Newbie question - Can using osc probe x 10 with dmm for check HV CRT

[Gyro]

Low breakdown voltage => small => SMD. Yes, I realise you wouldn't be so silly as to use one, but ... 😃

   Not Again! Why cite it as a serious objection then! Even if the OP used a string of 10 1/4W bog standard carbon film resistors it would just about manage 2kV (maximum resistor rating) and 4kV Overload / dielectric breakdown. With the chain of 1W wire ended resistors there's no way he's going to suffer breakdown as long as he puts them in a straight line in an insulating tube as I already advised him. Also remember that he's working on a low energy circuit too. He's not going to get to the market and be able to start quizzing sellers on datasheet voltage rating, wattage is all they will know.

We REALLY should be trying to support the OP to achieve what he wants to do, within his available resources, rather than engaging in pointless point scoring and pi**ing contests! Why don't YOU start addressing him directly with helpful suggestions rather than picking holes in my attempt to help him. 


@joeqsmith: That's a really instructive video that takes a serious and methodical approach to the problem, thanks. 

[chihaxinh]

Many thank for many advice

I think in easy question allway make hard answer like "Why does it rain? ? " ! Hehehe

So if you have any advice , i can learn and i can make or do it in next time ( for collect resistor, capacitor, isulation material ...etc ) .

You know, in my country it's very hard to buy good brand resistor, capacitor ...etc like vishay, bc phillip , epcos....

So when i ask and take answer , i need 1 2 week to collect item.

Yes, simply question : I really want to quick way check HV with DMM with simple material . For quick check HV Board CRT .

Many thank for your support !

PS: But any conversation about mearsurement technical , i allway hope to see

[tggzzz]

Low breakdown voltage => small => SMD. Yes, I realise you wouldn't be so silly as to use one, but ... 😃

   Not Again! Why cite it as a serious objection then! Even if the OP used a string of 10 1/4W bog standard carbon film resistors it would just about manage 2kV (maximum resistor rating) and 4kV Overload / dielectric breakdown. With the chain of 1W wire ended resistors there's no way he's going to suffer breakdown as long as he puts them in a straight line in an insulating tube as I already advised him. Also remember that he's working on a low energy circuit too. He's not going to get to the market and be able to start quizzing sellers on datasheet voltage rating, wattage is all they will know.

We REALLY should be trying to support the OP to achieve what he wants to do, within his available resources, rather than engaging in pointless point scoring and pi**ing contests! Why don't YOU start addressing him directly with helpful suggestions rather than picking holes in my attempt to help him. 

You have, responsibly, admitted you gave the OP incorrect advice and guidance (a watts rating instead of volts rating).
The OP is a newbie and might not appreciate that - and might blindly follow that guidance.
Dangerous voltages are involved; that's the whole reason for this thread.

Hence the points I made are necessary to prevent the OP (or anyone else reading the thread) endangering themselves by following your deficient guidance. Given that objective, I wonder how many people would regard that as "pointless point scoring"?

[Gyro]

Many thank for many advice

I think in easy question allway make hard answer like "Why does it rain? ? " ! Hehehe

So if you have any advice , i can learn and i can make or do it in next time ( for collect resistor, capacitor, isulation material ...etc ) .

You know, in my country it's very hard to buy good brand resistor, capacitor ...etc like vishay, bc phillip , epcos....

So when i ask and take answer , i need 1 2 week to collect item.

Yes, simply question : I really want to quick way check HV with DMM with simple material . For quick check HV Board CRT .

Many thank for your support !

PS: But any conversation about mearsurement technical , i allway hope to see

That's good  chihaxinh, I was worried that we might have scared you away!

It sounds as if tggzzz is best equipped to give you your 'shopping list - for the resistors at least, hopefully he will provide that to you shortly. 

Regarding the other things:

-an insulating plastic tube capable of taking the diameter of the string of resistors.
- some heatshrink sleeving,. High voltage test lead wire if you can get it.
- 4mm plugs to fit your meter (you don't want the connection between the probe and meter coming undone).
- Also a secure clip to make your ground connection (again you don't want the ground connection comming undone as your meter will float up to 2kV!).
- Anything else that tggzzz can think of.

