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Offline robintTopic starter

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scope differential probes
« on: October 13, 2023, 05:37:21 am »
Hi Guys, can you help this board in clearing up the rubbish spouted about making differential measurements in circuits using a scope :blah: :rant:

So this topic doesnt get convoluted - let us dispense here with concerns about electrical safety - that is probably well covered elsewhere. OBTW I deplore the cheap charlie way of just disconnecting the earth at the mains plug :rant:

What we are concerned with here is protection of your scope and also the viability of making differential measurement (where the gnd croc cant be attached to PE)

I have upgraded to a new DSO 4ch gear and (from my old school CRO scope bg) its quite a revelation and requires repurposing my habits so I can meddle with digital ccts safely.
I spent several weeks re-learning this new hi tech quantum leap stuff so bear with me if I stumble. First I was made firmly aware that my DSO is a sensitive instrument and easily damaged by ignorant use.  Fair play.

So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO.

Back to basics so many rubbish vids out there were informercials pushing Diff probe kit yet there is a simple way and that is to use 2 channels (with the croc gnd leads connected) so the probes are then free to use on ccts without danger of shorting out or blowing up your scope by careless application of the croc gnd lead.

It seems another way might be to use a cheap audio xformer 1:1 (£5) and use the primary as the device under test probes and the secondary for the scope leads. Total isolation. Of course this is no good for dc measurements (so use a DMM instead!) and will become inaccurate at mf-hf, but should be ok to poke around SWM PSUs.
Anything wrong with that? Pls advise

Any Techs out there care to share wisdom on this :-BROKE


 
« Last Edit: October 13, 2023, 05:41:04 am by robint »
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Offline modoran

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Re: scope differential probes
« Reply #1 on: October 13, 2023, 06:07:45 am »
I haven't seen anyone using audio transformers as probe input, this will load the circuit under test and get unexpected results, even blown it up.

Cheaper option will be to use an isolation transformer for the DUT power supply.
 
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Offline tautech

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Re: scope differential probes
« Reply #2 on: October 13, 2023, 06:50:19 am »
Back to basics so many rubbish vids out there were informercials pushing Diff probe kit yet there is a simple way and that is to use 2 channels (with the croc gnd leads connected removed) so the probes are then free to use on ccts without danger of shorting out or blowing up your scope by careless application of the croc gnd lead.
Correction made.

You should also fit the probe condoms to further isolate the reference (gnd) rings near the probe tip from accidental short circuits when the grabbers are removed.
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Offline tggzzz

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Re: scope differential probes
« Reply #3 on: October 13, 2023, 08:57:09 am »
So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO.
...
Any Techs out there care to share wisdom on this :-BROKE

If you think probes only cost as much as a cheap DSO, you haven't been looking :) Or is that :(

There are many types of differential probe. You need to characterise your requirements before this can be answered. What is are the differential and common mode voltages? What are the edge rates of digital waveforms or the bandwidth of analogue waveforms? What are the impedances where the waveforms are being measured?

You will probably find the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ helpful.
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Offline robintTopic starter

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Re: scope differential probes
« Reply #4 on: October 13, 2023, 11:01:31 am »
@tautech
(with the croc gnd leads connected removed)
I think the tiptster's purpose was that the croc leads didnt flop around - as they do and short to something bad
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Offline robintTopic starter

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Re: scope differential probes
« Reply #5 on: October 13, 2023, 11:06:02 am »
I haven't seen anyone using audio transformers as probe input, this will load the circuit under test and get unexpected results, even blown it up.

Cheaper option will be to use an isolation transformer for the DUT power supply.

These xformers are rated 600 ohm :-/O  What do techies use?
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Offline robintTopic starter

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Re: scope differential probes
« Reply #6 on: October 13, 2023, 11:17:58 am »
So I came to the area of Differential probes and was shocked to discover that they can cost nearly as much as my new DSO.
...
Any Techs out there care to share wisdom on this :-BROKE

If you think probes only cost as much as a cheap DSO, you haven't been looking :) Or is that :(

There are many types of differential probe. You need to characterise your requirements before this can be answered. What is are the differential and common mode voltages? What are the edge rates of digital waveforms or the bandwidth of analogue waveforms? What are the impedances where the waveforms are being measured?

You will probably find the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ helpful.

Yes indeed you can spend £2.5, £3k + on an MSO 200Mhz kit Agilent x2, Tek x4  Phew
My jolly DSO Sigol DS1054Z  £350  - its enough to make my eyes water and not want to hurt my new baby.

As for what user spec I am - say amateur (not prof techie) repairs/hacking  something like SMPS from a PC.  I am rarely involved in live Mains work and then I take proper precautions for personal safety - as you do. I am sufficiently schooled to know what I am doing there I hope which is why I hoped to divert comment away from HnS.  IMHO if you have NOT got those basics you shouldnt be anywhere near a scope, or any other sensitive expensive kit.
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Offline alm

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Re: scope differential probes
« Reply #7 on: October 13, 2023, 12:08:27 pm »
Have you tried two channels in subtract mode with the probe grounds connected together (you can wrap electrical tape around the alligator clips so they don't detach and touch the DUT)? What was the limitation you ran into? This mode should be safe for both the user and scope as long as you keep ground away from the DUT and stay within the voltage ratings of probes and scope. And it's very cheap for anyone with a 2+ channel scope and some identical probes.

It seems another way might be to use a cheap audio xformer 1:1 (£5) and use the primary as the device under test probes and the secondary for the scope leads. Total isolation. Of course this is no good for dc measurements (so use a DMM instead!) and will become inaccurate at mf-hf, but should be ok to poke around SWM PSUs.
As already mentioned, in addition to frequency response there is circuit loading and voltage rating of the transformer.

These xformers are rated 600 ohm :-/O  What do techies use?
CH1 - CH2 or an off-the-shelf differential probe. The relation to the price of the probe and your scope is irrelevant. What it's important is how much does the probe costs vs how important is the problem that it's solving, and what the alternatives are.
« Last Edit: October 13, 2023, 12:11:43 pm by alm »
 
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Offline robintTopic starter

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Re: scope differential probes
« Reply #8 on: October 13, 2023, 12:37:37 pm »
@tggzzz
Thnx for the link, OMG Pandora's box opened.  1ns rise time stuff or out of the procession :-DD
Jeez the comments went into rocket science  or praps those super switches used to detonate atomic bums. 

Small boy here shat upon from a dizzy height eh :clap:

I managed to ug my Rigol up to 100Mhz so herewith my humble attempt at rise time vs the https://www.eevblog.com/forum/testgear/pos-ebay-_scope-probes/msg398972/#msg398972.  Posted tests by https://www.eevblog.com/forum/profile/?u=15682 Grumpy doc
hmm pressing the rise time button I got 2.7us  (=bw 200kHz) after fiddling the probe pot.  Does this sound right?
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Offline robintTopic starter

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Re: scope differential probes
« Reply #9 on: October 13, 2023, 01:16:24 pm »
Have you tried two channels in subtract mode with the probe grounds connected together (you can wrap electrical tape around the alligator clips so they don't detach and touch the DUT)? What was the limitation you ran into? This mode should be safe for both the user and scope as long as you keep ground away from the DUT and stay within the voltage ratings of probes and scope. And it's very cheap for anyone with a 2+ channel scope and some identical probes.

It seems another way might be to use a cheap audio xformer 1:1 (£5) and use the primary as the device under test probes and the secondary for the scope leads. Total isolation. Of course this is no good for dc measurements (so use a DMM instead!) and will become inaccurate at mf-hf, but should be ok to poke around SWM PSUs.
As already mentioned, in addition to frequency response there is circuit loading and voltage rating of the transformer.

Thnx ALM

This seems the sensible answer (instead of being blinded by some pushing expensive diff probe kit - wonder why $ ? Use two channels +/-  I would clip the grn croc to their probes

These xformers are rated 600 ohm :-/O  What do techies use?
CH1 - CH2 or an off-the-shelf differential probe. The relation to the price of the probe and your scope is irrelevant. What it's important is how much does the probe costs vs how important is the problem that it's solving, and what the alternatives are.

I suppose one gotcha to be aware of is the common chassis earth (normally connected to PE but there are cowboys around). You now have 4 channels - an unheard of luxury in CRT scope days, so there may be a temptation to forget when using other channels elsewhere but thats show business :-DD
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Offline jonpaul

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Re: scope differential probes
« Reply #10 on: October 13, 2023, 01:20:23 pm »
Read and learn: 1960s..1990s Circuit Concets books written by Tektronix i staff.

Calssisc and authoritative even today.

https://www.w140.com/tekwiki/index.php/Concepts_Series

Oscilloscope Probe Circuits:

https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

Probe measurements

https://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Enjoy,

Jon
An Internet Dinosaur...
 
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Offline bdunham7

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Re: scope differential probes
« Reply #11 on: October 13, 2023, 01:37:21 pm »
Any Techs out there care to share wisdom on this :-BROKE

Sure.  There are many different situations that each require a specific solution to the probing problem.  An audio transformer would not be a good general solution, but if you wanted to see the output of a bridged audio amplifier (where you can't ground either side), it would be appropriate.  If you wanted to look at mains or a LF SMPS, an inexpensive HV differential probe might work well, but with higher frequencies you might have issues.  Using two channels plus math as an ad hoc differential probe might work well enough in certain situations but isn't great in general.  Tek IsoVu probes will work great in almost any case, but they're a bit pricey.

https://www.testequity.com/product/32064-1-TIVP1?gclid=Cj0KCQjw1aOpBhCOARIsACXYv-dYwymIuOL7XosEPk0bZxmwy8rYyVzTf0YvQHztzusndY59d7vENBUaAic6EALw_wcB

So you need to be discussing a specific use case to really get any meaningful answers here.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline tggzzz

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Re: scope differential probes
« Reply #12 on: October 13, 2023, 02:00:52 pm »
As for what user spec I am - say amateur (not prof techie) repairs/hacking  something like SMPS from a PC.  I am rarely involved in live Mains work and then I take proper precautions for personal safety - as you do. I am sufficiently schooled to know what I am doing there I hope which is why I hoped to divert comment away from HnS.  IMHO if you have NOT got those basics you shouldnt be anywhere near a scope, or any other sensitive expensive kit.

Don't think that £3k is the top end of probe prices: look at any major distributor, and sort on price. Or how about $5 every time a probe touches a circuit https://www.eevblog.com/forum/chat/what-do-you-use-prototyping/msg1309435/#msg1309435

Some of the phrases[1] you have used in this thread make me wonder whether you do know as much as you think you know. There are many subtle failure modes that aren't obvious to newcomers.

Some parts of SMPSs will be directly connected to the mains. That isn't an area for an amateur to try to cut corners with cheap or inadequate tools.

Mistakes, however short duration, can lead to loud noises, bad smells, - and deathly quiet.

Having said that, it is your scope, your UUT, and your health.

[1] e.g.  "I take proper precautions for personal safety - as you do". You have no idea what personal precautions I do - and don't - take.
« Last Edit: October 13, 2023, 02:26:54 pm by tggzzz »
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Offline robintTopic starter

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Re: scope differential probes
« Reply #13 on: October 13, 2023, 02:36:47 pm »
Thanx for your superior opinion
Q
Some of the phrases you have used in this thread make me wonder whether you know as much as you think you do. There are many subtle failure modes that aren't obvious to newcomers.

Some parts of SMPSs will be directly connected to the mains. That isn't an area for an amateur to try to cut corners with cheap or inadequate tools.

Mistakes, however short duration, can lead to loud noises, bad smells, - and deathly quiet.

UQ
Its always good to know that new members can benefit from such wisdom (as above) from elder statesmen and not waste their time on the all too regular rubbish that diverts and confuses important questions hoping that all may be helped (and avoid noxious smells)
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Offline tggzzz

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Re: scope differential probes
« Reply #14 on: October 13, 2023, 03:28:34 pm »
Thanx for your superior opinion
Q
Some of the phrases you have used in this thread make me wonder whether you know as much as you think you do. There are many subtle failure modes that aren't obvious to newcomers.

Some parts of SMPSs will be directly connected to the mains. That isn't an area for an amateur to try to cut corners with cheap or inadequate tools.

Mistakes, however short duration, can lead to loud noises, bad smells, - and deathly quiet.

UQ
Its always good to know that new members can benefit from such wisdom (as above) from elder statesmen and not waste their time on the all too regular rubbish that diverts and confuses important questions hoping that all may be helped (and avoid noxious smells)

Good luck, and have fun safely.

BTW: there's a useful "Quote" button at the top right of each post.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline robintTopic starter

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Re: scope differential probes
« Reply #15 on: October 13, 2023, 05:44:16 pm »
Read and learn: 1960s..1990s Circuit Concets books written by Tektronix i staff.

Calssisc and authoritative even today.

https://www.w140.com/tekwiki/index.php/Concepts_Series

Oscilloscope Probe Circuits:

https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

Probe measurements

https://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Enjoy,

Jon

WOW JP this should be made into a sticky - thats real gem stuff, basics dont change and that was in the days when there was pride in pursuit of excellence in making an honest product and providing genuine customer service. I served my time with an Industrial Instrument Automation Company whose proud boast was that they made equipment and guaranteed spares and service to last the life of the plant (20 years)
Imagine that document was produced in a typing pool (those legs) on mechanical Olivetti? typewriters using carbon paper.
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Offline tautech

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Re: scope differential probes
« Reply #16 on: October 13, 2023, 07:16:03 pm »
@tautech
(with the croc gnd leads connected removed)
I think the tiptster's purpose was that the croc leads didnt flop around - as they do and short to something bad
Sure, but they are made detachable so use it !

WRT to the cost differential probes......go price some current probes if this scares you.
It has always been this way however cheaper differential and current probes for the hobbyist are available now but weren't when I was setting up....we floated scopes < not ever doing that again and shelled out $200 for 2nd hand P6021 current probes.
Buying the scope is never the last cost.
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Offline alm

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Re: scope differential probes
« Reply #17 on: October 13, 2023, 08:17:36 pm »
Sure, but they are made detachable so use it !
Except that attaching them together will give you better performance. Tektronix made a special type of ground lead back when they were making passive probes specifically for use with the differential amplifiers used in the 7000 series scopes. They surely didn't design that because their customers couldn't figure out how to detach their ground lead.

Offline tautech

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Re: scope differential probes
« Reply #18 on: October 13, 2023, 08:31:09 pm »
Sure, but they are made detachable so use it !
Except that attaching them together will give you better performance.
When/if you need it.  ;)

A tech I delivered a DSO to never used probe reference leads and instead had a dedicated reference lead back to the scope which although not giving wonderful performance suited his needs and also allowed reference connection anywhere on the DUT and not be constrained by the length of the probe reference lead.

Of course with a proper differential probe their long leads also offer this advantage.

Quote
They surely didn't design that because their customers couldn't figure out how to detach their ground lead.
You'd be surprised by the # that initially don't know reference leads can be detached.
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Offline alm

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Re: scope differential probes
« Reply #19 on: October 13, 2023, 08:41:49 pm »
You'd be surprised by the # that initially don't know reference leads can be detached.
Back then probes shipped without ground leads attached. So I think the number of people knowing that they could be detached was higher then ;)

Offline jonpaul

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Re: scope differential probes
« Reply #20 on: October 13, 2023, 08:42:07 pm »
Bonjour a tous, mille mercis pour le mention.

Have  used diff probes since 1970s, on diff RF/IF tuners (Sequerra) and off line SMPS: a few moments please to clarify...

The FET input wideband probes like TEK P6046 are VERY delicate electrically and mechanically.
https://w140.com/tekwiki/wiki/P6046
Their 100 MHz  BW is seldom needed. The 25V peak V restriction, difficulty to repair a blow FET input, and huge size are limitations.

