Testing trigger voltage

(Dave Martindale) wrote:
(Floyd L. Davidson) writes:

The voltage you are measuring has *nothing* to do with it.

Correct, for both types of meters.

If you use a 2 volt scale, the meter will have 20M Ohms across
the terminals (whether you are measuring 2 volts or 20 volts).

Not for the DVM/DMM.

Wrong. What was specified was a 10M Ohm/Volt meter. If that is
true on a 2 Volt scale, then it *will* have 20M across the
terminals. Even for a DVM, if it is 10M Ohm/Volt on that scale.

Its input impedance is generally 10 or 11 Mohm on
*all* scales, because the same voltage divider is always bridged across
the input leads.

But what was *specified* was a 10M *Ohm/Volt* meter, not one that
is "generally 10 or 11 Mohm" input impedance.

On a 2 volt scale, your meter would be 5M Ohm/Volt; which is
clearly an entirely different beast.

The resistance is a shunt resistance, not a series
resistance, and it doesn't change with the scale selected. So it's just
"10 Mohm", not "10 Mohm/volt".

But that is *not* what was specified. Regardless, your meter
*can* (still) be specified in Ohms per Volt. It changes,
obviously, from one scale to another, which is not the same as
is common with older analog meters.

(The exception is some meters that remove the divider from the circuit
on the lowest range, often 200 mV, giving an impedance of hundreds or
thousands of Mohm).

Which is not of any significance to this discussion.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Prometheus wrote:
In article , Fred
McKenzie writes
It must be Analog Volt Ohmmeter, which explaines the reference
to 20K Ohms per volt.

Nah, it predates digital meters by so much that analogue was the
only sort and there was no distinction to make.

I assume that is terminology common in the UK. I never heard of
it in the US.

But then again... Have you ever heard of a "KS-Meter"???

Same thing!

The Bell System specifications for non-Western Electric
manufactured equipment were numbers like KS-14510 (as opposed to
a number starting with J- for WECo equipment). And KS-14510
happens to be the KS specification for a VOM (Volt Ohm Meter),
the common terminology used in the US by everyone not working
for Ma Bell. Typical Ma Bell people called it by the short term
"KS-Meter". They didn't measure voltage though. They just
looked for "battery" or "ground"... :-)

Of course *real* Ma Bell types didn't use a KS-Meter much,
because they had something called "Pics". That was a single
earphone with one test prod and one aligator clip leads. It was
also good for measuring "battery"! Just clip one lead to
something that was grounded, and tap the test prod on whatever
needed to be measured. If it was 24 volt signaling battery it
made a nice little "click". If it was 48 volt talk battery the
"click" was loud. If it was the plus or minus 130 volt battery,
it would simply blow your ears out... you'd hear nothing but
ringing for a few hours.

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

Leaving 'AVO' a trade name aside, I should have said an MC meter
draws current when measuring to 'drive' the meter, a digi meter will
only draw current through it's shunt resistances alone from the test
source and obviously not have to supply the movement.


Why do we want to measure the trigger voltage, what are we going to do
when we find out what it is...?

The trigger circuit requires a voltage from an internal battery to
supply a pulsed energy to the trigger coil, a coil with a low to high
turn ratio which charges a capacitor to around 2kV to strike the xenon
gas tube.

Older cameras used a battery around 12-15V, modern digi-cam uses only
3V - 2 AA or whatever...?

We start to increase the voltage to this coil what is going to
happen..? The coil 'might' suffer from internal flash-over or burn
out - the capacitor might become faulty due to over-voltage, the
drive circuitry may become stressed...

Just curious as to why we need to measure, yes Xenon tubes do wear
they'er only rated by their predicted flash cycles and components do
fail.

What about the Joule...thought thats how they was measured...?

Davy

On Sun, 26 Mar 2006 21:49:01 -0900, (Floyd
L. Davidson) wrote:

Prometheus wrote:
In article , Fred
McKenzie writes
It must be Analog Volt Ohmmeter, which explaines the reference
to 20K Ohms per volt.

Nah, it predates digital meters by so much that analogue was the
only sort and there was no distinction to make.

I assume that is terminology common in the UK. I never heard of
it in the US.

It's the name of a manufacturer rather than some obscure
terminology, but as the UK brand leader it became a generic
name to mean any multi-meter. When this sort of thing
happens the brand-leader often varies by country much to the
confusion of others, especially if the brand name is
generically in use to mean something else in their country.
My favourite "generic brand" example is Durex, a leading
brand of clear adhesive tape in Australia. In the UK it's a
leading brand of condoms... No jokes about coming to a
sticky end please.


--
John Bean

Anologue indeed .....thought AVO stood out like a sore thumb...
Ampere-Volt-Ohms, quite surprising the number of people haven't heard
of AVO....!

First made in 1923 was made by Atomatic Coil Winder & Electrical
Equipment Company - The Avometer Model 1 1923 -1927

Davy

In article , Dave Martindale
writes
"Allan" writes:
Yes, DVM is Digital Volt Meter. AVO is the trade name of a British company
(AVO Megger) that makes high quality multimeters and other lab grade test
equipment - considered by many as the multimeter of choice.

Never heard of them in North America, so AVO probably means nothing to a
lot of readers.

You probably know them better as "Megger", which is even what most of
the younger electricians over here refer to them as, these days:
http://www.megger.com/us/about/index.php

Megger products are manufactured at sites in Dallas, Texas; Valley
Forge, Pennsylvania and Dover, England, the latter being the company
head office.

