Resistance measurements

John-Delusional asshole wrote:

--------------------------------

I'm finding I get different results (vastly different in some cases) when
measuring the total resistance of a circuit with a) a DMM and b) an old
analog meter with a physical needle. And this doesn't only happen at high
impedance points, either. What could account for this?


** The fact you are totally clueless ?

Semiconductors are not resistors and there is no right value to measure.

DMMs are designed NOT to cause didoes or BJT junctions to conduct when using the ohms ranges.

Also, DMMs ohm ranges are very sensitive to any residual DC or AC voltage on a component while analogue meters are much less so.

Interesting fact: you can measure the resistance of a loudspeaker voice coil with either type, but not if the room is full of loud bass noise.

Think about it.




Come on Phil, lighten up.
Click to expand...

** Go fuck yourself, you vile scumbag.


The guy didn't know and asked the question.
Click to expand...


** Chris is a trolling moron, he got a way better answer than he deserved.

Nothing a retarded pig like you could ever appreciate.

Even something as simple as a light bulb baffles you.




..... Phil
 
Chris the retard troll wrote

------------------------------

It's no problem. I plonked Phil some time ago on the advice of other
posters here
Click to expand...

** That was another of your dumb mistakes.

Dime a dozen assholes like you make nothing else.



...... Phil
 
On Monday, July 17, 2017 at 5:41:43 PM UTC-4, Phil Allison wrote:

Even something as simple as a light bulb baffles you.




.... Phil
Click to expand...

Whooo! You're right about that. Especially them in-can-des-cent ones!! How does putting current through them there things produce both light *and* heat? Amazing!!!

When you get done bombing your trailer for your roach infestation can you favor us with one of your brilliant technical treatises on light bulbs please?
 
On Monday, 17 July 2017 19:36:52 UTC+1, Cursitor Doom wrote:
On Mon, 17 Jul 2017 07:59:41 -0700, Jeff Liebermann wrote:

Not high enough. If you want to measure really high resistances, such
as insulation leakage, you need a Megger (which is actually the name of
the company that makes them but has become somewhat of a generic term
for high voltage resistance testers):
https://www.google.com/search?q=megger+meter&tbm=isch
If you want to see if you really have water in the coax cables, you need
one of these insulation testers.

Yeah, I have one. They can still be used perfectly servicably if they're
within spec. The people at Megger tell me that every so often, an old
hand-crank version from the 50s or 60s will come in for re-calibration!
Click to expand...

How can they be calibrated when the voltage produced is so unsteady? Or do they have some form of regulation? I've used very basic cranked meggers, not those old ones, and the readings were wobbly.


NT
 
In article <4358cf68-083d-4f0e-b643-eaecd8d6b96a@googlegroups.com>,
tabbypurr@gmail.com says...
On Monday, 17 July 2017 19:36:52 UTC+1, Cursitor Doom wrote:
On Mon, 17 Jul 2017 07:59:41 -0700, Jeff Liebermann wrote:

hand-crank version from the 50s or 60s will come in for re-
calibration!

How can they be calibrated when the voltage produced is so unsteady? Or do they have some form of regulation? I've used very basic cranked meggers, not those old ones, and the readings were wobbly.
Click to expand...

The only one I ever used had a slip clutch that you cranked it so fast
and it would slip at that point to keep the speed sort of the same.
They may have the coils wound in certain ways to help keep the internal
voltages porportional to each other. They are not very accurate, nor do
they need to be at that high of resistance.
 
On Mon, 17 Jul 2017 16:50:18 -0700, tabbypurr wrote:

How can they be calibrated when the voltage produced is so unsteady? Or
do they have some form of regulation? I've used very basic cranked
meggers, not those old ones, and the readings were wobbly.


NT
Click to expand...

They specify how many RPM you should turn the handle at in the
instructions. Presumably they then have a motor to drive the thing at
that speed in the factory for calibration purposes.
In practice, the output gets progressively less 'wobbly' the faster you
go and they're intended to be spun-up pretty fast. Fuck knows how they
expect you to judge the specified RPM when taking measurements, though!
 
On Tuesday, 18 July 2017 22:26:19 UTC+1, Cursitor Doom wrote:
On Mon, 17 Jul 2017 16:50:18 -0700, tabbypurr wrote:

How can they be calibrated when the voltage produced is so unsteady? Or
do they have some form of regulation? I've used very basic cranked
meggers, not those old ones, and the readings were wobbly.

They specify how many RPM you should turn the handle at in the
instructions. Presumably they then have a motor to drive the thing at
that speed in the factory for calibration purposes.
In practice, the output gets progressively less 'wobbly' the faster you
go and they're intended to be spun-up pretty fast. Fuck knows how they
expect you to judge the specified RPM when taking measurements, though!
Click to expand...

Even at constant speed it's going at differing speeds at different parts of the turning cycle. The ones I used had very low handle speed and were far from steady. But they picked up bad insulation ok.


NT
 
On Tue, 18 Jul 2017 15:05:38 -0700, tabbypurr wrote:

Even at constant speed it's going at differing speeds at different parts
of the turning cycle. The ones I used had very low handle speed and were
far from steady. But they picked up bad insulation ok.
Click to expand...

Yup, and as Ralph has already said, they're really not designed for
precision accuracy. They tell you only what you really *need* to know and
no more and since they are like 60yr old technology, it's not fair to
compare them to the currently available stuff anyway.
 
On Tue, 18 Jul 2017 15:05:38 -0700 (PDT), tabbypurr@gmail.com wrote:

Even at constant speed it's going at differing speeds at different
parts of the turning cycle. The ones I used had very low handle speed
and were far from steady. But they picked up bad insulation ok.
NT
Click to expand...

Same here. Mine gives a reading, but varies somewhat with the crank
speed:
<http://www.11junk.com/jeffl/pics/Megger/slides/Megger-Major.html>
There's also a leather case and a tangle of wire leads. I just took
it for a spin with a handy 22M resistor. Something is wrong as I'm
getting very unstable readings. That's not surprising since the
insides are filthy. I've been afraid to clean it lest the insulation
crumbles.

There are some videos on YouTube mostly showing how to use a Megger.
<https://www.youtube.com/results?search_query=megger>

The technology is quite old and rather crude by todays standards, but
still useful for what it was originally intended (measuring high
resistances).

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
On Wed, 19 Jul 2017 00:02:50 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:
(...)

"What not to do with a Megger"
<https://www.youtube.com/watch?v=riYEUWa8trc>


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
oldschool@tubes.com schrieb:

[...]
I do have to ask, why these meters used BOTH the D cells and other
batteries as well. (In other words, more than one kind of battery). Why
didn't they just use one battery or one set of batteries for the whole
device?
Click to expand...

It's for measurement of high resistance values.

Sensitivity of the movement in a 20 kOhms/V AMM is 50 ľA. Current
through a 1.5 MOhm resistor will be 1 ľA with one AA cell (1.5 V):
hard to read on the scale.

With a 15 V battery there will be 10 ľA ...

HTH

Reinhard
 
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