Resistance measurements

C

Chris

Guest
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!
 
On Sunday, 16 July 2017 14:33:04 UTC+1, 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!

Are they calibrated?


NT
 
On Sunday, July 16, 2017 at 9:33:04 AM UTC-4, 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!


Do your DMMs and analogues agree with each other when using fixed resistors out of circuit?
 
On Sun, 16 Jul 2017 07:01:34 -0700, John-Del wrote:

Do your DMMs and analogues agree with each other when using fixed
resistors out of circuit?

Yes they do. Sorry, should have mentioned that in the OP.
 
In article <okfpn3$djj$10@dont-email.me>, cbx@noreply.com says...
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!

If you are measuring in circuit resistance of solid state circuits, then
you often will.

The analog meter has enough voltage/current to turn on the junctions of
diodes and transistors. The DMM will not have enough to turn them on.

If you have a diode out of the circut and use an analog meter you will
often see a small resistance in one direction and if you reverse the
leads a high resistance. The DMM will usually show a high resistance in
both directions unless you use the diode setting if the dmm has one.
 
In article <okfs1a$djj$11@dont-email.me>, cbx@noreply.com says...
On Sun, 16 Jul 2017 07:01:34 -0700, John-Del wrote:

Do your DMMs and analogues agree with each other when using fixed
resistors out of circuit?

Yes they do. Sorry, should have mentioned that in the OP.

Yes, and have reversed the leads as Ralph suggests...

Mike.
 
On Sun, 16 Jul 2017 10:41:02 -0400, Ralph Mowery wrote:

The analog meter has enough voltage/current to turn on the junctions of
diodes and transistors. The DMM will not have enough to turn them on.

If you have a diode out of the circut and use an analog meter you will
often see a small resistance in one direction and if you reverse the
leads a high resistance. The DMM will usually show a high resistance in
both directions unless you use the diode setting if the dmm has one.

Damnit, Ralph! You beat me to it. I was going to say that. ;-)
I have a couple of analogue meters too. They test for resistance at 15V
which is more than enough to turn on those semiconductors, but also more
than enough to destroy a lot of chips that can't tolerate much more than
5V. Horses for courses/different strokes and all that.
 
On 16/07/2017 14:29, 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!

Try with different probe leads and resistors with different termination
metals, maybe small dissimilar metals producing voltages that are
interpreted differently by the different metering systems
 
On Sun, 16 Jul 2017 16:36:56 +0100, N_Cook wrote:

Try with different probe leads and resistors with different termination
metals, maybe small dissimilar metals producing voltages that are
interpreted differently by the different metering systems

It's okay now I believe Ralph has nailed the problem entirely in his
post. (thanks, Ralph).
 
On Sun, 16 Jul 2017 13:29:07 -0000 (UTC), Chris <cbx@noreply.com>
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?

If you're doing in circuit resistance measurements, you're probably
measuring the resistance of a non-linear device such as a transistor
or diode. These will show different resistances at different applied
voltages. Umm... this assumes that you've unplugged the circuit that
you're testing and have discharged any BFC's (big fat caps).

Disconnect whatever you're measuring. Take one of the DVM's that has
the highest input resistance, set it to VOLTS, and measure the VOLTAGE
across the leads of the other meters. You'll find quite a bit of
variation. My guess(tm) is that the meter with the highest voltage,
will read the lowest resistance.

If you have an ESR (equivalent series resistance) meter, you can do in
circuit low resistance measurements without worrying much about the
effects of semiconductors. That's because the voltages involved are
so small, that the semiconductor doesn't even being to conduct, and is
therefore essentially out of the circuit.

There's really no way to "fix" the problem of measuring in circuit
resistances. If I want to accurately measure a resistor that's in a
circuit, I usually have lift one lead, and measure only that resistor.
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!

The analog meters (VOM) require more current in order to obtain a
resistance reading. More current means more applied voltage across
the leads, which means that the semiconductors in your test circuit
are well into conduction. Try measuring a resistor and diode in
parallel and you'll see the problem in action.

--
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 Sun, 16 Jul 2017 15:05:34 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Sun, 16 Jul 2017 10:41:02 -0400, Ralph Mowery wrote:

The analog meter has enough voltage/current to turn on the junctions of
diodes and transistors. The DMM will not have enough to turn them on.

If you have a diode out of the circut and use an analog meter you will
often see a small resistance in one direction and if you reverse the
leads a high resistance. The DMM will usually show a high resistance in
both directions unless you use the diode setting if the dmm has one.

Damnit, Ralph! You beat me to it. I was going to say that. ;-)
I have a couple of analogue meters too. They test for resistance at 15V
which is more than enough to turn on those semiconductors, but also more
than enough to destroy a lot of chips that can't tolerate much more than
5V. Horses for courses/different strokes and all that.

I have heard this several times about analog meters destroying chips,
because the chips cant tolerate over 5v. At the same time, I have never
seen any analog meter using more than two batteries. That's 3 volts.
I have several analog meters and they all use two AA batteries. Except
for the one mini meter which only has only one AA battery.

