HV differential probe versus isolation transformer

[Luxor92]

Hello, dear users. I've done a couple of designs: frequency control and TRIAC switcher. Both work, but, of course, I am curious to check performance by observing voltage transients on gate and terminals accordingly.. Until recently just wanted to purchase differential probe, but watched a video from Micsig demonstrating how HV diff probe distorts measured signal, when the signal is something more than just a sinewave and now have big doubts. Kindly asking for an advice from those of you who practice such measurements, what is actually more suitable, isolation transformer or HV diff probe? What are drawbacks and benifits of each choice. What do you prefere to use? My loads are: 80W fan for frequency control and 370W motor for switcher.

[tggzzz]

Start by reading the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ , especiallyt those marked "high voltage".

[Luxor92]

Start by reading the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ , especiallyt those marked "high voltage".

Useful articles, thanks alot! Apparently, it will take some of my time to look closely and absorb. Couldn't find any information about making "floating" measuremenst by isolating exactly the load itself (not an oscilloscope) yet. (P.S After reading app notes provided above I am not even sure how to name such measurements, since floating measurements are when non of  points connected to ground, and here oscilloscope would be connected to ground). If someone have anything to add, I am here to read 

[schmitt trigger]

Can you post a link to the Micsig video?
I have used diff probes extensively, and as long as you know the limitations, they work fine. Specially for 50/60 Hz signals.

But to your question, I always use an isolation transformer when working with power line circuits. When you are developing a circuit, sometimes I connect other equipment in addition to the scope or DMM. Thus better safe than sorry.

[Luxor92]

Can you post a link to the Micsig video?

Sure, here you are.

...they work fine. Specially for 50/60 Hz signals.

Yes for 50/60Hz signals, but as I mentioned in the first message, one of my tasks is to observe transients on gate of IGBT when switching: to check ringing and Miller's plato duration for example. On the linked video we can clearly see how even Tektronix differential probe has pretty poor performance when measuring step function: time [1:42] on the video. UPDATE: just noticed that they use 300mV/div on the video, so oscillations aren't so significant. But once again, it is Tektronix, I have budget for Micsig only, which I have heard do not have such bandwith as they declare:)

I always use an isolation transformer when working with power line circuits

You mean isolation transformer between load and mains even when using HV differential probes, right?

[james_s]

A HV differential probe is certainly the proper solution to this. Now that they are reasonably affordable there is not really any good reason to try to get around it.

[Martin72]

Nice video from micsig...

You mean isolation transformer between load and mains even when using HV differential probes, right?

Differential probes must be earthed.

[nctnico]

Can you post a link to the Micsig video?
I have used diff probes extensively, and as long as you know the limitations, they work fine. Specially for 50/60 Hz signals.

But to your question, I always use an isolation transformer when working with power line circuits. When you are developing a circuit, sometimes I connect other equipment in addition to the scope or DMM. Thus better safe than sorry.

That is how you create a death trap because the GFI will not kick in when you touch the live wires. This has been discussed to death already. A differential probe +GFI is the way to go. If you are worried about excessive currents then put a light bulb in series but even then there are much better ways. When I repair a switching mains power supply, I don't connect it to mains at all before I have tested the power converter stage is actually working. I use a bench PSU to apply power (typically around 12V) to the switcher controller and to the DC bus (several tens of volts). That gives enough information on whether the circuit is OK or not.

[Martin72]

A differential probe +GFI is the way to go.

+1

Word.

[james_s]

Can you post a link to the Micsig video?
I have used diff probes extensively, and as long as you know the limitations, they work fine. Specially for 50/60 Hz signals.

But to your question, I always use an isolation transformer when working with power line circuits. When you are developing a circuit, sometimes I connect other equipment in addition to the scope or DMM. Thus better safe than sorry.

That is how you create a death trap because the GFI will not kick in when you touch the live wires. This has been discussed to death already. A differential probe +GFI is the way to go. If you are worried about excessive currents then put a light bulb in series but even then there are much better ways. When I repair a switching mains power supply, I don't connect it to mains at all before I have tested the power converter stage is actually working. I use a bench PSU to apply power (typically around 12V) to the switcher controller and to the DC bus (several tens of volts). That gives enough information on whether the circuit is OK or not.

That's assuming you have a GFCI. I know they're used for the whole house in some areas, but in North America they're typically only used for wet locations. Nothing stopping someone from installing one in their lab but I would never assume it to be there.

[scoper007]

Looks like Micsig has new series HV diff probe! (Chinese web) https://www.micsig.com.cn/MDP/

[wasedadoc]

Can you post a link to the Micsig video?
I have used diff probes extensively, and as long as you know the limitations, they work fine. Specially for 50/60 Hz signals.

But to your question, I always use an isolation transformer when working with power line circuits. When you are developing a circuit, sometimes I connect other equipment in addition to the scope or DMM. Thus better safe than sorry.

That is how you create a death trap because the GFI will not kick in when you touch the live wires. This has been discussed to death already. A differential probe +GFI is the way to go. If you are worried about excessive currents then put a light bulb in series but even then there are much better ways. When I repair a switching mains power supply, I don't connect it to mains at all before I have tested the power converter stage is actually working. I use a bench PSU to apply power (typically around 12V) to the switcher controller and to the DC bus (several tens of volts). That gives enough information on whether the circuit is OK or not.

That's assuming you have a GFCI. I know they're used for the whole house in some areas, but in North America they're typically only used for wet locations. Nothing stopping someone from installing one in their lab but I would never assume it to be there.

Don't know about elsewhere but in the UK plug-in RCDs (our name for GFI) are readily available for about £10.  Just interpose between wall socket and plug. https://www.toolstation.com/rcd-adaptor/p12514

[schmitt trigger]

Can you post a link to the Micsig video?