[Gyro]

You have, responsibly, admitted you gave the OP incorrect advice and guidance (a watts rating instead of volts rating).
The OP is a newbie and might not appreciate that - and might blindly follow that guidance.
Dangerous voltages are involved; that's the whole reason for this thread.

Hence the points I made are necessary to prevent the OP (or anyone else reading the thread) endangering themselves by following your deficient guidance. Given that objective, I wonder how many people would regard that as "pointless point scoring"?

We'll I think others can judge the merits of the 'discussion' from now on!

In the meantime the OP has come back and clarified his needs and available resources again (thankfully). It's is time for you as a fellow super-contributor to 'Put up' and give him suitable and practical advice, relevant to his situation. Please do not let him down!

[tggzzz]

In the meantime the OP has come back and clarified his needs and available resources again (thankfully). It's is time for you as a fellow super-contributor to 'Put up' and give him suitable and practical advice, relevant to his situation. Please do not let him down!

Sigh.

I already have - and you have replied to it and thought it was correct useful advice! So clearly you were aware of it, and have forgotten it.

I don't think I can help you further at this point.

[Gyro]

I don't understand - the OP needs concrete advice on what resistors he should buy for a safe implementation at his location. You clearly have an opinion. Please provide appropriate advice to him!

[joeqsmith]

@joeqsmith: That's a really instructive video that takes a serious and methodical approach to the problem, thanks. 

Anyone working with potentially lethal circuits should.

A few things that may not have been clear from the video are effects of loading the circuit you are trying to measure.  The unit I made does have a metal shield internal to the box to help minimize noise.   It's fairly low impedance so noise was not much of an issue.   The higher bandwidth probe I showed is much higher impedance and required better shielding to get the performance I was looking for.   

One other video that may be worth watching.   Rather than using a 10X oscilloscope probe, maybe a low cost 100X probe would work.  Then again....   In this video, I tested a cheap HV probe I bought from Amazon. 

[Gyro]

That was an entertaining test to destruction and teardown, that internal construction was shocking, I wonder if there was any science in the specific length of insulating film wrapped inside the shield to obtain the right capacitance or just winged it.

@Joe, I'm still looking for support in advising the OP on resistor choice for his situation. Just to reiterate, he's in Vietnam and has limited access to decent components. His measurement need is 2kV DC (scope internals). My best is 9 or 10 (depending on attenuator style) 10Meg 1W axial resistors in a long series chain. The way I figure it is that minimum element rating for resistors of that size will be in the order of 350V with dielectric breakdown occurring at around 700V (say 500V accounting for high humidity). That should give an in-rating limit of about 3.5kV (for 10 resistors) and breakdown at 5kV. Obviously a higher safety margin would be preferable, but roughly x2 is probably all that can practically be achieved. My best advice to the OP would be to pick the largest physical body size available, with closer tolerance being an indicator of better quality. The whole assembly would have to be mounted inside a plastic tube and preferably overall heatshrinked too. Can you think of anything better / safer given the non-avaliability of an off-the-shelf product?

[David Hess]

I did not comment about the breakdown voltage of the 10 megohm resistors simply because I assumed 1 watt values were suggested for this very reason and the original poster is limited in what he has access to.

As far as accuracy, just measuring the resistors may not be enough.  As high a calibration voltage as available should be used because the effect of voltage coefficient of resistance on accuracy can be considerable.  For best accuracy when calibration is not available, it may be better to use an external shut resistor and high input impedance voltmeter so that at least some of the voltage coefficient of resistance cancels out.  Voltage derating the resistors helps minimize the voltage coefficient of resistance as well.

Have you considered that the meter's input resistance might not be a well-controlled value?
Have you considered 10*10Mohm + 100kohm, and measuring the voltage across the 100kohm resistor with a "10Mohm" meter. 1kV -> 1V, which is suitable for most DMMs.

I am not sure what kind of junk DMMs you are used to but all of mine have an input resistance tolerance of better than 1%.

