Resistance measurements

[Cursitor Doom]

On Sun, 16 Jul 2017 16:34:56 -0700, tabbypurr wrote:

voltmeters don't output anything. They should be fine measuring solid
state.

DMMs and VOMs give different readings in certain circumstances (see
Ralph's post). In GB (at least) for a long time the service manuals for
decent gear would provide expected voltage readings for 20k OpV analogue
meters, the AV0 model 8 specifically IIRC, which saved the technician the
extra bother of scratching his head when checking hi-z parts of a circuit.

 

[Phil Allison]

Chris 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.


....... Phil

 

[Ralph Mowery]

In article <okgudk$djj$40@dont-email.me>, curd@notformail.com says...

DMMs and VOMs give different readings in certain circumstances (see
Ralph's post). In GB (at least) for a long time the service manuals for
decent gear would provide expected voltage readings for 20k OpV analogue
meters, the AV0 model 8 specifically IIRC, which saved the technician the
extra bother of scratching his head when checking hi-z parts of a circuit.

Yes, I have seen some schematics specify the voltage was measured with a
certain ohms/volt meter. It does take some guess work or head
scratching out of the servicing. Makes a big difference when measuring
the screen votltage that is fed with a high value resistor and you use a
vtvm or a 20 k ohms/volt or a 5 k ohms /volt meter.

I don't think I have done any trouble shooting on a tube circuit in 20
or 30 years. Last thing I recall even looking at with tubes was my
Heathkit sb101 that was many years ago. Only other thing I have with
tubes are a couple of RF amplifiers and they use voltages that I don't
work on with any power on them.

 

On Sun, 16 Jul 2017 23:12:42 -0500, Jon Elson <elson@pico-systems.com>
wrote:

Chris 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?

I've got four DMMs and two analogs. The DMMs agree with the other DMMs
and the analogs agree with each other. But the different types don't
agree with each other!
I have some older DMMs. I get odd resistance readings the first time I set
it to Ohms scale. By wiggling the plugs on the probes at the meter end, and
occasionally working the range selector dial around a few times, I get a
stable Ohms reading of about 0.4 Ohms with the probes shorted. Then, I get
more sensible readings on circuits. So, these meters get poor contact on
the range selector switch and the bannana jacks. So, that is one thing to
check for.

Second, most DMMs use very low voltage to meke Ohms measurements. Analog
meters often used 9 or even 22 V batteries for the Ohms range, to push
enough current to move the needle on high resistance circuits.

If there are any seminconductors in your circuit, a DMM likely will not give
enough voltage to forward bias any junctions, while an analog meter will.

Jon

Sounds to me like you need to spray some contact cleaner in your test
meter switches, and clean the plug contacts too.

 

On Sun, 16 Jul 2017 16:30:03 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Sun, 16 Jul 2017 15:46:36 -0400, oldschool@tubes.com wrote:

Ok, I guess you have meters that I've never seen.

You haven't seen any of these?
http://www.simpson260.com
https://www.google.com/search?tbm=isch&q=simpson+260
These were probably the most common VOM's available. At one time, I
had about 5 of them. They've been replaced by more modern meters, but
I still keep one of them on the shelf, just in case.

Yep, I've seen several of them models. I just never owned any of them. I
generally used one of my VTVMs (Heathkit or Eico) for most in circuit
testing on tube gear. I have always been pretty satisfied with my older
Radio Shack VOMs. I blew up a few of them many years ago, but I learned
what NOT to do, and to pay attention and make sure I dont have the meter
on the ohms scale when I measure voltages.

I also have a GB Instruments model GMT-19A VOM sitting right next to me,
that I use for darn near everything, and it's held up well for many
years.

I am not real fond of digital meters. I find them confusing, since they
tend to pick up stray voltages and they also take awhile to "settle" on
a reading. I have some of them, and I use them for some things, but my
analog meter are usually the first ones I grab.

Seeing those Simpson meters makes me want to look for one to buy though.
Just seems there should be one on my bench.......

 

[Jon Elson]

Chris 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?

I've got four DMMs and two analogs. The DMMs agree with the other DMMs
and the analogs agree with each other. But the different types don't
agree with each other!