So we have this theory about these analog meters ruining chips for
exceeding 5v, but none of the meters can provide more than 3volts. This
makes me think that this theory is based on advertisers trying to sell
digital meters, or just an old wives tale with no backing. (Unless there
are some analog meters which use 4 batteries [6volts], or use a 9v
battery). If meters do exist, which are powered by more than 3v, I have
never seen them.
 
On Sun, 16 Jul 2017 13:20:54 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Sun, 16 Jul 2017 14:48:48 -0400, oldschool@tubes.com wrote:

...I have never
seen any analog meter using more than two batteries. That's 3 volts.
I have several analog meters and they all use two AA batteries. Except
for the one mini meter which only has only one AA battery.

The Simpson 260 VOM used a single D cells and a 15V battery (Eveready
417) for the Rx10K range. Later models switched to a 9V battery and
then to 4ea AA batteries. The Triplett 630 used a single D cell and a
30V (Eveready 413).

Ok, I guess you have meters that I've never seen. I have to wonder why
they need such high voltage to measure resistance. However, I would not
use those meters on modern circuits if I owned them.

My analog meters are all older Radio Shack meters, which I have owned
for many years. All (except the mini), have two AA batteries.

I also have a few VTVM's. I am not sure what they output, so I dont use
them on any solid state devices. But they are well suited for tube gear,
and can handle the high voltages in tube gear, which a lot of the
battery operated portable VOM's cant handle.
 
On Sun, 16 Jul 2017 14:48:48 -0400, oldschool@tubes.com wrote:

...I have never
seen any analog meter using more than two batteries. That's 3 volts.
I have several analog meters and they all use two AA batteries. Except
for the one mini meter which only has only one AA battery.

The Simpson 260 VOM used a single D cells and a 15V battery (Eveready
417) for the Rx10K range. Later models switched to a 9V battery and
then to 4ea AA batteries. The Triplett 630 used a single D cell and a
30V (Eveready 413).

--
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 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.
 
On Sunday, July 16, 2017 at 4:50:45 PM UTC-4, olds...@tubes.com wrote:
But they are well suited for tube gear,
and can handle the high voltages in tube gear, which a lot of the
battery operated portable VOM's cant handle.

Maybe the Harbor Freight free-with-any-purchase DMM can't handle higher voltage, but I've never seen any DMM that can't handle tube gear voltages. If you're talking about the old plate voltages of the horiz output or HV rectifier tube in televisions, no meter without an accessory HV probe will read those without some damage.

Unless you're doing peak and null, you shouldn't be using your analogue meters in my opinion.
 
In article <g7cnmc110cdt64ldsvmj3if2ed5f1jr06l@4ax.com>,
oldschool@tubes.com says...
I have heard this several times about analog meters destroying chips,
because the chips cant tolerate over 5v. At the same time, I have never
seen any analog meter using more than two batteries. That's 3 volts.
I have several analog meters and they all use two AA batteries. Except
for the one mini meter which only has only one AA battery.

So we have this theory about these analog meters ruining chips for
exceeding 5v, but none of the meters can provide more than 3volts. This
makes me think that this theory is based on advertisers trying to sell
digital meters, or just an old wives tale with no backing. (Unless there
are some analog meters which use 4 batteries [6volts], or use a 9v
battery). If meters do exist, which are powered by more than 3v, I have
never seen them.

The Simpson 260 series used a 1.5 V D cell and several voltages for the
highest resistance range. Very old ones used a 15 volt battery I think,
some used several AA batteries and the two I have use a standard 9 volt
battery. Tripplit made a similar VOM. I have not looked inside one of
those, but would bet it had 2 batteries similar to the Simpson.

The caution before ICs came out was not to use the lowest current range
for solid state devices. That 1.5 volt battery could supply enough
current to destroy some simiconductors. Not voltage, but current.

The caution was way before the DMMs and ICs came out.
 
In article <s5gnmcl9ukiegiv2q861m73b2ih8mll1ev@4ax.com>,
oldschool@tubes.com says...
The Simpson 260 VOM used a single D cells and a 15V battery (Eveready
417) for the Rx10K range. Later models switched to a 9V battery and
then to 4ea AA batteries. The Triplett 630 used a single D cell and a
30V (Eveready 413).

Ok, I guess you have meters that I've never seen. I have to wonder why
they need such high voltage to measure resistance. However, I would not
use those meters on modern circuits if I owned them.

My analog meters are all older Radio Shack meters, which I have owned
for many years. All (except the mini), have two AA batteries.

I also have a few VTVM's. I am not sure what they output, so I dont use
them on any solid state devices. But they are well suited for tube gear,
and can handle the high voltages in tube gear, which a lot of the
battery operated portable VOM's cant handle.

Radio Shack tells all. Anyone that says old school should have heard of
and maybe used the Simpson or Tripplit meter. Don't recall the number
of the Triplett as I only used one in school over 40 years ago.

The RS meters may have a meter of around 30 uA and the others have 50 uA
meters. It takes more voltage to operate them in the higher resistance
ranges. I don't know what the RS meters have for full scale of the
resistance ranges, but it may not be as much.

Even the Free HF meter I have says it will do 1000 VDC and 700 VAC. Not
too much common tube equipment has voltages over that, unless much
higher and special HV probes are needed. I think my old Heathkit VTVM
may do 1500 Volts. Have not had it out in years except to give it a
check a year or two ago to see if it still works.
 
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.

I have to wonder why
they need such high voltage to measure resistance. However, I would not
use those meters on modern circuits if I owned them.

Most (not all) VOM's did NOT have amplifiers with gain. Therefore,
resistance measurements needed to be using the basic meter
sensitivity. The meter sensitivity and battery voltage put a limit on
the highest resistor value that could be accurately measured.

The meter face usually had the meter sensitivity. In this example:
<https://cdn6.bigcommerce.com/s-a1x7hg2jgk/images/stencil/500x659/products/42383/204330/50126_2__62784.1490313691.jpg?c=2>
It says "20,000 ohms per volt" (on DC scales) which is the same as:
1 / 20K ohms/volt = 50 ľA full scale
You could probably read 1/10th of full scale accurately. So, what's
the largest resistance that you could read at 1/10th of full scale,
using a 9V battery?
R = E / I = 9V / 5*10-6A = 45M
Good enough to measure common resistors of the 1960's. However, if
you tried it with a 1.5V battery, you would get:
R = E / I = 1.5 / 5*10-6A = 7.5M
That's too low, because there were plenty of resistors up to 22M in
older tube sets, that such a meter could not measure.

My analog meters are all older Radio Shack meters, which I have owned
for many years. All (except the mini), have two AA batteries.

I also have a few VTVM's. I am not sure what they output, so I dont use
them on any solid state devices. But they are well suited for tube gear,
and can handle the high voltages in tube gear, which a lot of the
battery operated portable VOM's cant handle.
--
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 Sunday, 16 July 2017 21:50:45 UTC+1, olds...@tubes.com wrote:
On Sun, 16 Jul 2017 13:20:54 -0700, Jeff Liebermann <jeffl@cruzio.com
wrote:

On Sun, 16 Jul 2017 14:48:48 -0400, oldschool@tubes.com wrote:

...I have never
seen any analog meter using more than two batteries. That's 3 volts.
I have several analog meters and they all use two AA batteries. Except
for the one mini meter which only has only one AA battery.

The Simpson 260 VOM used a single D cells and a 15V battery (Eveready
417) for the Rx10K range. Later models switched to a 9V battery and
then to 4ea AA batteries. The Triplett 630 used a single D cell and a
30V (Eveready 413).

Many old meters used 9v or 15v batteries as well as a 1.5v.

Ok, I guess you have meters that I've never seen. I have to wonder why
they need such high voltage to measure resistance.

To enable the highest R range to work

However, I would not
use those meters on modern circuits if I owned them.

they're fine, just don't use the top R range on anyhing delicate.

My analog meters are all older Radio Shack meters, which I have owned
for many years. All (except the mini), have two AA batteries.

I also have a few VTVM's. I am not sure what they output, so I dont use
them on any solid state devices. But they are well suited for tube gear,
and can handle the high voltages in tube gear, which a lot of the
battery operated portable VOM's cant handle.

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


NT
 
In article <easnmcp9is7gu1eiqbu0bs76i89dhsqi4f@4ax.com>,
jeffl@cruzio.com says...
Most (not all) VOM's did NOT have amplifiers with gain. Therefore,
resistance measurements needed to be using the basic meter
sensitivity. The meter sensitivity and battery voltage put a limit on
the highest resistor value that could be accurately measured.

The meter face usually had the meter sensitivity. In this example:
https://cdn6.bigcommerce.com/s-a1x7hg2jgk/images/stencil/500x659/products/42383/204330/50126_2__62784.1490313691.jpg?c=2
It says "20,000 ohms per volt" (on DC scales) which is the same as:
1 / 20K ohms/volt = 50 ľA full scale
You could probably read 1/10th of full scale accurately. So, what's
the largest resistance that you could read at 1/10th of full scale,
using a 9V battery?
R = E / I = 9V / 5*10-6A = 45M
Good enough to measure common resistors of the 1960's. However, if
you tried it with a 1.5V battery, you would get:
R = E / I = 1.5 / 5*10-6A = 7.5M
That's too low, because there were plenty of resistors up to 22M in
older tube sets, that such a meter could not measure.
The Simpson did have a 20 meg mark on the scale. It is almost worthless
at that resistance. About all that can be told is that the reistor is
not totally open. Around 2 to 5 meg ohms is abut the best anyone can
tell close to the resistance. The resistance scale is similar to a log
scale so as the resistance value goes up and below about 1/4 scale the
values start getting very close together.

I have not looked into the 260 working in many years. I know of the 20
K per volt and how it works on the DC ranges, but not sure where it
comes into play on the ohms scale for this meter.

The meter movement is just under 50 uA and has a pot that I think goes
across it so the first step in calibrating one is to set it for 50 uA
full scale.
 

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