Author Topic: DIY Differential probe again again - please help :-)  (Read 6002 times)

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Offline ogden

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Re: DIY Differential probe again again - please help :-)
« Reply #25 on: October 06, 2019, 05:32:49 pm »
Sorry if I have not explained that in a good enough manner, I did try at my comment number 6:
THAT project do not need differential probe. It does not even need scope. Thou may benefit from proper signal gen, not necessarily 500MHz. There is nothing wrong about being beginner, but before drawing moon rocket plans, you shall *build* at least one working paper plane first. So I strongly suggest you to carefully consider following advice:

copy a successful design *exactly*.
 

Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #26 on: October 06, 2019, 05:48:43 pm »
Sorry if I have not explained that in a good enough manner, I did try at my comment number 6:
THAT project do not need differential probe. It does not even need scope. Thou may benefit from proper signal gen, not necessarily 500MHz. There is nothing wrong about being beginner, but before drawing moon rocket plans, you shall *build* at least one working paper plane first. So I strongly suggest you to carefully consider following advice:

copy a successful design *exactly*.
You are completely correct that it may not need a probe but, as a beginner, I would like to see where what is happening, on top of that do I plan on building a simple PSU for it, just to learn what PSU "signal" do to that circuit, what make the 500MHz behave wrong, bad or you know. :-) So selected a project that is easy-ish to make, had max MHz my scope can handle to learn what happens and how do my scope react... Or in short, just to see what is going on all sort of places in the circuit.

LOL I love the way you say it about not building a space rocket before even learn to fold a paper plane:-) But again as a beginner, it is hard to know what is a paper plane project and what is a moon landing:-)
« Last Edit: October 06, 2019, 06:02:15 pm by FriedMule »
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Offline ogden

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Re: DIY Differential probe again again - please help :-)
« Reply #27 on: October 06, 2019, 06:08:37 pm »
So selected a project that is easy-ish to make, had max MHz my scope can handle to learn what happens and how do my scope react... Or in short, just to see what is going on all sort of places in the circuit.

Well.. Everything is fine while you do not require your 65V common mode range for high speed differential probe. That is very "unusual spec" if I can say so. Either stick with common mode range of existing DIY diff probe designs or do not call it "easy-ish to make". Please do not get me wrong - I am not trying to discourage you from your idea. Sometimes brave ideas of unaware are surprisingly successful :D Yet I still afree agree that you better start with building *exact* copy of working diff probe, then think of increasing it's common mode voltage range.

Why do you think 1:10 500MHz scope probes cost so much? - because of probe manufacturer's cartel? ;)
« Last Edit: October 06, 2019, 06:16:25 pm by ogden »
 

Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #28 on: October 06, 2019, 07:05:14 pm »
So selected a project that is easy-ish to make, had max MHz my scope can handle to learn what happens and how do my scope react... Or in short, just to see what is going on all sort of places in the circuit.

Well.. Everything is fine while you do not require your 65V common mode range for high speed differential probe. That is very "unusual spec" if I can say so. Either stick with common mode range of existing DIY diff probe designs or do not call it "easy-ish to make". Please do not get me wrong - I am not trying to discourage you from your idea. Sometimes brave ideas of unaware are surprisingly successful :D Yet I still afree agree that you better start with building *exact* copy of working diff probe, then think of increasing it's common mode voltage range.

Why do you think 1:10 500MHz scope probes cost so much? - because of probe manufacturer's cartel? ;)
Thanks:-) The 65V is due to it's max I do work with and why not build it to be able to handle max. (my innocent thought) By easy-ish I meant the project board at 500MHz, not the probe:-) I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities. So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.

I have two channels and would like to measure on maybe the PSU while also look at the 500MHz board, so that's why.
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Offline ogden

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Re: DIY Differential probe again again - please help :-)
« Reply #29 on: October 06, 2019, 07:23:18 pm »
I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities.
Urban myth. If you have ANY 500MHz "original/brand" probe, then buy cheap 500MHz probe from China (if any) and compare. You will see that "you get what you pay for" saying makes sense.