You cannot always directly measure the input resistance of a DMM.  Modern DMMs may have problems doing this because of charge pumping from their analog to digital converters.  Old DMMs which use external input buffers lacking chopper stabilization before their analog to digital converter do not suffer from this issue.

[joeqsmith]

That was an entertaining test to destruction and teardown, that internal construction was shocking, I wonder if there was any science in the specific length of insulating film wrapped inside the shield to obtain the right capacitance or just winged it.

@Joe, I'm still looking for support in advising the OP on resistor choice for his situation. Just to reiterate, he's in Vietnam and has limited access to decent components. His measurement need is 2kV DC (scope internals). My best is 9 or 10 (depending on attenuator style) 10Meg 1W axial resistors in a long series chain. The way I figure it is that minimum element rating for resistors of that size will be in the order of 350V with dielectric breakdown occurring at around 700V (say 500V accounting for high humidity). That should give an in-rating limit of about 3.5kV (for 10 resistors) and breakdown at 5kV. Obviously a higher safety margin would be preferable, but roughly x2 is probably all that can practically be achieved. My best advice to the OP would be to pick the largest physical body size available, with closer tolerance being an indicator of better quality. The whole assembly would have to be mounted inside a plastic tube and preferably overall heatshrinked too. Can you think of anything better / safer given the non-avaliability of an off-the-shelf product?

Maybe you could post some clear pictures detailing your construction techniques to help guide them along.   The high voltage parts will show the life derate curves in the data sheets.  Something to consider.   

Yea, that 100X probe is not something I would recommend even though it was not very expensive.  I gave up and rolled my own.   I use them mostly for 2KV and under. 

[tggzzz]

As far as accuracy, just measuring the resistors may not be enough.  As high a calibration voltage as available should be used because the effect of voltage coefficient of resistance on accuracy can be considerable.  ... Voltage derating the resistors helps minimize the voltage coefficient of resistance as well.

Good point; I should have mentioned that.

Have you considered that the meter's input resistance might not be a well-controlled value?
Have you considered 10*10Mohm + 100kohm, and measuring the voltage across the 100kohm resistor with a "10Mohm" meter. 1kV -> 1V, which is suitable for most DMMs.

I am not sure what kind of junk DMMs you are used to but all of mine have an input resistance tolerance of better than 1%.

I use Flukes, and have never made measurements where the tolerance of the input resistance is important. Hence I've never looked at that specification.

I have no idea of the OP's equipment, and hence merely noted that the tolerance should be considered before relying on it.

You cannot always directly measure the input resistance of a DMM.  Modern DMMs may have problems doing this because of charge pumping from their analog to digital converters.  Old DMMs which use external input buffers lacking chopper stabilization before their analog to digital converter do not suffer from this issue.

Just so; ISTR I mentioned that point.

[Gyro]

Maybe you could post some clear pictures detailing your construction techniques to help guide them along.   The high voltage parts will show the life derate curves in the data sheets.  Something to consider.   

In terms of construction, I was simply thinking 10 (or 9) resistors soldered end to end and enclosed in a plastic tube. The actual detail would depend on what the OP has available, if flexible then the ends could be constricted by cable ties, otherwise retained by epoxy or other means. On reflection I would avoid heatshrink, my picoammeter experiments have shown it to be relatively conductive and I don't know how this would scale with voltage. Termination at each end would be flexible leads so that the OP could solder to the Vk test point rather than using it handheld (less risk of direct contact). For the voltage divider approach I guess this would simply be a matter of hardwiring the 100k resistor across the DMM input jacks.

In terms of life derating curves, I very much doubt that the OP will have access to relevant datasheets, he has already said that he doesn't have access to quality branded parts. We are very much depending on physical size (creepage path length) as an indication of voltage capability. This sounds like pretty much a one-time test anyway.

As far as accuracy, just measuring the resistors may not be enough.  As high a calibration voltage as available should be used because the effect of voltage coefficient of resistance on accuracy can be considerable.  ... Voltage derating the resistors helps minimize the voltage coefficient of resistance as well.

Good point; I should have mentioned that.

I did mention it, but only in terms of something that we don't have control over as it depends on the resistors that the OP has access to. I don't see a way around this as he will not have access to a higher calibration voltage.

Have you considered that the meter's input resistance might not be a well-controlled value?
Have you considered 10*10Mohm + 100kohm, and measuring the voltage across the 100kohm resistor with a "10Mohm" meter. 1kV -> 1V, which is suitable for most DMMs.

I am not sure what kind of junk DMMs you are used to but all of mine have an input resistance tolerance of better than 1%.

I use Flukes, and have never made measurements where the tolerance of the input resistance is important. Hence I've never looked at that specification.

I have no idea of the OP's equipment, and hence merely noted that the tolerance should be considered before relying on it.

This is why I still favour measuring the current through the resistor chain rather than a voltage divider (either external divider or using the DMM input resistance as the lower part of the divider). If the OP uses his lowest current range, assuming that it is low enough (200uA) then the meter impedance ceases to be an issue. It is simply measuring current through a pre-measured resistor chain. Additionally there is no added complication of an external 'bottom resistor'. Admittedly it does limit resolution as 2kV will come out as 20.0uA and still doesn't address the voltage coefficient, but I don't know how we can, other than recommending buying physically large metal film.


It seems that we are mainly dependent on details of the DMM at this point...

@chihaxinh: Can you please give us some details of your DMM? Does it have a manufacturer and model number that we might be able to find outside your country? Do the instructions specify the input resistance on voltage ranges (eg. 10M ohm +/-1%)? What is it's lowest current range?

[chihaxinh]

Hi Gyro

I using Agilent 34401a - Hioki 3238 - Sanwa PC5000a - So with Agilent i have 10M input impedance in 100mv to 100V range - 1M in > 100V
Hioki 3238 same Agilent
Sanwa fix in 10Meg input impedance

I alway interesting method measurement like tube , solid state curve , Inductor , Capacitor measurement ...etc

@Tggzz
So i hope  tggzzz draw clearly method you want show me ( using jfet or opamp for reading current of 30Meg resistor )

[Gyro]

Ah, you have access to rather better test equipment than I had realized your description of component availability and Newbie question tread title  . In fact you have rather more accurate test equipment (the Agilent) than I have! 

It seems that there are more measurement options available than I had thought. It sounds as if you can very easily calibrate a resistor chain (with decent components?) and accurately measure the current through them.

[joeqsmith]

I using Agilent 34401a - Hioki 3238 - Sanwa PC5000a

I am surprised you can get your hands on some decent meters but not a probe or resistors.   

Sounds like this is what is being recommended. 

[chihaxinh]

Ah, you have access to rather better test equipment than I had realized your description of component availability and Newbie question tread title  . In fact you have rather more accurate test equipment (the Agilent) than I have! 

It seems that there are more measurement options available than I had thought. It sounds as if you can very easily calibrate a resistor chain (with decent components?) and accurately measure the current through them.

Hi Gyro
I'm not graduation major  Electrical and Electronic Engineering , my job it's IT system admin . So Electronic it's my hobby , every thing i learn by reading ebook, forums...etc==> i'm newbie

In time , I like simple method for quick test ( dont care about accurate ) . So if i learn best method , high acurrate i will buy some part and buy in next time.

You know

I am surprised you can get your hands on some decent meters but not a probe or resistors.   

Sounds like this is what is being recommended. 

Yes, i buy all from one factory bankrupt . If you buy new , it's very very expensive in VN .
In VN can buy from Mouser, Digikey but take 15-20 day for shipping. Some time when need repair equiment , i dont have time for wait delivery !

[tggzzz]

@Tggzz
So i hope  tggzzz draw clearly method you want show me ( using jfet or opamp for reading current of 30Meg resistor )

Some of the issues are mechanical, i.e. ensuring adequate materials and construction techniques. Some of the issues relate to component characteristics. While I can mention important characteristics, there is no point in my specifying components, since I would almost certainly specify a part number that you could not obtain.

It would be unwise of me to quickly tell a beginner how to do something potentially hazardous, unless I was there to check what they actually did. I would probably miss out a detail that is obvious to me but not to you. I don't know you, don't know how much you do and don't know, and don't know which bits of of any instructions might become "lost in translation". There's an old engineering saying: it is impossible to make something beginner-proof, since beginners are so damn ingenious

[Gyro]

I using Agilent 34401a - Hioki 3238 - Sanwa PC5000a

I am surprised you can get your hands on some decent meters but not a probe or resistors.   

Sounds like this is what is being recommended. 

Pretty much, yes. The resistors shown in the video look like 1/4W metal film, so an element limiting voltage of about 250V (though higher breakdown). I was suggesting physically larger ones, both for higher voltage capability and hopefully better voltage coefficient. The ptfe tape wrap is a good idea, I've found that some heatshrink is conductive at the nA level at relatively low voltages (carbon black pigment content?). I would have liked to have seen some strain relief, but for short term use it is probably ok. The deformation of the heatshrink would hold the resistor chain intact and plenty of layers shown. Oh, I hate seeing people heatshrinking with a flame.

Looking at the Sanwa PC5000a pdf specifications (nice 50000 count meter + optional extra digit)...
- They confirm that the DC voltage input resistance is 10M ohms on all ranges but no tolerance stated. Accuracy 0.05% rdg+2 digits on 500V range (1kV range is 0.1% rdg.)
- It has a 500uA current range, resistance burden 100R (negligible in this context), resolution 10nA, 0.15% rdg+20 digits.

Given the resistor quality and voltage coefficient limitations of the resistor chain, I guess in practice it could be used either on the voltage range (9 resistor chain) or the current range (10 resistor chain).

I can't think of any particular benefit, given the OP's stated requirements and component sourcing, of building a complete external divider (with 100k) and risking the high impedance Agilent or Hioki. Not until he wants to do a proper permanent setup anyway - it's not worth it until you use decent precision resistors and build a proper enclosure (Edit: and some protection).

[David Hess]

This is why I still favour measuring the current through the resistor chain rather than a voltage divider (either external divider or using the DMM input resistance as the lower part of the divider). If the OP uses his lowest current range, assuming that it is low enough (200uA) then the meter impedance ceases to be an issue. It is simply measuring current through a pre-measured resistor chain. Additionally there is no added complication of an external 'bottom resistor'. Admittedly it does limit resolution as 2kV will come out as 20.0uA and still doesn't address the voltage coefficient, but I don't know how we can, other than recommending buying physically large metal film.

If I wanted an accurate measurement and had no high voltage calibration source and inadequate specifications for the resistors, then what I would do is use a high input resistance DMM (electrometer) or buffer with an external series/shunt resistor divider.  This way the voltage coefficient of resistance for the resistive divider using stacked identical resistors should cancel out to some extent; each resistor has the same voltage across it.

If you buy resistors intended for high voltage applications, they will have a specified voltage coefficient of resistance and this allows you to voltage derate them for the accuracy you require.

[Gyro]

Sure David, I 100% agree, but unfortunately above 20V the high impedance voltmeters (the Agilent and Hioki) will switch in their 10M ohm resistive dividers and no longer be high impedance. That forces a change to a 100:1 divider to get to (barely scrape) the 20V limit. In that case he would have different resistor values (and maybe types) for the 10M and 100k resistors, with unequal voltage distribution. The only way to do 100:1 by stacking identical resistors to get equal voltage distribution would have to be a chain of 100 1M resistors (as far as I can see anyway).

If you buy resistors intended for high voltage applications, they will have a specified voltage coefficient of resistance and this allows you to voltage derate them for the accuracy you require.

Again, agreed but the OP doesn't seem to have time or ability to get properly specified resistors at the moment... a mistake in my view.

Chris.

Edit: I have to admit, I am beginning to lose a little bit of my sympathy (probably an unworthy of me ) since learning of the OPs high end test equipment - I'd be tempted to sell either the Agilent or the Hioki to fund the purchase of other useful gear. Even that Sanwa is no cheapee, in fact it looks like a gem of a hand-held, accurate, feature packed and with a very high resolution option.

[tggzzz]

Sure David, I 100% agree, but unfortunately above 20V the high impedance voltmeters (the Agilent and Hioki) will switch in their 10M ohm resistive dividers and no longer be high impedance. That forces a change to a 100:1 divider to get to (barely scrape) the 20V limit. In that case he would have different resistor values (and maybe types) for the 10M and 100k resistors, with unequal voltage distribution. The only way to do 100:1 by stacking identical resistors to get equal voltage distribution would have to be a chain of 100 1M resistors (as far as I can see anyway).

If you have a 1000:1 ratio (10*10M+1*100k), then there would only be a negligable 2V across the 100k. There would be no requirement to match the voltage coefficient of resistance; only that of the 10M resistors need be assessed. And I'd measure that voltage with a 3.5 digit handheld DMM.

But personally I'd be more concerned about leakage/arcing/breakdown, particularly w.r.t. mechanical construction.

[Gyro]

If you have a 1000:1 ratio (10*10M+1*100k), then there would only be a negligable 2V across the 100k. There would be no requirement to match the voltage coefficient of resistance; only that of the 10M resistors need be assessed.

Doesn't that just bring us back to the problem of the unknown voltage coefficient of the 10M resistors, which David was trying to compensate by using a chain of identical resistors? Without that if becomes pretty academic whether you put the bottom 100k (with its own tolerance) in, or just measure the current through the chain (which is what the 100k is doing, acting as a shunt is doing anyway). It doesn't sound as if the OP actually has anything as humble as a 3.5 digit DMM.

[tggzzz]

If you have a 1000:1 ratio (10*10M+1*100k), then there would only be a negligable 2V across the 100k. There would be no requirement to match the voltage coefficient of resistance; only that of the 10M resistors need be assessed.

Doesn't that just bring us back to the problem of the unknown voltage coefficient of the 10M resistors, which David was trying to compensate by using a chain of identical resistors? Without that if becomes pretty academic whether you put the bottom 100k (with its own tolerance) in, or just measure the current through the chain (which is what the 100k is doing, acting as a shunt is doing anyway). It doesn't sound as if the OP actually has anything as humble as a 3.5 digit DMM.

The keyword in my statement, which I even emphasised is "match".  For some reason you chose to omit that in your quote, and might not have even read it before quoting.

Apart from that, your "Doesn't that just bring us back..." statement merely reiterates what is in my statement; therefore I don't see the point of your question.

[Gyro]

Huh?  Actually I just cut and pasted your text into a quote box, it wasn't worth hitting quote button on your reply as it was the next one up - I've no idea why the italicize didn't go with it, maybe it doesn't if you just paste into reply window unless you <ctrl>I a word, you really think I'd waste my time of retyping the stuff you're writing?  .

I didn't say anything about the 100k resistor having to match (about time you started reading!). All I said was that it had it's own tolerance, not VOLTAGE tolerance just tolerance - 1%, 5% whatever! (EDIT: It's an extra source of error).

What I was saying was that whatever is there or not there, it doesn't help us with the fundamental issue of the voltage coefficient of the 10M resistor chain - the only suggestion that does is David's (using identical resistors for the divider and high impedance measurement of the bottom one. As I pointed out this good idea unfortunately fails because the Agilent and Hioki do not maintain their high impedance input up to a high enough voltage before cutting in their 10M resistive dividers.

I don't know why you are so keen to keep putting in the 100k resistor. has it not occurred to you that as soon as you introduce the quote "3.5 digit handheld DMM" with its 10M input resistance across The 100K you immediately add a 1% error to the divider Just like that or didn't you do the sums? 

The Sanwa meter is quite capable of reading the current through a chain of 10 x 10M resistors with 10nA resolution and sufficient accuracy to match any 10M resistors is likely to buy! No need for a 100k resistor, no need for the extra connections to include it, no need to consider it's tolerance (%!). Just a simple chain of connections.

Now I'm sure David understands this, I'm sure Joe understands this, I'm even pretty sure that the OP understands this.

@tggzzz at this point you are either being deliberately obtuse, have some troll type need to keep this thread circling or some desperate self gratification need to have the last word? However I'm finding the Return on Invested effort becoming distinctly negative.