Sure, here you are.

As a friend of mine used to say: there is never a problem with spending money, when you have enough of it.
The video shows that the optical coupled probe has superior performance characteristics, but also with a superior price tag.
I routinely use the exact same model Tektronix diff probe as in the video, and can tell you that as you are aware of its limitations, it is an excellent probe.

One of the limitations is common mode rejection. For some reason, when I probe an off-line SMPS, I have found that employing a good isolation transformer increases the CM rejection. I use a medical grade 171 series transformer from Hammond, which has an electrostatic shield between primary and secondary.

And I am perfectly aware of the electrical risks.

[AVGresponding]

When I used to work on UPS's, especially bigger ones, isolation transformers were never a possibility. We used Fluke Scopemeters (I now prefer Tek THS700 series), and a great deal of care.

[PwrElectronics]

At my employer, we've been working on motor drive inverters and DC-DC converters running off 700VDC for a number of years.

We use differential probes for nearly all HV measurements and for looking at gate signals, etc.  Scopes are grounded to earth via the normal AC power plugs.

In the early days of working on the 700V stuff, we didn't have much in the way for diff probes and I recall trying to get a look at the high side gate signals using a TEK THS720A scope.  With isolated inputs, it was not rated for that sort of voltage but since it is battery powered, I figured just run off battery with only that 1 probe connected and use a wood stick to press the control buttons, etc.  Basically, the scope was floated.

For reasons unclear, as the DC voltage increased, the gate waveform I was looking at became more and more distorted.  Later was able go get a diff probe and that issue was not present.

Nearly 30yrs ago I worked at a plant that made power supplies (mostly off-line types).  The line repair techs (repaired test failures) used to chop off the earth pin on their AC powered bench scopes and float the scope to whatever the internal DC link voltage on the power supply was when troubleshooting things.  That could often be 200V.  They got away with it but for sure was not a safe practice.  That place at that time didn't really have any sort of safely rules or training....

[AVGresponding]

At my employer, we've been working on motor drive inverters and DC-DC converters running off 700VDC for a number of years.

We use differential probes for nearly all HV measurements and for looking at gate signals, etc.  Scopes are grounded to earth via the normal AC power plugs.

In the early days of working on the 700V stuff, we didn't have much in the way for diff probes and I recall trying to get a look at the high side gate signals using a TEK THS720A scope.  With isolated inputs, it was not rated for that sort of voltage but since it is battery powered, I figured just run off battery with only that 1 probe connected and use a wood stick to press the control buttons, etc.  Basically, the scope was floated.

For reasons unclear, as the DC voltage increased, the gate waveform I was looking at became more and more distorted.  Later was able go get a diff probe and that issue was not present.

Nearly 30yrs ago I worked at a plant that made power supplies (mostly off-line types).  The line repair techs (repaired test failures) used to chop off the earth pin on their AC powered bench scopes and float the scope to whatever the internal DC link voltage on the power supply was when troubleshooting things.  That could often be 200V.  They got away with it but for sure was not a safe practice.  That place at that time didn't really have any sort of safely rules or training....

Yeah, in fairness, the highest voltage we worked with was 450VDC or so, and nowadays I don't usually poke anything more exciting than an SMPS, so no more than 350VDC or so.

The THS700 is only rated for those kind of voltages with the P5102 probe, and the probe at frequencies below about 2 or 3 MHz, as its impedance drops quite a bit once you go past that.

[HarryDoPECC]

These are not too pricey and can be quite good as long as you keep in mind that anticipated rise times must also fit in the bandwidth.

https://w140.com/tekwiki/wiki/A6902

[Luxor92]

I've left the thread for a while, thought it will be dead soon, but noticed so many answers when back to here. Actually to the moment I explored the situation with attention (thanks to tgzzz for provided articles). Basically there is nothing better than diff probe. Isolation amp will be more expensive + bulky. Indirect grounding - too exotic and also expensive, I guess. Other options even't don't wanna consider (isolated channel osc etc). Recently found a video, part of it explains how to calculate error related to CMRR when measuring hight side gate signal of half-bridge. So we can work with that now)  Anyway, it's interesting to read about your experience 

[tggzzz]

Glad to be of some help, and it is good to see you have been able to reach a conclusion as to what suits your purposes

Thanks for reporting back; too few new posters do that!

[Luxor92]

Looks like Micsig has new series HV diff probe! (Chinese web) https://www.micsig.com.cn/MDP/

I like that it has shorter cables.. but wonder why 1:10 ratio is so unpopular. If we look at new MDP1500 and it has 2% accuracy in 1:50 mode, and 150 diff volts maximum. So we have ±3 volts which is for MOSFET or IGBT  driving 20 volts signal already ±15%. Not good :/

[alm]

I like that it has shorter cables.. but wonder why 1:10 ratio is so unpopular. If we look at new MDP1500 and it has 2% accuracy in 1:10 mode, and 150 diff volts maximum. So we have ±3 volts which is for MOSFET or IGBT  driving 20 volts signal already ±15%. Not good :/

I prefer the lower attenuation ratios, too. For me measuring a 50V signal is more likely than a 3 kV signal. Though for the really low voltage (tens of volts common mode, a few volts differential mode) there are also low-voltage differential probes like this for low voltage, higher frequency signals.

[Bitstream]

On the Micsig MDP series differential probes, Micsig’s website states they “ Originated from Micsig's cutting-edge SigOFIT™ technology”. Does that mean the MDP probe head is optically connected to the box that plugs into the scope?  Or is it just Micsig being clever with wordsmithing their SigOFIT term?  Micsig has the MOIP probes that they clearly state are optically isolated, but they are about 10x the price of the MDP probes.

Has anybody done a tear down of an MDP probe?