Quote
So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.
Dreamer :)
65V differential probe that compares to 2GHz 100V Agilent probe may take years of your "career" and all your savings. Better just buy one.
[edit] Failure to complete my 10 minute homework even after so many posts just confirms that probe you are dreaming about is above your paygrade. Sorry.
« Last Edit: October 06, 2019, 07:41:29 pm by ogden »
 

Offline rstofer

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Re: DIY Differential probe again again - please help :-)
« Reply #30 on: October 06, 2019, 07:27:36 pm »
I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities. So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.

Don't forget the cast of dozens that worked on the design, the management that lead the project and up the chain.  Then there is the cost of safety certifications and the ever important QA department. Those are substantial costs to be recovered from what is bound to be a low volume device.

It's easy to build a Rigol DS1054Z and sell it for $400 when they are going to sell millions of units.  I doubt that Keysight has many customers for their $300,000 model.  I rather expect it is built to order.

The EEVblog store has Dave's differential probe for sale for A$350 but the bandwidth is just 70 MHz.  It has a voltage range switch for dealing with up to 700V differential.

https://www.eevblog.com/product/hvp70/

Sapphire makes Dave's probe and many others that get rebadged by the scope manufacturers.  Look at the list here and see if what you need is even manufactured.  For the higher voltages, 100 MHz looks like the limit.

https://www.nbn.cz/Sapphire/Sapphire%20Instruments%20CO_%20LTD%20produts_files/pproducts.htm

« Last Edit: October 06, 2019, 07:30:38 pm by rstofer »
 

Offline rstofer

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Re: DIY Differential probe again again - please help :-)
« Reply #31 on: October 06, 2019, 07:40:07 pm »
LOL I love the way you say it about not building a space rocket before even learn to fold a paper plane:-) But again as a beginner, it is hard to know what is a paper plane project and what is a moon landing:-)

Not really!  Just create a spreadsheet of specs and price.  Specs would include Common Mode (AC+DC) Max Voltage, Differential Max Voltage, Switchable Voltage Ranges, Bandwidth and so on.  Maybe one column for each.

You might include a column for Safety Certifications and another for Warranty. 

For voltages less than 50V (peak to ground), I might not get too concerned about safety certs but beyond that, where voltages become life threatening, I would have a lot more concern.  That's why I keep my projects at of below +-15V.



 

Offline MagicSmoker

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Re: DIY Differential probe again again - please help :-)
« Reply #32 on: October 06, 2019, 08:01:48 pm »
You don't need a differential probe unless you are either looking at differential signals (I gave examples earlier) or a signal which is elevated from ground, such as the gate drive to the upper switches in a bridge converter (that likely didn't make any sense, but f it, it's a good example).

Neither condition applies for that battery power RF thingamajig, nor does the need for >500MHz of bandwidth.

I find it amusing when a beginner/clueless noob can't see why something costs so much to make. Well, here's a clue when it comes to >500MHz active/differential probes: the stray capacitance of two 1cm teflon-insulated wires twisted together (called a "gimmick", btw) is about 4pF, and that meager amount of capacitance has a reactance (AC equivalent to resistance) of 80 Ohms at 500MHz. Heck, the stray capacitance of a typical chip resistor might be in the half-a-puff range, so equivalent to about 640 Ohms. THAT is why differential probes are so difficult to design once either/both a high voltage division ratio and/or high bandwidth are required.


EDIT - forgot to put the frequency in for the reactance examples.
« Last Edit: October 06, 2019, 08:47:49 pm by MagicSmoker »
 

Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #33 on: October 06, 2019, 08:45:41 pm »
I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities.
Urban myth. If you have ANY 500MHz "original/brand" probe, then buy cheap 500MHz probe from China (if any) and compare. You will see that "you get what you pay for" saying makes sense.

Quote
So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.
Dreamer :)
65V differential probe that compares to 2GHz 100V Agilent probe may take years of your "career" and all your savings. Better just buy one.
[edit] Failure to complete my 10 minute homework even after so many posts just confirms that probe you are dreaming about is above your paygrade. Sorry.
It makes a lot of sense, my thought just comes from so many other things you can buy that alone is expensive due to brand.
I think that most probes are above my paygrade, even the "cheap" one:-)
LOL I love the way you say it about not building a space rocket before even learn to fold a paper plane:-) But again as a beginner, it is hard to know what is a paper plane project and what is a moon landing:-)

Not really!  Just create a spreadsheet of specs and price.  Specs would include Common Mode (AC+DC) Max Voltage, Differential Max Voltage, Switchable Voltage Ranges, Bandwidth and so on.  Maybe one column for each.

You might include a column for Safety Certifications and another for Warranty. 

For voltages less than 50V (peak to ground), I might not get too concerned about safety certs but beyond that, where voltages become life threatening, I would have a lot more concern.  That's why I keep my projects at of below +-15V.
What a fantastic idea, thanks, it "forces" me also to dig deeper in what I am about to jump into, so that is a thing I will do from today and on! :-)
Yes above 50V is some nasty stuff and can even be dangerous already from 40V or even lower if all things goes wrong!
I did / do think that most electronic, made by Agilent and other is much more expencive then needed to be, due to name, manufacturing and the 1000V capabilities. So I did thought, DIY and low voltage (65V) would be much cheaper and maybe not as difficult as a 2GHz 1000V Agilent you can buy.

Don't forget the cast of dozens that worked on the design, the management that lead the project and up the chain.  Then there is the cost of safety certifications and the ever important QA department. Those are substantial costs to be recovered from what is bound to be a low volume device.

It's easy to build a Rigol DS1054Z and sell it for $400 when they are going to sell millions of units.  I doubt that Keysight has many customers for their $300,000 model.  I rather expect it is built to order.

The EEVblog store has Dave's differential probe for sale for A$350 but the bandwidth is just 70 MHz.  It has a voltage range switch for dealing with up to 700V differential.

https://www.eevblog.com/product/hvp70/

Sapphire makes Dave's probe and many others that get rebadged by the scope manufacturers.  Look at the list here and see if what you need is even manufactured.  For the higher voltages, 100 MHz looks like the limit.

https://www.nbn.cz/Sapphire/Sapphire%20Instruments%20CO_%20LTD%20produts_files/pproducts.htm
Yes I have looked at these also but the price did frighten me and then did I think: "why not make it myself, it is a few components and I do not require GHz+ or high voltage, so even cheaper" LOL was I wrong:-)

My reason for talking about differential probe, is that, that's what I did find out after several weeks of searching, but is it what I need?
I "just" want to be able to measure two places where the ground is not the same or where I am not sure if a common ground connection will give me a wrong result.

EDIT: Sorry for this maybe infantile addition but I imagine some circuit where it measures the difference between ground and probe point, send that voltage out to the scope, so lets say DC difference of 15V gives the scope a 10:1 or 1.5V or AC 2VP to P sends 0.2V to the scope.
« Last Edit: October 06, 2019, 08:55:14 pm by FriedMule »
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Offline The Senate

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Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #35 on: October 06, 2019, 11:45:47 pm »
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Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #36 on: October 08, 2019, 05:11:55 am »
in the above article abut an isolated circuit there is talk about two isolated chips, the one has Isolated Power, the other not, what is that? I have looked at Wikipedia, and other places but I only find articles about sport and other like that, the wii page do not talk about Isolated Power while it do talk about digital isolator's.
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Offline ogden

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Re: DIY Differential probe again again - please help :-)
« Reply #37 on: October 08, 2019, 06:07:57 am »
in the above article abut an isolated circuit there is talk about two isolated chips, the one has Isolated Power, the other not, what is that? I have looked at Wikipedia, and other places but I only find articles about sport and other like that, the wii page do not talk about Isolated Power while it do talk about digital isolator's.

You shall try internet search instead of wikipedia. https://lmgtfy.com/?q=AMC1200. findchips.com is another good place to find chips. In short: It is  ±250 mV-Input Basic Isolated Amplifier for Current Sensing. Kind reminder - that isolated probe has <= 60Khz bandwidth and it is not differential.

