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measuring very high resistance

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blippy

New Member
i have a 1.8 Mega Ohm resitor, my meter can read up to 2 mega ohm, but when i try and read the value with the meter, i get a very inconsistant reading - sometimes as low as 0.8 mega ohm.

Or the reading is usually about 1.78 Mega ohm, but it fluctuates in value - it wont give a steady reading - always fluctuating between 1.7 - 1.9 Mega ohm. can anyone tell me why?

i tried an new battery - not that. It reads perfectly the lower value resistors of around 3k no problem?

I wanted to use a 1.8 mega ohm as a shunt , in parallel, with a very high value resitor to measure the higher values (above 2 megaohm), to save me buying a new meter
 

tresca

Member
From what i can rememeber, when you measure resistance, you are actually measuring your internal resistance (2M) in parrallel with the resistor you are trying to measure.

So, if you have a 1.8M // 2M then the measured resitance is ~947k. Which would account for your 0.8M.

You can not get an accurate reading if the measured resistance is close to the internal resistance.

Maybe try measuring it indirectly ? Have a voltage source in series with the 1.8M and measure current, and apply ohms law ?
 

Sceadwian

Banned
Get a better one? Mine reads up to 20 meg and it's not that expensive.
 

Willbe

New Member
With the resistor in series with 120vac you should read 120/1.8 = 70 uA.

With the resistor in series with 180K and 120v, across the 180K resistor you should read 120[(180/(180 + 1800)] volts.

Doodahmath.
 
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Hero999

Banned
You're not touching the probes of the meter whilst measuring the resistor are you?

That will obviously cause a lower reading.

1.78M is reasonable though considering the tolerance of the resistor and accuracy of the meter.
 

tcmtech

Banned
Most Helpful Member
I agree. I have a high pot tester that uses a 1000 volt testing referance. It can go up to 1500 meg. My beatup old digital meters go up to 20 meg. 1.8 meg is not that much.
 

blippy

New Member
as i mentioned, the idea was to use the 1.8Meg, or similar range, in parallell with an even higher resistance to measure resistors well above the meters range of 2meg- quite high value resistors.

It seems the meter is too inacurate to do it or i need a decent power supply. I tried it with a car battery but it doesnt seem to work.
 

blippy

New Member
i just managed it now - the meter is 2megaohm max, but managed to measure 6.8megaohm, 10megaohm fine. I have an smd resistor that was anonymous and it reads 300megaohm, which seems a bit high for an smd resistor , but i checked it and it seems right.

nice way to save some cash,,using a shunt, instead of buying an expensive meter.
 

Sceadwian

Banned
I checked Digikey real fast, they sell SMD resistors up to 100Gohms. I have no idea what you would use something like that for.
 

blippy

New Member
well, as i said, the reason for it was because i didnt fancy buying another meter. anyway it works, because i tested known values........
 

Hero999

Banned
It sounds like a pretty crappy meter to me.

Generally 1M is the highest value would normally use, up to 10M is still not uncommon but I don't know why one would want a 100GΩ resistor.
 
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blippy

New Member
well i dont know what giga ohms have to do with it, i was talking about mega ohms!

the meter has a max of 2 mega ohms,,but i have a 10 mega ohm resistor,, and managed to get a perfect reading on it, so better than buying a new one for the odd resistor. i have also got accurate readings on very high value insulation resistors.
 

unclejed613

Well-Known Member
Most Helpful Member
one thing that will stabilize your readings at such high resistances is to keep your fingers off of the probes....... :)

my fluke dmm has a conductance range and can measure resistance up to 200MΩ (but the result has to be "inverted", i.e. 1/x)

however if you want to "roll your own" megohm meter, you can build a constant current source and set it to 1µA, 0.1µA, or 0.01µA. then it's as simple as putting the DUT (device under test) in series with the current source and reading the voltage across the DUT.


i used to use very high (10 and 20 MΩ) resistors as standoffs when doing "ugly construction". this is a circuit assembly method where you start with a ground plane and build "up". also known as "dead bug" construction.
 
