Author Topic: Newbie question - Can using osc probe x 10 with dmm for check HV CRT  (Read 13765 times)

0 Members and 1 Guest are viewing this topic.

Offline chihaxinhTopic starter

  • Regular Contributor
  • *
  • Posts: 126
  • Country: vn
Hi every one

When repair OSC analog , i need checking HV relative with CRT ( 1.6KV ) so my DMM have max 1000V , can i using OSC Probe in mode x 10 with DMM to mesurement High Voltage ?

I done have DMM HV Probe !

Thank you so much !
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #1 on: August 10, 2016, 09:19:40 am »
Sorry, but no. Don't even think about it. The 9M series resistor in the scope probe will have a much lower breakdown voltage than your meter and you will probably kill both.

It is possible to build a HV probe if you take care, particularly for relatively low voltages and low energy like a scope supply. You need to create a chain of high value resistors, used within their voltage rating, in a non-conducting tube. Something like 9 10M 1W resistors (90M total) in a plastic tube. Don't go above 10M because you will want to accurately measure their resistance values (to 'calibrate it'). Don't go above a few kV with it, and low power circuits only.

There are a few designs on the Web, be careful of insulation, particularly in a humid country like Vietnam.

P.S. You might want to use 10 x 10M resistors and use the meter on its current range instead (giving 10uA/kV). That wouldn't rely on the input resistance of your meter being exactly 10M.
« Last Edit: August 10, 2016, 09:24:26 am by Gyro »
Best Regards, Chris
 

Offline chihaxinhTopic starter

  • Regular Contributor
  • *
  • Posts: 126
  • Country: vn
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #2 on: August 10, 2016, 11:23:21 am »
Many thank Gyro !

I want to measurement maximum 2kv . HV before multiple to 14Kv go to drirect CRT .

I searching on internet , so have 2 type DIY HV Probe , One type using 2 resistor to make resistor bridge , and other type using only 1 resistor will mixing with DMM input impedance .

What a best type for DIY HV probe ?

Thank you so much !
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #3 on: August 10, 2016, 11:57:36 am »
You're welcome chihaxinh,

Unfortunately that introduces another problem  - the 2kV before the multiplier will be high frequency AC (the multiplier works as a charge pump using diodes and capacitors). I assumed that you were trying to measure the cathode bias circuit (often in the -1 - 2 kV DC range).

Trying to measure high frequency AC will cause problems, both with the parasitic capacitances of the resistor string and the frequency response of your meter.

In this case you might be forced to measure the DC voltage at the anode cap directly. This requires a much longer resistor chain to avoid voltage breakdown and reduce the load current. It is clearly possible (as it is achieved by commercial products and internet designs) but does require more care in construction and insulation... and more resistors.

In terms of the type, my preference would be for the single resistor (chain) type. Having the 2 resistor (divider) type means that you are losing some current through the bottom resistor, which increases the measuring load... and you still have to compensate it for the meter input resistance anyway.

As I indicated in my 'P.S.' using the meter in current mode is a good option, you no longer have to worry about meter input resistance (it is very low), and you are directly measuring the current through the resistor chain (in uA). It is both accurate and reminds you how much you are loading the voltage source. This is how the self-contained EHT testers with the moving coil meters (used to) work.

Sorry if I misled you a little regarding ease of construction, but 2kV high frequency AC is a very different problem to 2kV DC.
« Last Edit: August 10, 2016, 12:00:17 pm by Gyro »
Best Regards, Chris
 
The following users thanked this post: chihaxinh

Offline chihaxinhTopic starter

  • Regular Contributor
  • *
  • Posts: 126
  • Country: vn
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #4 on: August 10, 2016, 01:00:22 pm »
Many thank Gyro

I'ts my mistake ! So i review schematics it's correct 2kV AC before multiple to 14Kv . And Vk it's -1975V ,a and Vp1 -625V , ok so we can bypass for mearsurement AC High Voltage.

Let's me diy DC HV probe in next time , i need repair focus function first !

Many thank you so much again !

when not sure i don't know ask someone so i ask in forum , Sory for take forum resource
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #5 on: August 10, 2016, 04:50:49 pm »
Hi chihaxinh,

From the circuit it looks as if everything is taken from the same transformer and the regulation (via IC1) is taken from Vk. That means that if Vk is correct then your EHT will be correct to if there were a fault in the multiplier then you would get some indication... probably a nasty smell.

