Author Topic: Nead an insulated probe for oscilloscope  (Read 5025 times)

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Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #25 on: June 06, 2019, 03:41:51 am »
great replyes! :-)

Lets say I am using probe 1 to measure input ripple before the full bridge rectifier and the smoothing of the DC 5V input and put probe 2 in the other end, to measure how much ripple there are left. If I do understand it correctly, this will give some strange result, due to the common ground connector?
How would you do that?
Why ?
What could you possibly learn from looking at the LV AC ripple from the transformer. We all know it will be at mains frequency.
What matters is the ripple at the PSU output under various states of load and for which you'd just connect across the output terminals (after checking 0V doesn't create ground loop issues with a DMM) and use a 1x probe for the additional sensitivity it offers.
For a low impedance source like a PSU rail the capacitive loading of a 1x probe is never an issue.
As Electro Detective says, I would like to have the ability to measure two places in a circuit and then tweak, tinkerer and play. Why, because I think that will be a great way for me to learn what happens in a circuit, before and after, a way to follow the power, from input to output, trough an opamp, logic circuit or what not. :-)
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Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #26 on: June 06, 2019, 03:49:28 am »
Am I understanding right if I am saying that I can either build a crappy probe that will be of no use anyway, or dig deep in my pocket once for all and get a decent one?

That pretty much sums it up.

The old way was to use an isolation amplifier between the probe and oscilloscope:

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

Quote
But what about that 500X attenuation, would that not eliminate the 2uV noise I may look for in an circuit?

A high voltage differential probe will not be useful in that case but there may be other ways if that is your requirement.  An isolated probe will not measure 2 microvolts noise either.

An active differential probe might be able to resolve 2 microvolts under the right circumstances.

You will most likely never see 2 uV signals on a oscilloscope, regardless of which probe you use.

A Tektronix 7A22 differential amplifier can resolve 2 microvolts over a range of +/-1 volt but only at a bandwidth significantly lower than its rated 1 MHz.  Doing better than this even 40 years later would be a significant challenge but it could be done.
TThe 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.
Simply a way to poke around.
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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #27 on: June 06, 2019, 04:08:42 am »
The 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.

High voltage probes are available with lower attenuation factors yielding lower noise and that is the modern solution but not particularly inexpensive.  Even a 500x attenuation probe might be acceptable for making a floating measurement of everything interesting across a bridge rectifier.  Dave's probe is x10 and x100 and will certainly work.

Either a Tektronix 7A13 (100 MHz) or 7A22 (1 MHz) could make that measurement easily and I have used them to do it.  But they are not an inexpensive method either.

An isolated probe would also work great for this but today that pretty much means buying an oscilloscope which includes isolated inputs as a feature.

Bob91343 mentioned combining two oscilloscope channels by subtracting one from another and that will work well in this case.  Some oscilloscopes are better at this than others.
« Last Edit: June 06, 2019, 04:10:52 am by David Hess »
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #28 on: June 06, 2019, 04:52:54 am »
The 2uV was an error from my side, I was trying to illustrate that I think that a 500X atteniuatet probe, may not be usefull for my usage of playing with beginner circuits at maybe 1.5V to 15V or to show ripple in a DC output from a power supply circuit. Sinse I have about no idea of what I am doing, am I looking for a way to look before and after some components, or maybe save my circuit, if I by error connect plus to oscilloscope ground.

High voltage probes are available with lower attenuation factors yielding lower noise and that is the modern solution but not particularly inexpensive.  Even a 500x attenuation probe might be acceptable for making a floating measurement of everything interesting across a bridge rectifier.  Dave's probe is x10 and x100 and will certainly work.

Either a Tektronix 7A13 (100 MHz) or 7A22 (1 MHz) could make that measurement easily and I have used them to do it.  But they are not an inexpensive method either.

An isolated probe would also work great for this but today that pretty much means buying an oscilloscope which includes isolated inputs as a feature.

Bob91343 mentioned combining two oscilloscope channels by subtracting one from another and that will work well in this case.  Some oscilloscopes are better at this than others.
What is bothering me is that I may have to pay more then a beginner scope costs:-)
I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.
I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)
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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #29 on: June 06, 2019, 05:01:32 am »
What is bothering me is that I may have to pay more then a beginner scope costs:-)
I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.
I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)

That is exactly how this works:

http://w140.com/tekwiki/wiki/A6902
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #30 on: June 06, 2019, 05:32:04 am »
Thanks I'll read about that system:-)

Just found this US patent: https://patents.google.com/patent/US6603891
Sounds complicated but perhaps something useful?
Is it even possible to build?
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Offline capt bullshot

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Re: Nead an insulated probe for oscilloscope
« Reply #31 on: June 06, 2019, 05:39:25 am »
What is bothering me is that I may have to pay more then a beginner scope costs:-)
I my naivity, I had hoped for a simpl-ish solution on a circuit, that could isulate my 1x/10x pasive probes for my noob usage.
I thought that a few mega Hertz and voltage under what the scope is build fore, could be handled by some sort of transformer, optocoupler or something like that, could handle it:-)

You wrote about measuring the AC input of a bridge rectifier and the output of a 5VDC regulator. One can achieve this with an 4ch beginners scope, using 3 probes. One single ended for the DC part, and the two others for the AC part, scope does subtract these channels.

Otherwise, I'm really often watching more than one signal in a circuit, and have access to various kinds of isolated channels on scopes and differential probes, but rarely need them. Most of the measurements can be done with all scope inputs connected to one common GND in the circuit. Speaking your simple example, I'd connect one channel to the regulators output and the other one to its input to see the rectified ripple - this usually gives you the same information as watching the AC side.

Of course, there are situations where you _need_ an isolated input or a differential probe - e.g. watching gate driver signals on large IGBT modules, or debugging PFC stages and the like. Most small signal measurement can be achieved with your standard single ended probes.

Edit: BTW, the isolation technique used by the A6902 is the same as the THS720 uses - a HF transformer path and a LF optocoupler. The TPS20xx somehow replaced the optocoupler part by transformer and encoding / modulation stuff. Splitting and re-combining the frequency band can be hard to get right. You'd still have to place the step attenuator (say the 10mV 20mV ... 20V / div switch) on the isolated side.

The most modern Cleverscope does it the digital way, placing the ADC on the isolated side. Same did I when I developed rather low frequency isolated input channels for power line transient recording usage in a former job.
« Last Edit: June 06, 2019, 05:48:03 am by capt bullshot »
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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #32 on: June 06, 2019, 05:39:44 am »
The A6902 service manual at that page has a detailed description and full schematic.  It works like you suggested with a parallel path using transformer isolation at high frequencies up to 20 MHz and a linear optocoupler for low frequencies down to DC.  It would not be difficult to duplicate but it would be a lot of work.
 

Offline capt bullshot

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Re: Nead an insulated probe for oscilloscope
« Reply #33 on: June 06, 2019, 05:51:40 am »
Thanks I'll read about that system:-)

Just found this US patent: https://patents.google.com/patent/US6603891
Sounds complicated but perhaps something useful?
Is it even possible to build?

Tek did a probe that uses optical fibers only to transfer power, the signal and some control data. I've seen this probe working, it was quite bulky and you wouldn't ask for the price tag ;)
Otherwise quite impressive specs.
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Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #34 on: June 06, 2019, 08:41:24 am »
The A6902 service manual at that page has a detailed description and full schematic.  It works like you suggested with a parallel path using transformer isolation at high frequencies up to 20 MHz and a linear optocoupler for low frequencies down to DC.  It would not be difficult to duplicate but it would be a lot of work.
Wary interesting!
If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached?
You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-)
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Offline capt bullshot

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Re: Nead an insulated probe for oscilloscope
« Reply #35 on: June 06, 2019, 09:55:44 am »
The A6902 service manual at that page has a detailed description and full schematic.  It works like you suggested with a parallel path using transformer isolation at high frequencies up to 20 MHz and a linear optocoupler for low frequencies down to DC.  It would not be difficult to duplicate but it would be a lot of work.
Wary interesting!
If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached?
You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-)

Since this is quite an old design, using parts that you cannot buy anymore, you'd have to duplicate the working principle, not the schematic.

With some luck, an A6902 might appear at the evil bay within the next ten years ;)

Edit: not within ten years, but today (you see me surprised):
https://www.ebay.de/itm/TEKTRONIX-A6902B-Isolator/233187964339

« Last Edit: June 06, 2019, 10:02:25 am by capt bullshot »
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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #36 on: June 06, 2019, 05:08:12 pm »
If we ignore the part of 3000V and instead say maybe 100V, could that not be made with SMD 1206 in a much smaller box with maybe some standard probes attached?

The A6902 actually works with standard 1 megohm passive oscilloscope probes.  It included special probes which had reinforced insulation for safety.  Modern oscilloscopes which have isolated inputs use the same type of probes with reinforced insulation for the same reason.

Honestly if you are measuring after the line isolation transformer but before the rectifiers, lifting the ground connection to your oscilloscope is relatively safe.  However this only gives you one isolated domain; all of the oscilloscope probes share the same common connection to each other.  Also the common mode capacitance from the probe commons to Earth ground still exists and would be high enough to disturb some measurements.  The A6902 specifications say less than 100 picofarads of common mode capacitance which is only that low because the A6902 was designed to minimize it.

Quote
You write "not difficult to duplicate but a lot of work" ehm I am not sure I understand? To me it look like a wary difficult task to duplicate the schematic, since I have about no idea on what does what, so I have to duplicate every line and comma. :-)

I mean the concept and design is straightforward for someone skilled in the art.  A modern implementation would be considerably simplified because integrated operational amplifiers are now commonly available with sufficient performance to replace most if not all of the discrete circuitry. (1)

I think a transformer would still be required for the high frequency path.  Modern isolation amplifiers are only good up to 500 kHz or so.  But an isolation amplifier could replace the low frequency linear optocoupler path and yield better performance.

(1) I am not a fan of using shunt feedback amplifiers in oscilloscope applications because of poor overload recovery time but at only 20 MHz this would not be a problem.
 

Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #37 on: June 06, 2019, 05:19:35 pm »
Honestly if you are measuring after the line isolation transformer but before the rectifiers, lifting the ground connection to your oscilloscope is relatively safe.

Incidentally, Tektronix also made the A6901 Ground Isolation Monitor for exactly this application.

http://w140.com/tekwiki/wiki/A6901
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #38 on: June 06, 2019, 05:52:24 pm »
LOL WOW sometime luck is smiling to you! :-)
I hope that it is still there when I have all the money.

I can see that handheld scopes have galvanic isolated channels, would it be possible to build one, I mean a unit where you plug the standard passive probe in one end and a coax out in the other end to the scope, maybe with external power source?

https://hackaday.com/2016/03/31/diy-active-sub-ghz-differential-scope-probe/
« Last Edit: June 06, 2019, 06:43:52 pm by FriedMule »
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Offline ggchab

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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #40 on: June 06, 2019, 08:56:25 pm »
I can see that handheld scopes have galvanic isolated channels, would it be possible to build one, I mean a unit where you plug the standard passive probe in one end and a coax out in the other end to the scope, maybe with external power source?

That is what the A6902 is.  The probe plugs into one side and the oscilloscope plugs into the other.
 

Offline FriedMuleTopic starter

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Re: Nead an insulated probe for oscilloscope
« Reply #41 on: June 06, 2019, 09:34:09 pm »
I can see that handheld scopes have galvanic isolated channels, would it be possible to build one, I mean a unit where you plug the standard passive probe in one end and a coax out in the other end to the scope, maybe with external power source?

That is what the A6902 is.  The probe plugs into one side and the oscilloscope plugs into the other.
Okay yes, sorry for being so noob:-)
Do you think it is realistic for me to try to build it?

This also seems interesting: http://www.paulvdiyblogs.net/2017/10/differential-amplifier-probe-buy-or-diy.html
Do look interesting, again, thinking of I can manage to build anything like that?
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Offline David Hess

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Re: Nead an insulated probe for oscilloscope
« Reply #42 on: June 07, 2019, 03:45:30 am »
That is what the A6902 is.  The probe plugs into one side and the oscilloscope plugs into the other.

Okay yes, sorry for being so noob:-)
Do you think it is realistic for me to try to build it?

Break it down into parts and start with the simple stuff.  We have the full description and schematic of the A6902.

Could you design and build a high impedance amplifier which accepts a probe signal?  That is the first stage.  If not, then no although I can think of old oscilloscopes which could provide this part inexpensively.  It just gets more complicated after this.

 


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