Oscilloscope

[PCBONEZ]

Re: Oscilloscope

Because in the primary the "ground" reference is the negative side of the filter cap(s), which is rectified mains. If you connect the ground of the scope to the negative of the filter caps, you make a direct short across the mains (hot to earth) and you blow the power supply's fuse.

And adding an additional transformer on the AC side of the rectifier changes that - how?
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[PCBONEZ]

Re: Oscilloscope

I must admit I am accepting it at the moment but not truly understanding. I will have to think more. Having looked at an smps schematic, the bridge rectifier is before the transformer which is not something I have encountered before.

Basic SMPS block diagram flow path.

Input filter
Rectifier AC-DC [High Volts]
Switching Section [makes HV AC at Higher Frequency]
Step-Down Transformer
Rectifiers AC-DC [OP Volts]
Output Filters

In modern gear there is a ground in just about every stage.
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[godin]

Re: Oscilloscope

I must admit I am accepting it at the moment but not truly understanding. I will have to think more. Having looked at an smps schematic, the bridge rectifier is before the transformer which is not something I have encountered before.

[PCBONEZ]

Re: Oscilloscope

Without isolation you CANNOT probe in the primary of a SMPS with an earthed chassis scope - only with one powered by batteries and without a metallic frame.

That a regurgitation of the same old 'ancient knowledge' -what- that's been repeated since at least the '50's.
[You back when much equipment and even most buildings didn't have grounds...]
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As I already expressed I can't think of a single -why- for that -what- when it comes to modern grounded equipment.

.. And it seems neither can anyone else. - Including Tektronix..
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Please explain your -why-.
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[godin]

Re: Oscilloscope

Thanks, I guess the safe thing to do is always check to see if there is a voltage between what you are about to clip to and the ground clip!

[Th3_uN1Qu3]

Re: Oscilloscope

Because in the primary the "ground" reference is the negative side of the filter cap(s), which is rectified mains. If you connect the ground of the scope to the negative of the filter caps, you make a direct short across the mains (hot to earth) and you blow the power supply's fuse.

[godin]

Re: Oscilloscope

Without isolation you CANNOT probe in the primary of a SMPS with an earthed chassis scope - only with one powered by batteries and without a metallic frame.

I thought I understood this now, but I still do not understand the reason for this.
Would you mind explaining please?
Many thanks

[godin]

Re: Oscilloscope

I can't tell you without tracing the circuit or seeing the schematic.
I never considered it before and I don't have anything handy to look at.
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Measuring the resistance between the Negative DC Output and the AC Return [Neutral] Input will tell you is it's Isolated completely.
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I can't see floating the DC ground inside a Laptop as a good idea because they can be connected to external AC powered LPT/Serial/USB devices that do [or might] have the DC Negative at earth ground potential.
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Yes it is completely open circuit from input to output

[Th3_uN1Qu3]

Re: Oscilloscope

Using an Isolation Transformer on modern grounded chassis equipment will make the work MORE dangerous, not less, by Isolating [Floating] the AC Return above (Earth) Ground Potential.

Not any more dangerous than not connected the ground lead to earth, which a lot of people do. The only thing between hot and earth is an Y cap, and that can bite a little, but it is far from dangerous. Without isolation you CANNOT probe in the primary of a SMPS with an earthed chassis scope - only with one powered by batteries and without a metallic frame. Never ever disconnect the earth ground connection of a mains powered scope, significant risk of electrocution exists if you do so. That not mentioning that the measurements will be screwed up anyway by the large common mode currents.

An alternative is differential probing like you mentioned. But if you only got one channel on your scope it obviously won't work.

[PCBONEZ]

Re: Oscilloscope

I understand what double insulated is, it was just that I had observed that there was no connection to earth of the outputs from it. That is from the output plug to mains earth is open circuit and therefore isolated from it. I thought it might fit the bill not by definition of being double insulated, just that it happened to have this property.

I can't tell you without tracing the circuit or seeing the schematic.
I never considered it before and I don't have anything handy to look at.
.
Measuring the resistance between the Negative DC Output and the AC Return [Neutral] Input will tell you is it's Isolated completely.
.
I can't see floating the DC ground inside a Laptop as a good idea because they can be connected to external AC powered LPT/Serial/USB devices that do [or might] have the DC Negative at earth ground potential.
.

