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Old 11-04-2019, 06:34 PM   #1
johnfin
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Default High resistance measurement

Any old school way i can check a 600meg ohm resistor. Maybe like putting a frequency through it and using a scope?
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Old 11-04-2019, 06:57 PM   #2
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Default Re: High resistance measurement

just do the search for 'how to measure very high resistance' You will see many ways of testing but I am not sure if you do have the instrument at home to accomplish the task or not.
https://www.youtube.com/watch?v=kk--xLxP95c
https://www.youtube.com/watch?v=NPx7-60HVr0
https://www.scientific-devices.com.a...easurement.pdf
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Old 11-06-2019, 11:36 AM   #3
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Default Re: High resistance measurement

Good advice, buy a tester. A guy like edison would home brew a test. They didnt have digital meters 100 years ago.
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Old 11-06-2019, 11:40 AM   #4
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Default Re: High resistance measurement

Use ohms law to figure out how much voltage you would need to get a measureable amount of current. You would likely need over 5KV.

Last edited by R_J; 11-06-2019 at 11:46 AM..
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Old 11-06-2019, 12:14 PM   #5
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Default Re: High resistance measurement

Once you understand the concept so you will understand what are needed to accomplish what you are trying to do that is why I provide the links, then you either buy the tester or just have to brew up power supply to force enough current for your instrument to be able to read accurately, I.E. in one of the example the current is in PICO amp range (if you do not have high Voltage source) which I doubt that your meter can do.
Or try reading the Vdrops of the meter: http://www.giangrandi.ch/electronics.../gigaohm.shtml
600M Ohms will need high Voltage source. Do you have that?
Then you also need to know what the breakdown Voltage of the resistor is that you are trying test.

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Old 11-06-2019, 12:23 PM   #6
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Default Re: High resistance measurement

there's a choice of either a decent amount of current (which means you'll need to deal with high voltage) or deal with tiny currents (which means you'll need to deal with high noise)... In the olden ages high voltage is the only way to go. In modern electronics, it's now possible to work with tiny currents.

Your scope is completely unsuited for the task unless it's a CRT type and you want to use the second anode voltage ... which hopefully won't do dielectric breakdown...

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Old 11-06-2019, 02:46 PM   #7
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Default Re: High resistance measurement

I just use a multimeter with known input resistance, say 20MEG.
Set a bench power supply as high as I got, say 40VDC and measure the voltage. Then use the voltage divider equation to figure out the resistor's value.
A multimeter reading of 1.290V for 600MEG in series with 20MEG DMM.
Using a 9V battery, would read 0.290V for 600MEG, but susceptible to AC hum and interference, so you need a shielded, grounded work area.
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Old 11-06-2019, 04:07 PM   #8
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Default Re: High resistance measurement

Quote:
Originally Posted by budm View Post
Once you understand the concept so you will understand what are needed to accomplish what you are trying to do that is why I provide the links, then you either buy the tester or just have to brew up power supply to force enough current for your instrument to be able to read accurately, I.E. in one of the example the current is in PICO amp range (if you do not have high Voltage source) which I doubt that your meter can do.
Or try reading the Vdrops of the meter: http://www.giangrandi.ch/electronics.../gigaohm.shtml
600M Ohms will need high Voltage source. Do you have that?
Then you also need to know what the breakdown Voltage of the resistor is that you are trying test.
When you're dealing with impedances this high, the "instrument" becomes part of the measurement -- it can no longer be considered as an approximation of an ideal meter (infinite input impedance). Can you characterize the input of your instrument to ensure it is >> the impedance you're trying to measure? (Or, at least KNOWN?!)

E.g., in theory, creating a divider with another resistance IDENTICAL to the one you are measuring should yield a voltage of 50.0000% of the supply impressed. Yet, if you measure between that point and ground you'll get less than half the impressed voltage. If you then measure between that point and supply, you'll also get less than half the impressed voltage. And, if you add these two measurements, you'll scratch your head wondering where the "missing voltage" went!

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Old 11-06-2019, 04:19 PM   #9
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Default Re: High resistance measurement

In that link, the Author claims the DMM has 10M input Z, so you need to know your true spec of your instrument, not just going by what it is claimed to be, so that is another issue. Op is trying to do the so called old fashion way so all the variable must be known and accounted for.
The meter Z will of course will affect the reading I.E. you have two 10M resistors in series, when you put your meter across that 10M resistor, the Vdrops on that resistor will no longer be at 50%.
When I was working on the tube stuff in the old day, I have to keep in mind what my 25K/Volt meter will affect the circuits.