I have some older DMMs. I get odd resistance readings the first time I set
it to Ohms scale. By wiggling the plugs on the probes at the meter end, and
occasionally working the range selector dial around a few times, I get a
stable Ohms reading of about 0.4 Ohms with the probes shorted. Then, I get
more sensible readings on circuits. So, these meters get poor contact on
the range selector switch and the bannana jacks. So, that is one thing to
check for.

Second, most DMMs use very low voltage to meke Ohms measurements. Analog
meters often used 9 or even 22 V batteries for the Ohms range, to push
enough current to move the needle on high resistance circuits.

If there are any seminconductors in your circuit, a DMM likely will not give
enough voltage to forward bias any junctions, while an analog meter will.

Jon

 

[Phil Allison]

Jon Elson wrote:

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

I have some older DMMs. I get odd resistance readings the first time I set
it to Ohms scale. By wiggling the plugs on the probes at the meter end, and
occasionally working the range selector dial around a few times, I get a
stable Ohms reading of about 0.4 Ohms with the probes shorted. Then, I get
more sensible readings on circuits. So, these meters get poor contact on
the range selector switch and the bannana jacks. So, that is one thing to
check for.


** A little WD40 or similar will fix that in seconds.

A short squirt under the edge of the switch followed by a few complete rotations is all you do. Clean up any excess.

Wet a cloth to do the 4mm plugs and a cotton bud to do the sockets.


...... Phil

 

[Mike Coon]

In article <okgudk$djj$40@dont-email.me>, curd@notformail.com says...

On Sun, 16 Jul 2017 16:34:56 -0700, tabbypurr wrote:

voltmeters don't output anything. They should be fine measuring solid
state.

DMMs and VOMs give different readings in certain circumstances (see
Ralph's post). In GB (at least) for a long time the service manuals for
decent gear would provide expected voltage readings for 20k OpV analogue
meters, the AV0 model 8 specifically IIRC, which saved the technician the
extra bother of scratching his head when checking hi-z parts of a circuit.

For amusement I fetched the Avo Model 7 that I inherited from my father
(goodness knows when he got it!). The plate on the back specifies 1.5V
cell for < 1M, 2x4.5 for 1M, and external voltage sources for 10m and
40M ranges.

It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!

Mike.

 

[Cursitor Doom]

On Mon, 17 Jul 2017 09:08:01 +0100, Mike Coon wrote:

> It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!

Yes, and built like a tank! I have a Model 7, too, IIRC it was made in
1943 and is still going strong. I'll wager when it was first produced,
techs of the day marvelled at how light and portable it was.

 

On Sun, 16 Jul 2017 21:24:23 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 16 Jul 2017 15:46:36 -0400, oldschool wrote:

Ok, I guess you have meters that I've never seen.

You've never seen any AVO/Megger meters?? Like this for example:

http://www.ebay.co.uk/itm/Bakelite-AVO-Model8-MK5-Multi-Meter-Excellent-
Condition-100-Tested-/152599935777?hash=item2387aa3b21:g:1csAAOSw241YZTX1

They've been exported all over the world for decades!

They typically use one 15V battery (about the size of a AA cell) and a
single 1.5V D cell.

This is one I've never seen. I have seen Simpson and Triplett vom's but
not this one.

I dont think I'd want any of these that need these special batteries. I
have looked at some of the specs for the older Simpson 260 meters and
some of the early models had oddball batteries too. The series 5 has D
and AA batteries, and the series 6 an above have D and 9V.

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?

My Radio Shack and GB meters only have 2 AA cells, and work fine.

 

On Monday, 17 July 2017 09:31:17 UTC+1, Cursitor Doom wrote:

On Mon, 17 Jul 2017 09:08:01 +0100, Mike Coon wrote:

It also weighs 5lb (2+kg). I guess in those days servicemen were MEN!

Yes, and built like a tank! I have a Model 7, too, IIRC it was made in
1943 and is still going strong. I'll wager when it was first produced,
techs of the day marvelled at how light and portable it was.

My 1920s v/i meter weighs a small fraction of that. Avos were high impedance accurate bench meters.


NT

 

[Cursitor Doom]

On Mon, 17 Jul 2017 02:50:34 -0700, tabbypurr wrote:

My 1920s v/i meter weighs a small fraction of that. Avos were high
impedance accurate bench meters.

"High impedance" back then, yeah.

 

[Cursitor Doom]

On Mon, 17 Jul 2017 05:00:55 -0400, oldschool wrote:

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?

Dunno. I'm sure someone here will, though.
Fortunately, although 15V batteries are largely unobtainium these days,
10x1.5V AAA cells will get you there and the battery compartment is
capacious enough (AVOs are anyway) to house them all comfortably.

 

On Monday, 17 July 2017 11:21:54 UTC+1, Cursitor Doom wrote:

On Mon, 17 Jul 2017 02:50:34 -0700, tabbypurr wrote:

My 1920s v/i meter weighs a small fraction of that. Avos were high
impedance accurate bench meters.

"High impedance" back then, yeah.

They were, you could always get lower resistance meters for a lot less. Why it took so long for analogue meters to get FETs I don't know. My 1920s meter is moving iron, so the resistance is dreadful & it's nonlinear. But its worst shortcoming is that the case is the -ve electrode, you hold it in your hand and it's bare metal. AND it's low resistance, so measuring radio/TV HT was always a fun experience. Maybe they figured if they killed their customers there wouldn't be any requests for refund.


NT

 

On Monday, 17 July 2017 11:05:20 UTC+1, olds...@tubes.com wrote:

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?

A 1.5v cell is high capacity, high current & cheap. 15v batteries are low capacity, low current & never cheap.


NT

 

[Jeff Liebermann]

On Mon, 17 Jul 2017 05:00:55 -0400, oldschool@tubes.com wrote:

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?

The Rx10K scale needs high voltage and low current to measure large
resistors. The Rx1 scale needs low voltage and high current to
measure the small resistors. One could use the high voltage 15v
battery for measuring low resistances, but you would be replacing 15v
batteries at an a alarming rate. Be thankful that the ancient
designers of the VOM did not include a different battery for each
resistance scale.

--
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

 

[Jeff Liebermann]

On Mon, 17 Jul 2017 10:17:54 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Mon, 17 Jul 2017 02:50:34 -0700, tabbypurr wrote:

My 1920s v/i meter weighs a small fraction of that. Avos were high
impedance accurate bench meters.

"High impedance" back then, yeah.

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.

I have an old and ugly meter, which has a hand crank generator. It
produces enough voltage to have given me a rather nasty shock. It's
fairly difficult to electrocute oneself while operating the crank, but
I managed. Some modern Megger models still have such hand cranks
generators:
<http://www.tequipment.net/Megger212160.html>
<http://us.megger.com/extended-range-insulation-resistance-testers-210170-and-210600->
These small testers will deliver 1000V in order to measure up to 2000
Mohms. Now, does anyone still want to complain about 15v batteries?

--
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

 

[John-Del]

On Sunday, July 16, 2017 at 8:36:16 PM UTC-4, Phil Allison wrote:

Chris 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.


...... Phil

Come on Phil, lighten up. The guy didn't know and asked the question. We aren't all born full of knowledge; it's acquired by experience and asking questions.

Remember when you asked your mother why you have no friends, and she told you that you're obnoxious, pig headed and your feet smell like a fetid swamp? Same thing..

 

[Cursitor Doom]

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!
The one I have is the 250V model which is relatively unusual here in the
UK as almost all of them here are 500V. We would typically use the 500V
version for testing our 240V domestic wiring. I guess the 250V model was
intended for export to countries which use 110/120V.

The current range of Meggers are quite expensive, IRO $1200 but at least
you don't have to crank them any more.

 

[Chris]

On Mon, 17 Jul 2017 09:47:17 -0700, John-Del wrote:

Come on Phil, lighten up. The guy didn't know and asked the question.
We aren't all born full of knowledge; it's acquired by experience and
asking questions.

It's no problem. I plonked Phil some time ago on the advice of other
posters here so I rarely get to see any of his unfortunate, socially-
embarrassing outbursts. ;-)

Remember when you asked your mother why you have no friends, and she
told you that you're obnoxious, pig headed and your feet smell like a
fetid swamp? Same thing..

LOL!!! Most amusing. >:*D