[PCBONEZ]

Re: Oscilloscope

Here's a good FAQ's an isolation transformers (Chapter 4 Section 4)

http://www.repairfaq.org/REPAIR/F_tshoot.html

Isolation Transformer section in that link:
- If you don't put the proper emphasis on the correct part of the FIRST sentence then that can get you into trouble.
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The First sentence with proper emphasis:
>> An isolation transformer is very important for safely when working on live chassis equipment. <<

Live chassis equipment = Hot chassis design = No chassis-to-earth grounding.
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[As KeriJane suggested earlier] - It's only for really old designs like old 'Live/Hot chassis' TV sets.
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Using an Isolation Transformer on modern grounded chassis equipment will make the work MORE dangerous, not less, by Isolating [Floating] the AC Return above (Earth) Ground Potential.
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That basically means you DON'T want to be using an Isolation Transformer for anything to do with a PC.
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[PCBONEZ]

Re: Oscilloscope

That was very informative, it doesn't float the ground.

No but if you use it on a grounded chassis it DOES float the AC Return in the device under test which actually makes using the Iso-Xfmr MORE dangerous than not using one.
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[godin]

Re: Oscilloscope

That was the case with the one in the video I posted, but the guy disconnected the earth from the isolated side to make into what he called a technicians isolation transformer as opposed to the domestic one that it was manufactured to be.

[Mad_Professor]

Re: Oscilloscope

Here's a good FAQ's an isolation transformers (Chapter 4 Section 4)

http://www.repairfaq.org/REPAIR/F_tshoot.html

That was very informative, it doesn't float the ground.

[cmj21973]

Re: Oscilloscope

Here's a good FAQ's an isolation transformers (Chapter 4 Section 4)

http://www.repairfaq.org/REPAIR/F_tshoot.html

[godin]

Re: Oscilloscope

I read the Tektronix link this morning and also found it very informative.
It does not mention the other option of isolating the equipment being tested from ground. I think this is what people are advocating and not floating the scope but using the isolation transformer on the equipment to be tested. If you only connect the earth clip to earth it would be safe but may affect measurements. This seems to make sense to me as a novice.
I am as always willing to be corrected.

[KeriJane]

Re: Oscilloscope

I think the need for an isolation transformer questionable.
It's become a 'pat answer' and honestly it's never made sense to me, particularly from a safety standpoint.
I can't see how floating a ground intentionally makes things safer. - Doesn't make sense.

We never used them in the Navy - that I can remember anyway.
Scope probe grounds were ALWAYS connected to chassis-earth ground.
[Seems strange calling it earth ground when you're floating on water....]
If we needed to measure a hot to a hot we used two probes and a differential amp in the scope and the grounds on both probes were connected to earth ground.

Further:
Tektronix advises against isolation transformers - see page 2.

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If someone has an explanation of why they are needed that doesn't involve an 'I Believe' button then I'm all ears.
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Thank you for the Tektronix reference.

I also learned that probe grounds must always be connected to chassis ground and never, ever to a circuit path. Just use the ADD or differential between channels feature or get a differential probe.

Aren't isolation transformers a leftover from the bad old days of hot chassis radios and televisions?

For the children around here, yes, they used to build consumer electronics with chassis connected directly to power! They were often wired to the wide, "neutral" blade of the plug but it wasn't always so..... You could get a dangerous or deadly shock from just touching the chassis, if maybe a knob came off and uncle "deadly" Smedly had replaced the cord or plug...

When testing such equipment, wouldn't an isolation transformer be a really good idea? How did the techs deal with hot chassis scoping way back then?
Could one ground the chassis (and scope) while connected to an isolation transformer?

[godin]

Re: Oscilloscope

I understand what double insulated is, it was just that I had observed that there was no connection to earth of the outputs from it. That is from the output plug to mains earth is open circuit and therefore isolated from it. I thought it might fit the bill not by definition of being double insulated, just that it happened to have this property.

[PCBONEZ]

Re: Oscilloscope

Ground loops CAN cause problems with signal quality in some situations by way of EMI, but that's not a safety issue and it's not going to blow up your scope.
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[PCBONEZ]

Re: Oscilloscope

For work on laptops isolation still seems to be relevant. The -ve terminal on the power supply has continuity to mains earth in a non double insulated one.
On the other hand I have a double insulated laptop power supply with a 2 pin mains plug and this has no continuity from -ve to mains earth on its output. In this instance is the power supply acting as an isolation transformer and is it safe to use the scope on a laptop with a double insulated supply?

Double Insulated doesn't mean what you think it does.
All Double Insulated means is that no -single- failure can result in dangerous voltage becoming exposed.
There is a sundry or ways they can achieve that.
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The lack of a ground wire does not mean much electrically [voltage potential wise - in a way that matters to an o'scope] when the neutral in the building's wiring is connected to earth ground in your breaker box anyway.
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