you have to use.
the ch1-ch2 is cheaper but you have half the samples per second, and only one channel, using 2 probes
and floating the oscilloscope you have one and a half channel, with one with full sampling rate, but you get parasitic capasitance and inductance, and some dangers above 40V.
i got it right, didn't i?

just one more question, how much inductance and capasitance would that be? my scope says 13pf, but how much does the chasis add? so as to know how "dangerous" could it be to my circuits.

ps:but i still don't get the "...stressing the oscilloscope's power supply." part though

 

[The Electrician]

ps:but i still don't get the "...stressing the oscilloscope's power supply." part though

In the U.S., the grid voltage in a home is 120 VAC from a standard outlet, and in other countries it could be 230 VAC.

The power transformer in the scope is designed with the assumption that such voltages will be what the insulation system from primary to secondary must withstand. Tektronix's warning is undoubtedly aimed at workers in, for example, industrial locations where the user might be attempting to measure on a 440 VAC line. Then the scope's transformer will be exposed to a higher than rated voltage from primary to secondary.

If you float the scope in a home environment and attempt to measure on the grid, you won't be greatly overstressing the scope's transformer, but you will be creating a dangerous situation. If you are attempting to measure a point that has a higher than grid voltage, such as, for example, between the plates of a push-pull output stage in a high power audio amplifier, then you will be stressing the scope's transformer.

There are metal parts on a floated scope that are a common source of unexpected danger even if the scope is made of plastic. The shell of the BNC connectors that connect to the scope's inputs are typically metal. Tektronix makes special probes for use with its TPS2000 series isolated input scopes. The BNC connectors to the scope inputs are shrouded with insulating material, but most scope probes don't have this insulation; their BNC shells are bare metal.

Just imagine how easy it would be if you have the scope floating to decide to connect another probe. You would be holding the metal shell of the BNC connecter and when you pushed it onto the input connector on the scope, you would at that moment connect your hand to whatever the ground clips of the other probes were connected to. Depending on what other things you were touching you could be seriously injured.

 

[squishy36]

I've used a variety of methods for making differential voltage measurements without the differential amplifier (float the scope off ground, isolation transformer, two channel difference). They all can work but, as others have indicated, most of them are not for newbies. Remember, it only takes one fatal accident to ruin your day -- and mother nature doesn't come with training wheels. Thus, except for experienced folks who only do occasional measurements of this type, I recommend you either use the two channel difference method or buy a suitable differential amplifier. And, like Electrician says, there are metal parts on all scopes I've ever used -- and it could be easy to forget that they had a significant voltage on them when floating the scope.

 

[52613 sfw356645eh5ah6j75]

ok, thanks for the help, seems i'll be using the channels' diference for now, better safe than sorry. And because i'm a bit short of cash so double win

 

[KeepItSimpleStupid]

Isolate what your measuring.