This means that you can still use a 'low kV' probe to check that Vk (DC) is correct.

Please don't apologise for asking questions. If nobody asks questions then nobody gets to see the answers and maybe learn something. Even worse, they may do something dangerous without asking first.  :)

Chris


P.S. For your focus problem, it is worth checking the values of all the high value resistors in the focus circuit because they often drift with age.
« Last Edit: August 10, 2016, 05:18:44 pm by Gyro »
Best Regards, Chris
 

Online David Hess

  • Super Contributor
  • ***
  • Posts: 16549
  • Country: us
  • DavidH
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #6 on: August 11, 2016, 01:29:05 am »
I agree with Gyro, a x10 probe will not work because the probe itself will breakdown somewhere above about 600 DC volts.

Some x100 probes are good to 2000 volts or higher and could be used but you have to do it right.  The probe expects the 1 megohm input resistance of an oscilloscope and your digital multimeter probably has a 10 megohm input resistance.  That is easy enough to solve by placing a 1.11 megohm resistor across the digital multimeter input when using the oscilloscope probe.

Gyro's suggestion of 9 x 10M resistors in series with your multimeter is probably easier than buying a x100 probe and 1.11 megohm resistor unless you already have the x100 probe.  I picked up a Fluke 80k-6 inexpensively on Ebay for making these kinds of measurements.
 

Offline chihaxinhTopic starter

  • Regular Contributor
  • *
  • Posts: 126
  • Country: vn
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #7 on: August 11, 2016, 01:53:43 am »
Many thanks Gyro and David Hess

9x10M Ohm it's correct for Osc probe , but it's correct for DMM ?

I check resistor Voltage Divider need 10x 10Meg series with 10Meg input impedance DMM . You can see picture, if correct i go to market and buy resistor in weekend .

Very helpful if running well with DMM . :D
 

Online David Hess

  • Super Contributor
  • ***
  • Posts: 16549
  • Country: us
  • DavidH
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #8 on: August 11, 2016, 02:27:39 am »
It is 9 x 10 megohm resistors in series for the multimeter and 9 x 1 megohm resistors in series for a 1 megohm oscilloscope input which is what a x10 probe does but with 1 x 9 megohm resistor.

10 resistors in series yield this input current:

Iin = Vin / (10 * R)

Output voltage is input current through one resistor:

Vout = Iin x R

Substituting Iin:

Vout = Vin * ( R / (10 x R) )
Vout = Vin * (1 / 10)
Vout = Vin / 10
 
The following users thanked this post: chihaxinh

Offline chihaxinhTopic starter

  • Regular Contributor
  • *
  • Posts: 126
  • Country: vn
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #9 on: August 11, 2016, 02:39:07 am »
Ah i deeper understand , hehe . 9 x10 M + 10 Meg input impedance = 10 resister series and we mesurement voltage drop in one resistor   ;D

Sory for my concern !   :palm:

 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #10 on: August 11, 2016, 07:57:15 am »
I've just bought two things you might like to have, but no, you can't have them :)

The 40kV meter is conceptually simple: a precision FET amplifier measures the current through a 30Gohm (30000Mohm) resistor and shows the result on a conventional analogue meter. The tricks are in the mechanical construction, i.e. probe handle to keep 40kV away from the operator, ditto flexible probe wire, and preventing breakdown in or around the resistor. The resistor is potted in a block that's about 2cm * 4cm * 6cm.

The 12kV scope probe is also conceptually simple (1000:1 divider plus frequency response compensation), but again the mechanical construction is vitally important. The manuals at http://w140.com/tekwiki/wiki/P6013 have exploded diagrams showing the internal construction.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #11 on: August 11, 2016, 09:46:49 am »
Ah i deeper understand , hehe . 9 x10 M + 10 Meg input impedance = 10 resister series and we mesurement voltage drop in one resistor   ;D

Sory for my concern !   :palm:

Yes, you've got it.  :-+

The 9 x 10M resistors + the meter input resistance will give you a divide by 10 divider, so your meter will display 200V for 2kV input.

If you want to get best accuracy, then measure the actual resistance of each 10M resistor (they might be +/-5% tolerance or more). Then add all your resistance measurements together to see how close it is to 90M total. For example, if the actual total is 89M then your meter display will be 1% high. With up to 5% tolerance resistors you  probably don't need to bother, but if you can only find 10% tolerance ones at the market then becomes more important that you calculate the actual value.