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Hero999

Banned
well i dont know what giga ohms have to do with it, i was talking about mega ohms!

the meter has a max of 2 mega ohms,,but i have a 10 mega ohm resistor,, and managed to get a perfect reading on it, so better than buying a new one for the odd resistor. i have also got accurate readings on very high value insulation resistors.
What tolerance is your 10M resistor?

If it's quite wide, i.e. 5% then the reading you'll get will be less accurate because of the error the DVM adds.

I would recommend using a 1% tolerance resistor at worst.
 

unclejed613

Well-Known Member
Most Helpful Member
From what i can rememeber, when you measure resistance, you are actually measuring your internal resistance (2M) in parrallel with the resistor you are trying to measure.

So, if you have a 1.8M // 2M then the measured resitance is ~947k. Which would account for your 0.8M.

You can not get an accurate reading if the measured resistance is close to the internal resistance.

Maybe try measuring it indirectly ? Have a voltage source in series with the 1.8M and measure current, and apply ohms law ?
digital meters use the method of placing a constent current source in series with the test probes and reading the resulting voltage. older VOM's used to measure current through a resistor with a known voltage source. the oldest method (and still in use in calibration labs) is to use a resistance bridge where the voltage between 2 legs of a bridge are compared. the bridge is nulled between a calibrated potentiometer and the unknown resistance.
 

lasielle

New Member
Hi all,

for all those wondering about high value resistors, please remember that electronics is a huge field and no-one can be familiar with them all. Just because you have never seen a high meg resistor doesn't mean they aren't vital components to people working in other fields. High value resistors are often used in integrators, pico-ammeters or electrometers. Pico-ammeters commonly use a resistor in the feedback loop of an amplifier to convert the input current into a voltage. 1 pico-amp X 100 Gigohm = 100 millivolts. Electrometers are used in many research labs, and in electro-chemistry. Pico-ammeters are used with ion chambers for measuring various types of ionoising radiation eg. gamma, X-ray, neutrons, beta particle beams Hig meg resistors, some encapsulated in glass tubes to avoid leakage currents from environmental humidity are quite common in nuclear instrumentation and analytical instruments. An ion chamber contains a filling gas, which may be at high or low pressure and a central electrode. When ionising radiation passes through the chamber, it ionises the gas inside (it can also be air) the high voltage across the tube (can be from 30 to 2000 volts) causes the gas ions to drift and hit the wall and electrode. That gives you a minute current to be measured. For example, ion chambers are used in hospitals with linear accelerators that make high energy X-rays for caner treatment. The linacs aren't highly stable and are usually measured each day to ensure the doctors give a patient enough dose to kill the cancer without burning any more of healthy tissue than necessary. High accuracy pico-ammeter is used with various ion chamber to do this. Ion chambers can be tiny, from 1cc (medical) or big, to 2 litres for nuclear research.

The idea of using a 1.8Meg in parallel with higher values is an inventive solution to a problem caused by available equipment limitations. Being able to find solutions to problems when you are doing field repairs is a valuable skill. So long as you remember that your accuracy will be poorer than the basic instrument accuracy, it will work ok. Another solution is to get ten of the same resistor if quantity is available, parallel them and read their value.

Cheers, Colin
 

unclejed613

Well-Known Member
Most Helpful Member
high value resistors are also used as voltage dividers for very high voltages. the focus and screen voltages for a CRT TV or monitor are derived from the anode voltage of the CRT. the voltage divider is usually inside the flyback transformer and is a voltage divider of usually 500Meg or so total resistance. the focus voltage pot is usually 5 or 10meg, and the screen voltage pot usually 2 meg.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
high value resistors are also used as voltage dividers for very high voltages. the focus and screen voltages for a CRT TV or monitor are derived from the anode voltage of the CRT. the voltage divider is usually inside the flyback transformer and is a voltage divider of usually 500Meg or so total resistance. the focus voltage pot is usually 5 or 10meg, and the screen voltage pot usually 2 meg.
This is (or was - as CRT's are dead now) only true for some sets, those with a single EHT winding and a half wave rectifier.

Many sets (probably most?) used either an EHT tripler, or a diode-split transformer, with the focus and A1 voltage taken off the first tap of the tripler.
 
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