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Old 11-06-2019, 04:20 PM   #10
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Default Re: High resistance measurement

Quote:
Originally Posted by redwire View Post
I just use a multimeter with known input resistance, say 20MEG.
Set a bench power supply as high as I got, say 40VDC and measure the voltage. Then use the voltage divider equation to figure out the resistor's value.
A multimeter reading of 1.290V for 600MEG in series with 20MEG DMM.
Using a 9V battery, would read 0.290V for 600MEG, but susceptible to AC hum and interference, so you need a shielded, grounded work area.
Are you saying that when you measure the Vdrops across the 600M Ohns resistor with your DMM, it shows 1.290V?
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Old 11-06-2019, 05:01 PM   #11
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Default Re: High resistance measurement

Actually I think this is the way that's being described:

Equipment needed:
- 10MΩ impedance or so DMM that can also measure millivolts.
If you don't know the exact impedance of the DMM, you'll also need a resistor of known value, say 10MΩ.
- low impedance voltage source that does not drift within the time of your testing. Usually a good battery is good enough for government work. You may need multiple voltages if you go offscale.
- Calculator, unless you can do multiplication and long division with a writing implement and a pad of paper.

Step 1:
Measure voltage of your voltage source using the multimeter. Record that.
Say you're using a 9V battery, you will record 9.00V.

Step 1a:
If you don't know the impedance of your multimeter, now without changing the range, measure the voltage with the 10MΩ resistor in series with the voltage source. (If you trust the ohm scale of your meter, you can use the ohm range to measure this 10MΩ resistance just to be sure, if it's not 10MΩ then substitute it in.)

9V+ ---- [your resistor in both cases] ---- [ + DMM - ] ---- 9V-

In all cases you are measuring the voltage drop across your DMM's internal impedance. In no case are you measuring across the external resistance.

Now with the resistor in series, the voltage will be lower. Say you read 4.50 volts due to the resistor voltage divider made with the resistor and your meter. You can calculate the impedance of the multimeter by

4.5 volts [measured]=9 Volts [your battery you measured in step 1] * x Ω / (x + 10MΩ [your known resistor] ) - solve for x which is your meter impedance in that mode.

In this case X will calculate out to 10MΩ.

Note that multimeters may have different impedances for different voltage ranges. You may need to do this measurement on different ranges if you have to change ranges. Note that you will have a problem - if you overload a range, you can't use that range.

Also note that multimeters have different input impedances. There has been a standard of DMMs for 10MΩ but my Fluke is 11MΩ and I have a HFT DMM that measures 9MΩ ... I used to have a VTVM and it measured 11MΩ which is probably why the Fluke is 11MΩ.

I also have a JFET input Tektronix DMM that I leave in 10MΩ mode. If I set it into JFET input mode, this will result in the meter itself becoming a problem in measuring its input impedance as it can be damaged with high voltages.

Step 2: Measure the voltage with the unknown resistance in series with the multimeter, record the voltage. Say you get 148mV. Remember do not change range, unless you calculated the impedance of that range as well. If you get 0 volts you will need a higher low impedance voltage source and go back to step 1. A possibility is two 9V's or more in series.

Step 3: Calculate your unknown resistance.

0.148 V [your reading] = 9.00 V [your battery you measured in step 1] * 10MΩ [your meter impedance in the range you're using] / ( x [your unknown resistance] + 10MΩ [your meter resistance again] )

Once again solve for x. x would turn out to be 607108108 which is the calculated ohmage of the unknown resistance.

Without a sensitive multimeter that measures in the millivolts in this case, you would need a higher voltage than 9V. 148mV is tough to measure accurately with a 20KΩ/volt analog multimeter but may be close enough.

Once again do not use autorange if your meter supports it.

Note that the higher the unknown resistance, the more jitter you will get in your voltage readings. This jitter is noise and you may need to shield or especially stop touching the resistor and leads to get a stable reading.