P.S. A trap that many people fall into... Don't hold the 10M resistor leads to the probe tips with your fingers when measuring them. Your body resistance will make the reading wrong.  :D
« Last Edit: August 11, 2016, 09:51:55 am by Gyro »
Best Regards, Chris
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #12 on: August 11, 2016, 10:09:29 am »
The 9 x 10M resistors + the meter input resistance will give you a divide by 10 divider, so your meter will display 200V for 2kV input.

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.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 11632
  • Country: us
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #13 on: August 11, 2016, 11:43:11 am »
Short clip showing an attenuator I made to work with my meter. 


Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #14 on: August 11, 2016, 12:03:22 pm »
The 9 x 10M resistors + the meter input resistance will give you a divide by 10 divider, so your meter will display 200V for 2kV input.

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.

That's why my other suggestion (1st and 3rd replies) was to use the lowest current range and measure the current through 10 x 10M resistors... To make meter resistance an insignificant factor. Assuming a 200uA lowest range 2kV would give 20.0uA (3 significant digits on a 3 1/2 digit DMM).

You're right 10x 10M + 100k forms a 1000:1 divider. Paralleling a 10M input resistance meter across the 100k drops it to 99k which would cause 2kV input would display 1.978V on the meter.

Hopefully the OP has the meter instruction leaflet and it specifies input resistance and tolerance.

Now we've probably really confused him.  :palm:
« Last Edit: August 11, 2016, 12:05:26 pm by Gyro »
Best Regards, Chris
 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 11632
  • Country: us
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #15 on: August 11, 2016, 12:13:10 pm »
Best thing for DC would be to just find a used one in good condition or get a new one.  I'm too cheep for that and attempted to make my own wideband probe.  I use it with both the meters and the scope.   There were 5 parts to the video showing the progress.   These two show the schematics and then the final probe in operation.   May give you some idea what is involved.   




Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #16 on: August 11, 2016, 12:25:09 pm »
@joeqsmith:
The problem is that due to the OP's location, he may not have ready access to used HV probes, he is looking for a quick, cheap and easy way of allowing his meter to read 2kV (Edit: DC now... Vk).

@tggzzz:
What about the following (assuming the 9 x 10M series resistor approach).
- Find a reasonably stable approx 10 volt source.
- Measure it accurately with the DMM on its 10V range.
- Add the probe and measure the resulting divided by 10 voltage.

This is relying on the DMM having the same input resistance on the 100V, 10V and 1V ranges, but not that unreasonable an assumption.
« Last Edit: August 11, 2016, 12:27:23 pm by Gyro »
Best Regards, Chris
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #17 on: August 11, 2016, 02:06:17 pm »
@joeqsmith:
The problem is that due to the OP's location, he may not have ready access to used HV probes, he is looking for a quick, cheap and easy way of allowing his meter to read 2kV (Edit: DC now... Vk).

@tggzzz:
What about the following (assuming the 9 x 10M series resistor approach).
- Find a reasonably stable approx 10 volt source.
- Measure it accurately with the DMM on its 10V range.
- Add the probe and measure the resulting divided by 10 voltage.

This is relying on the DMM having the same input resistance on the 100V, 10V and 1V ranges, but not that unreasonable an assumption.

The uncertainty about that assumption is unnecessary and - so far as I can see - offers no significant advantages.

N.B. some DVMs have input resistances that vary with time, since they are bootstrapped to minimise the input current, and it takes time for the bootstrap to settle. However, that is unlikely to be relevant to the OP's problem.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #18 on: August 11, 2016, 03:25:15 pm »
Quote
N.B. some DVMs have input resistances that vary with time, since they are bootstrapped to minimise the input current, and it takes time for the bootstrap to settle. However, that is unlikely to be relevant to the OP's problem.

Sure, input bias drift is an obvious source of error for a high impedance bench DMMs with bootstrapped inputs (like my old Datrons for instance) but definitely not relevant for a simple handheld DMM like the OP has. His thread title is "Newbie question - Can...". He's looking for simple advice. If we're going to that level then voltage coefficient of the resistors is probably a more relevant error source (that we can't do anything about) we don't need to be worrying him with such esoterics.

Sorry if that came out as a rant, but I think we (as a group) have a tendency to go off into long technical discussions that don't match the specific needs of the questioner.