[edit] BTW Elektor article explains everything in details, including that chip (attach)
« Last Edit: October 08, 2019, 06:37:23 am by ogden »
 

Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #38 on: October 08, 2019, 07:41:07 am »
in the above article abut an isolated circuit there is talk about two isolated chips, the one has Isolated Power, the other not, what is that? I have looked at Wikipedia, and other places but I only find articles about sport and other like that, the wii page do not talk about Isolated Power while it do talk about digital isolator's.

You shall try internet search instead of wikipedia. https://lmgtfy.com/?q=AMC1200. findchips.com is another good place to find chips. In short: It is  ±250 mV-Input Basic Isolated Amplifier for Current Sensing. Kind reminder - that isolated probe has <= 60Khz bandwidth and it is not differential.

[edit] BTW Elektor article explains everything in details, including that chip (attach)
I have tried google and there are a lot  of links, but not any explanation on what Isolated Power means in a digital isolator.
Yes I saw that about the 60KHz and it is only an isolated probe but I am not sure what I need, isolated or differential.

My goal is to be able to measure two different places at once, i.e before the bridge rectifier and before a chip much later in the circuit.
Or short, no common ground between the two measurements.
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Offline BravoV

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Re: DIY Differential probe again again - please help :-)
« Reply #39 on: October 08, 2019, 07:55:41 am »
My goal is to be able to measure two different places at once, i.e before the bridge rectifier and before a chip much later in the circuit.
Or short, no common ground between the two measurements.

Imo, what you want there is not cheap or easy, let alone DIY project for beginner.

Usually, when the times there is need to measure this kind of measurement points, the best is to use a fully isolated oscilloscope, mostly handheld and not cheap.

No, ordinary scope can't do it, and even using HV differential scope, you need two of them.  :-[

Once you own it, you can freely poke and clip the probe's ground clip at anywhere you like as below example.


Offline ogden

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Re: DIY Differential probe again again - please help :-)
« Reply #40 on: October 08, 2019, 08:01:22 am »
I have tried google and there are a lot  of links, but not any explanation on what Isolated Power means in a digital isolator.
That digital isolator IC have isolated dc-dc power converter inside. Did I answer your question? You may ask further questions.

I am not sure what I need, isolated or differential.
My goal is to be able to measure two different places at once, i.e before the bridge rectifier and before a chip much later in the circuit.
For that you do not need differential probe, nor 500MHz bandwidth. As you ask surprisingly simple questions, I am afraid that instead of building something safety-related, you shall consider buying one or two HV diff probes: https://www.eevblog.com/product/hvp70/. Alt least then there will be less chances of you killing yourself.
 

Offline FriedMuleTopic starter

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Re: DIY Differential probe again again - please help :-)
« Reply #41 on: October 08, 2019, 08:18:28 am »
I have tried google and there are a lot  of links, but not any explanation on what Isolated Power means in a digital isolator.
That digital isolator IC have isolated dc-dc power converter inside. Did I answer your question? You may ask further questions.

I am not sure what I need, isolated or differential.
My goal is to be able to measure two different places at once, i.e before the bridge rectifier and before a chip much later in the circuit.
For that you do not need differential probe, nor 500MHz bandwidth. As you ask surprisingly simple questions, I am afraid that instead of building something safety-related, you shall consider buying one or two HV diff probes: https://www.eevblog.com/product/hvp70/. Alt least then there will be less chances of you killing yourself.
Thanks for your concern! the "funny" thing is that I know way more about main power and have done several installations, while I know nearly nothing below 220V.
I am not concerned about my safety, since I will take safety precautions that can be considered total over"kill":-)
I have seen what power can do when it want to and that's scary!! 400V 1000A can do things a hand grenade only can dream of!

But on the other side, will I keep it below 50V, my example was bad but just to illustrate that I want to neasure one place with the normal probe and then the other with the isolated probe. One of the hard ting to do I to search or explain what I want when I don't know the therms, words and possibility, on top of that is my English wary bad and do not make it easier.

When I have found out what i need, isolated or diff, then begins my search again for articles, diagrams and gerber files. :-)
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