Last edited by eccerr0r; 11-06-2019 at 05:22 PM..
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Old 11-06-2019, 07:43 PM   #12
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Default Re: High resistance measurement

Quote:
Originally Posted by budm View Post
In that link, the Author claims the DMM has 10M input Z, so you need to know your true spec of your instrument, not just going by what it is claimed to be, so that is another issue. Op is trying to do the so called old fashion way so all the variable must be known and accounted for.
The meter Z will of course will affect the reading I.E. you have two 10M resistors in series, when you put your meter across that 10M resistor, the Vdrops on that resistor will no longer be at 50%.
Exactly. But, if you know (for sure) one of the resistor values, you can make OTHER measurements to determine the effect of the meter on the circuit. I.e., get a set of linear equations that you can then solve for the TWO unknowns -- the "big resistor" and the "input impedance".
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Old 11-06-2019, 09:07 PM   #13
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Default Re: High resistance measurement

Quote:
Originally Posted by budm View Post
Are you saying that when you measure the Vdrops across the 600M Ohms resistor with your DMM, it shows 1.290V?
No, the DMM is in series with the unknown resistor. This is the test circuit and the math formula, changed for a DMM with 10MEG input resistance on DCV.

I'd tried putting the DMM on current range instead, but 40V into 600MEG is only 0.0667uA and too low a reading.
I've measured up to 5GIG resistors this way, testing a high voltage probe because the resistors got old and drifted.
It's because a DMM can measure mV accurately that this method works, although AC hum and noise needs to be kept to a minimum.
Attached Images
File Type: png Measure high value R with DMM.PNG (47.2 KB, 4 views)

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Old 11-06-2019, 10:02 PM   #14
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Default Re: High resistance measurement

Quote:
Originally Posted by Curious.George View Post
Exactly. But, if you know (for sure) one of the resistor values, you can make OTHER measurements to determine the effect of the meter on the circuit. I.e., get a set of linear equations that you can then solve for the TWO unknowns -- the "big resistor" and the "input impedance".
So what is the issue? I am not scratching my head.
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Old 11-06-2019, 10:18 PM   #15
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Default Re: High resistance measurement

Quote:
Originally Posted by budm View Post
So what is the issue? I am not scratching my head.
  • The meter is part of the circuit and has an effect on the measurement.
  • The characteristics of the meter are (typically) only NOMINALLY known (what's its tolerance? does it vary with temperature? frequency? etc.).
  • Awareness of both of the above is sufficient to allow someone to remove the effect of the meter from the measurement and get at the actual value of interest.

The same sort of reasoning applies to other measuring devices (e.g., 'scope probes)

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Old 11-06-2019, 10:25 PM   #16
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Default Re: High resistance measurement

So? I worked for 2 years for Fairchild semiconductor calibration lab, not new to me. Tell that to OP and how to accomplish the task OLD SCHOOL WAY, that is what OP wants to hear..OP wants to be like Edison.

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Old 11-06-2019, 11:48 PM   #17
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Default Re: High resistance measurement

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So? I worked for 2 years for Fairchild semiconductor calibration lab, not new to me. Tell that to OP and how to accomplish the task OLD SCHOOL WAY, that is what OP wants to hear..OP wants to be like Edison.
Did I say it was "new to you"? :>

As to the OPs motivation... <shrug> Maybe he's limited to the tools he has available?

These are the sorts of questions I use to quiz job applicants (techs and engineers). It is discouraging how many fail to understand the role the non-ideal instrument plays in the measurement's accuracy. Even more amusing to watch them MAKE the measurement(s) in the lab!

And, how many more can't sort out how to resolve these issues without thinking that they have to resort to "yet another (better!) instrument".

Amusingly, more engineer candidates fail -- likely due to Dunning-Kruger. At least some technician candidates see the "trick" in the question -- usually, those who have "been around for a while" and/or been exposed to a variety of electronic designs. (in their defense, engineering candidates often aren't tasked with troubleshooting many different types of circuits so have very little first-hand experience)
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Old 11-08-2019, 01:30 AM   #18
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Default Re: High resistance measurement

I take my comment back about the oscilloscope, there is a possibility you could use an oscilloscope in the normal sense. The 1MΩ input impedance would be something that you'd need to take into account, and likely you'd use probe/scope/jig capacitance as when dealing with almost 1GΩ, to keep times reasonable you'd probably need to work with picofarads that scope already provides. This unfortunately would be very error prone, plus you'd still need a known resistor or other way to calibrate your scope with its probe for this measurement. Your scope will need to have some sort of time measurement capability as well. Then you could use the frequency generator, or rather use it as a pulse generator, to "measure" or rather estimate resistance by finding the RC delay.

You could also build an oscillator with the resistance, and use the scope to measure frequency, once again you'd need to be pin the nano/picofarads to make reasonable times to measure.

I wouldn't call these "Edison" methods, but rather jury rigging an nonideal solution that's completely inaccurate if you have a multimeter at your disposal...
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