For off line SMPS, bridge, the probes are designed for such use, with > 1 kV  rating and dedicated probes.

Fluke, Teledyne, Keysight, TEK,We use similar to  Yokogwa  701927

https://tmi.yokogawa.com/us/solutions/products/oscilloscopes/oscilloscope-accessories/701927-differential-probe-150-mhz1400-v/

Since 1970s we designed, debugged and consulted on many off line SMPS, PFC, EBU and seldom used these costly devices.


Use of battery operated portable scopes and   a 2 kVA 1:1 mains isolation transformer on the primary side.
eliminated the need for such probes.

Just my experience and notes...

 HAVE AN ABSOLUTELY FANTASTIC DAY

Jon
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Offline tautech

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Re: scope differential probes
« Reply #21 on: October 13, 2023, 08:48:33 pm »
You'd be surprised by the # that initially don't know reference leads can be detached.
Back then probes shipped without ground leads attached. So I think the number of people knowing that they could be detached was higher then ;)
Yup, that had crossed my mind however today some would get into trouble probing without a reference and we'd just have to offer more support.
Scopes are pretty hardy these days since Tek learned to place robust PCB ground planes on the TDS200 series and not have worldwide recalls for fused traces.
A few sparks or melted reference leads are lessons newbies never forget.
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Offline robintTopic starter

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Re: scope differential probes
« Reply #22 on: October 13, 2023, 09:46:56 pm »
JP merci mille fois :phew:

Have followed the Tek Probe doc 128pp, I stopped at active probes - not going down there.
I am suitably apprised and now have the wisdom to know how little I know.  Will tackle Probe measurements soonest tout de suite :scared:

You suggested that a battery powered dso can solve many problems.  Its a pity the my Rigol DS1054z doesnt have a battery supply port (maybe its hackable?).  In the meantime its power is 17VA so it could be run from a typical PC UPS (with PE disconnected) - has anyone done this?  Also my scope is a total plastic case with only the channel i/p sockets exposed (of course care not to connect ethernet or usb cables - which I would do with Mains power around.)
Around Covid time I rigged up an sine wave 24v inverter connected to a 50 Ah Lead battery deep cycle (what a con trick)  Dont get me started on LABs blood pressure alarm!!!  In essence whatever is claimed as Ah capacity divide by 4 - that how bad they are in practice.
Anyway it was there in case of a prolonged power cut. My APC UPS would allow me say 15mins for an orderly shut down.  I could run my router and a laptop to maintain comms to the outside world for a few hours say - never yet put to the test.  But I could use this stand alone system to power my scope - ca 0.7A from a 17 Ah battery bank, so I might get a day's use say?

Is this a techie solution? |O
« Last Edit: October 13, 2023, 09:48:59 pm by robint »
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Offline tautech

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Re: scope differential probes
« Reply #23 on: October 13, 2023, 09:51:22 pm »
Is this a techie solution? |O
Half arsed, yes.
But it only offers 1 isolated channel from PE as all BNC's shells share a common connection.

Scope chassis and BNC's become elevated to reference lead connection potential = risky to the operator.
« Last Edit: October 13, 2023, 10:00:57 pm by tautech »
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Offline tggzzz

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Re: scope differential probes
« Reply #24 on: October 13, 2023, 09:57:38 pm »
You suggested that a battery powered dso can solve many problems.  Its a pity the my Rigol DS1054z doesnt have a battery supply port (maybe its hackable?).  In the meantime its power is 17VA so it could be run from a typical PC UPS (with PE disconnected) - has anyone done this?  Also my scope is a total plastic case with only the channel i/p sockets exposed (of course care not to connect ethernet or usb cables - which I would do with Mains power around.)

Battery power is beneficial, but not sufficient.

Yes, people have run scopes with PE disconnected. But that's the correct answer to the wrong question.

Read the references in the page I gave you; they will help you understand the widely different classes of scopes and probes, and the reasons it is necessary for them to exist.
There are lies, damned lies, statistics - and ADC/DAC specs.
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