The OP probably needs, what?, 2-5% measurement accuracy for checking his analogue scope supplies? It's a question of whether a 10:1 divider using the 10M input impedance of his DMM or a 1000:1 divider. He will need to measure the values of the resistors for both to achieve even that level of accuracy though. I have to say, I'm now leaning towards the 1000:1 even though it's a bit more complex to build than a simple series probe, however I think both will do the job. I guess he can test them both with a lower voltage source, either 10V in 1V out or 10V in 10mV out (input impedance on the bottom range is sometimes different to the others on low-end meters though).


@chihaxinh: You should probably purchase at least 10 x 10M resistors (get some spares) and make sure you have some 100ks too. Also see what information the instructions for your DMM have for the input resistance. (There's no point in building something more complicated than you need).
Best Regards, Chris
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #19 on: August 11, 2016, 04:08:25 pm »
When selecting resistors, be very careful that the breakdown voltage is appropriate. Not much use having a 10Mohm resistor with a breakdown voltage of 100v!

Once one resistor breaks down, more stress is put on the others in the chain - making them more likely to break down. And that's a way 2kV could propagate to a DMM and your fingers.

IMHO, 5% (~120V) would be undesirably inaccurate in this application. 1% (24V)would be acceptable, and it is readily achievable.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #20 on: August 11, 2016, 04:51:17 pm »
That's why I specified 1W resistors in Reply #1  ;)

The OP has the service manual, It will tell him the required measurement accuracy.
« Last Edit: August 11, 2016, 04:53:54 pm by Gyro »
Best Regards, Chris
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #21 on: August 11, 2016, 05:28:39 pm »
1W may or may not be sufficient. A quick look at Farnell indicates there are 10Mohm 1W 200V resistors, which is inadequate for the OP's requirement.

If you need to withstand voltage, specify the voltage! Anything else either indicates or engenders dangerous misunderstandings.
« Last Edit: August 11, 2016, 05:30:11 pm by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 9410
  • Country: gb
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #22 on: August 11, 2016, 08:38:50 pm »
Fair point tggzzz, I've never come across a 1W resistor rated that low.

EDIT: What on earth did you find that was that low? Farnell is down for maintenance at the moment but looking at RS, even bog standard 0.25W TE connectivity carbon film are 200V limiting element voltage (400V overload) and 0.5W carbon / 0.6W metal film are 350V element limiting (700V overload). The lowest 1W I found were also 350V (700V overload). That's in line with my expectations.  Are you talking smd or something? I'm really curious about what you found that was that bad? I'd like to know so that I can avoid it!
« Last Edit: August 11, 2016, 09:32:29 pm by Gyro »
Best Regards, Chris
 

Online tggzzz

  • Super Contributor
  • ***
  • Posts: 19281
  • Country: gb
  • Numbers, not adjectives
    • Having fun doing more, with less
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #23 on: August 11, 2016, 10:07:41 pm »
Fair point tggzzz, I've never come across a 1W resistor rated that low.

EDIT: What on earth did you find that was that low? Farnell is down for maintenance at the moment but looking at RS, even bog standard 0.25W TE connectivity carbon film are 200V limiting element voltage (400V overload) and 0.5W carbon / 0.6W metal film are 350V element limiting (700V overload). The lowest 1W I found were also 350V (700V overload). That's in line with my expectations.  Are you talking smd or something? I'm really curious about what you found that was that bad? I'd like to know so that I can avoid it!

Low breakdown voltage => small => SMD. Yes, I realise you wouldn't be so silly as to use one, but ... 😃
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 11632
  • Country: us
Re: Newbie question - Can using osc probe x 10 with dmm for check HV CRT
« Reply #24 on: August 12, 2016, 12:15:07 am »
@joeqsmith:
The problem is that due to the OP's location, he may not have ready access to used HV probes, he is looking for a quick, cheap and easy way of allowing his meter to read 2kV (Edit: DC now... Vk).

@tggzzz:
What about the following (assuming the 9 x 10M series resistor approach).
- Find a reasonably stable approx 10 volt source.
- Measure it accurately with the DMM on its 10V range.
- Add the probe and measure the resulting divided by 10 voltage.

This is relying on the DMM having the same input resistance on the 100V, 10V and 1V ranges, but not that unreasonable an assumption.

Reason for the video showing the attenuator.   


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf