Does this power supply have an isolated output?

[Flyback]

From my finding s so far, it looks like scopes and signal generators virtually always have the output common connected to earth ground.....................................but it looks like virtually everyting else does not (unless as you say, you have the shorting bar option)

Scopes and sig gens deal with high frequencies, so i think this is the thing, to reduce interference to the frequencies/waveforms.

KERT did phone me up and ask me who i was, and whether i would be using this supply................but it was just a sales call and the tech call would follow...............i hope i havent asked a stupid question and now they are going to tell my boss that i'm a drongo and should never be allowed to touch their power supply!

 

[Flyback]

Anyway, some time ago, i went with my boss to a customer site, 120 miles away. When we got there, we found the test wouldnt work because of our scope output being at earth ground. They wouldnt lend us an isolation transformer, or anything, and so we just cut off the earth ground and did the test, and were careful not to touch the output common etc.

What would you have done in this situation?

Please dont say you would have taken a battery scope because i asked for one and was told no. And that company used to put LEDs in parallel with no current matching circuitry (not even resistors) because the driver stages were cheaper.....so as you can imagine, they wouldnt pay for stuff like battery scopes , etc.

Also, please dont say you would isolate the electric drive, because the company (our customer) expected us to work with it as it was, we were their supplier, and they werent going to put themselves out for us...............when i grovellingly asked their engineer if i could borrow one of their battery scopes , he gave me one of THOSE looks.

Could we have damaged the scope?...there was no way we were going back 120 miles without the test results. We would have got our hides booted around the office. The customer concerned was our biggest customer, and we needed these results. The test was just to see if a few lamps could be powered off of an electric drive auxiliary PWM output....we needed the scope to look at the PWM frequency and duty cycle.

Also, as already mentioned, it is not easy to use an isolation transformer in the mains path to an smps under test, this is because the smps under test is fed like this.............................Wall socket > Mains autotransformer > Power meter > Smps under test.

If you use an isolation transformer in this path then you adversely affect the power meter readings..........this is due to the strays of the isolation transformer....just feel how hot an isolation transformer gets when its plugged in even on no load.
So if we cannot isolate the path to the smps, then we must float the scope, and cut off the earth lead to it.

If you dont "earth-cut" the scope then in a busy department , where a variety of smps under test are being tested, you will keep getting people forgetting that the scope gnd is at earth and they will connect it to the smps primary side and Bang!!!!!

This is why one of the worlds biggest lighting companies float their scopes as a department rule..all the designers do it, everybody, and they have done so for 25 years, and not one injury or fatality.

I remember this consultant contractor coming in and giving our gaffer hell about it......................he said it could be dangerous if there is "inductance between the earth at the installation inlet and the neutral at the bench's".
He was gone in a few weeks.

So, could you tell if this practice could damage the scopes?, which incidentally were those expensive Lecroy ones.

If i write to my ex-gaffer to tell him why he is wrong, how should i word it?, given that productivity is very important there, and they used to stress when you tried to order solder, let alone diff probes and isolation transformers etc....not least, they have worked like that for 25 years.

I hope i will not be kicked off the forum, because this is the reality of life in so many companies today..i would say the majority. I know it is said i "refuse" to learn, but i am just trying to make the best of my lot, and i don't want to be sacked becasue i refuse to "earth cut", thats just how things are, i cannot stop this practice, i must live with it, so i'm trying to find out more of the whys and wherefores, and hopefully other silent readers will be feasting on the words of your valuable posts.

 

[KeepItSimpleStupid]

What would you have done in this situation?

Dual channel scope? Use differential mode. A-B or A+B (with invert B pulled).

 

[ChrisP58]

Floating the ground on a scope, or anything else that has a protective earth connection, is like driving without your seatbelt fastened. It isn't a guaranteed injury, it's just that you have willingly bypassed one safety barrier that would protect you if something goes wrong. You can still be safe as long as you take extra precautions, like driving r-e-a-l-l-y slow.

Beyond that, when you are onsite with a customer is when you need to put on your most professional face. Showing up without the proper equipment to do the test, the right way, puts a black mark on your reputation. It would tell me that my supplier really doesn't know what they are doing. And maybe that is the core problem with your company. If your engineers are cutting corners on safety issues, what else are they cutting? And having an expensive Lecroy scope, but not a differential or isolation probe, sounds to me like someone was buying flash to look good, without the understanding to use it properly.

Having the right tools and equipment is just the cost of doing business.

 

[Flyback]

This company was a fellow USA company, and they look after us, and give us advice, and help us along.....I have found this quite common with US companies, they'll give out rollockings to their fellow USA suppliers where needed, but will generally help them out if they can.

So you agree that cutting the earth out of a plug supplying a scope, won't actually damage the scope in any way?

By The way, the KERT KAT5VD bench power supply has its output common NOT connected to earth ground...it is isolated from earth ground....KERT told me this today.

Dual channel scope? Use differential mode. A-B or A+B (with invert B pulled).

-i tried this once, and the reading was horrendously noisy, i dont believe this method really works on the majority of smps waveforms.

 

[ronv]

You need good and properly compensated probes for the A-B measurements.

I think we have probably all floated a scope to measure something at line voltage, but I hope we all know the risk involved to ourselves and others.

If you need to make many measurements like this I would buy a cheaper LeCroy and spent the extra $1000 on a good differential probe. We all want to see you back here again.

 

[KeepItSimpleStupid]

The non-ground connection really makes the differential measurement impossible.

You should be aware as to how much your floating the scope. 30 V or 3000 kV?

Instead of cutting the third prong, I would have used the 3-prong adapter with the ground wire removed.

But long term, I'd have the company invent in a differential probe. Some more affordable ones are here: **broken link removed**

I did most of my lab troubleshooting using a battery powered 500 kHz scope with 1 mV to 50 V per division in a laboratory. It only had built-in probes.

I absolutely HATED the 30-60 MHz Phillips scopes that someone bought. The traces were always blurry and not sharp.

 

[ronsimpson]

I usually don't float the scope but float the work. I have many isolation transformers. When the line is isolated then where ever I attach the ground lead of the scope becomes ground.

Maybe I did not say that clear. Run power to your project through a 1:1 isolation transformer.
I have 110/220 to 110/220 transformers in boxes with on/off switch, V and I meters, fuse/breaker and some of the transformers also have a know to output 0 to 250V. I work on switching power supplies!

 

[KeepItSimpleStupid]

Right:

If at all possible, float the work.

 

[Flyback]

But long term, I'd have the company invent in a differential probe. Some more affordable ones are here: **broken link removed**

...thanks, looks good, but the 100MHz "TA042" one costs £330. (I'd need at least 100MHz so as to be able to see, for example, the peak of the drain voltage spike in an offline flyback converter)
I am also concerned that these differential probes dont have BNC connection to the probe...in fact most diff probes dont seem to have this, and it doesnt seem to make sense?

Run power to your project through a 1:1 isolation transformer.

Yes, i'd like to , but 1:1 isolation transformers tend to affect the power meter reading at the mains side of the SMPS..(unless of course , a very expensive isolation transformer is purchased)
As you know, the SMPS is fed like this......230VAC wall socket > autotransformer > Power meter > SMPS.
..We cannot add a 1:1 isolation transformer in here as well, as it will affect the power meter readings, due to the strays of the 1:1 transformer.....(as you know, just feel how hot a 1:1 isolation transformer gets if left plugged in even on no load)

You should be aware as to how much your floating the scope. 30 V or 3000 kV?

Not 3KV, just voltage at primary side of mains powered SMPS. Also, high side Gate drive waveforms of, say, a full bridge converter.

The non-ground connection really makes the differential measurement impossible.

-If i was doing the differential measurement, ie "A-B" or using a diff probe, then i would not need to cut the earth out of the scopes wall plug, i appreciate that.

I have heard that when the scope gets its earth connection cut out of its plug, then any measurements made are affected by the capacitance of the scope "gnd" connection to elsewhere? I am not sure about this though?

I think we have probably all floated a scope to measure something at line voltage, but I hope we all know the risk involved to ourselves and others.

.Thanks, am aware of the safety risk, but does it potentially damage the scope?

 

[kubeek]

...thanks, looks good, but the 100MHz "TA042" one costs £330.

That is like one tenth of the price of a half-decent dual-channel digital scope. Definitely not "too much" for any big company.

I have heard that when the scope gets its earth connection cut out of its plug, then any measurements made are affected by the capacitance of the scope "gnd" connection to elsewhere? I am not sure about this though?

Imagine the circuit when you connect the scope. There will be some resonably large capacitance between the primary and secondary of the transformer of the scope, I would dare to guess around 1nF. Using Thevenin and superposition this basically gives you ~0.5nF between the probe ground and mains earth. If you connect the probe ground to a wrong place, it can skew your measurements a lot.

 

[KeepItSimpleStupid]

Thanks, am aware of the safety risk, but does it potentially damage the scope?

Depends primarily as to how for you can float the scope and a tough question to answer.

 

[Flyback]

Using Thevenin and superposition this basically gives you ~0.5nF between the probe ground and mains earth. If you connect the probe ground to a wrong place, it can skew your measurements a lot.

...from this, if both scope and test circuit are both floating, then presumably this capacitance doesnt apply? (i.e. if both scope and test circuit are supplied via 1:1 isolation transformer and the earth is not taken to either scope or test circuit)

 

[kubeek]

...from this, if both scope and test circuit are both floating, then presumably this capacitance doesnt apply? (i.e. if both scope and test circuit are supplied via 1:1 isolation transformer and the earth is not taken to either scope or test circuit)

I would guess the isolation transformer secondary still has some capacitance to ground, so there will be a capacitive divider. Of course differential probes will have some parasitic capacitance too, but that would be about 2 orders of magnitude lower, i.e. 10-20pF.

As for the possible damage you asked before, If the scope floats too high you can exceed the isolation voltage of the scope's supply and then bad things will start to happen.