I don't think the traditional analogue AVO meter, referred to by the OP,
was ever available in the US, but certainly their DVM range of Avo
meters a
http://www.megger.com/us/products/Pr...9&Description=

Interestingly, although their original moving coil meters, such as this
one http://www.richardsradios.co.uk/avo8.html
were the reference standard of most labs, due to the high input
impedance and low load resistance of their voltage and current
measurement ranges respectively, their digital AVO range has slightly
lower in input impedance at 7.8MOhms than most DVMs, such as those by
Fluke and others.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

John Bean writes:

It's the name of a manufacturer rather than some obscure
terminology, but as the UK brand leader it became a generic
name to mean any multi-meter. When this sort of thing
happens the brand-leader often varies by country much to the
confusion of others, especially if the brand name is
generically in use to mean something else in their country.
My favourite "generic brand" example is Durex, a leading
brand of clear adhesive tape in Australia. In the UK it's a
leading brand of condoms... No jokes about coming to a
sticky end please.

My favourite example of this: Digital Equipment made a very popular line
of large minicomputers called Vax. At the same time, some UK
manufacturer of vacuum cleaners made one called Vax - I don't know if
that was the company name or just the model name.

Anyway, at one time the vacuum cleaner was featured in ads whose text
read "Nothing sucks like a Vax!". American computer-literate types were
amused. Apparently Digital Equipment was not.

Dave

(Floyd L. Davidson) writes:

Not for the DVM/DMM.

Wrong. What was specified was a 10M Ohm/Volt meter. If that is
true on a 2 Volt scale, then it *will* have 20M across the
terminals. Even for a DVM, if it is 10M Ohm/Volt on that scale.

The point is that there are no DVMs that work this way. The reference
to 10 Mohm/volt is simply a mistake. The DVM was actually rated 10
Mohm, not 10 Mohm/volt.

But what was *specified* was a 10M *Ohm/Volt* meter, not one that
is "generally 10 or 11 Mohm" input impedance.

Your statement is logically true, but it applies to no real DVMs.
My point is that this spec is a mistake - there is no such device.
Real DVMs, of the sort that might be used to make this measurement, have
a fixed input impedance.

On a 2 volt scale, your meter would be 5M Ohm/Volt; which is
clearly an entirely different beast.

No, the spec is simply wrong. A "5 Mohm/volt" meter would have that
spec apply to all ranges, so it would be 10 Mohm on 2 V, 100 Mohm on 20
V, and so on. Real DVMs are a fixed impedance on all ranges, so they
are specified as "10 Mohm" rather than "10 Mohm/volt".

The resistance is a shunt resistance, not a series
resistance, and it doesn't change with the scale selected. So it's just
"10 Mohm", not "10 Mohm/volt".

But that is *not* what was specified. Regardless, your meter
*can* (still) be specified in Ohms per Volt. It changes,
obviously, from one scale to another, which is not the same as
is common with older analog meters.

Specifying impedance that way is nearly useless if the number changes
for every range. The point of specifying analog meters in terms of
ohms/volt is that one number told you the impedance on any range,
rather than having a table of impedances with one line per range. With
DVMs, the single "10 Mohm" spec applies to all ranges, so it is useful,
while an impedance expressed in ohms/volt would have to have a table of
values different for every range (and the product of range and
ohms/volt would always be 10 M).

Even when comparing a DVM and analog meter, it makes more sense to
convert the analog spec in ohms/volt into an actual value in ohms on a
particular range, and compare that to the DVM impedance, than it does to
convert the DVM spec into an equivalent ohms/volt for comparison to the
analog meter. It's mathematically well-defined, just not very useful,
and not what is done in practice.

Dave

(Dave Martindale) wrote:
(Floyd L. Davidson) writes:

Not for the DVM/DMM.

Wrong. What was specified was a 10M Ohm/Volt meter. If that is
true on a 2 Volt scale, then it *will* have 20M across the
terminals. Even for a DVM, if it is 10M Ohm/Volt on that scale.

The point is that there are no DVMs that work this way. The reference
to 10 Mohm/volt is simply a mistake. The DVM was actually rated 10
Mohm, not 10 Mohm/volt.

The point was (and is) that you are not talking about the same
scenario that I was. Your comment was in error.

But what was *specified* was a 10M *Ohm/Volt* meter, not one that
is "generally 10 or 11 Mohm" input impedance.

Your statement is logically true, but it applies to no real DVMs.

It was never intended to apply to any specific DVM. It applied
to what the OP stated. And yes, that is the logical thing to
address... :-)

My point is that this spec is a mistake - there is no such device.
Real DVMs, of the sort that might be used to make this measurement, have
a fixed input impedance.

So?

On a 2 volt scale, your meter would be 5M Ohm/Volt; which is
clearly an entirely different beast.

No, the spec is simply wrong. A "5 Mohm/volt" meter would have that
spec apply to all ranges, so it would be 10 Mohm on 2 V, 100 Mohm on 20
V, and so on. Real DVMs are a fixed impedance on all ranges, so they
are specified as "10 Mohm" rather than "10 Mohm/volt".

The *fact* is that a 5M Ohm/Volt 2V scale and a 10M Ohm input
impedance are *the* *exact* *same* *thing*.

The resistance is a shunt resistance, not a series
resistance, and it doesn't change with the scale selected. So it's just
"10 Mohm", not "10 Mohm/volt".

But that is *not* what was specified. Regardless, your meter
*can* (still) be specified in Ohms per Volt. It changes,
obviously, from one scale to another, which is not the same as
is common with older analog meters.

Specifying impedance that way is nearly useless if the number changes
for every range.

No it is not. It is just more wordy. It says *exactly* the same
thing.

The point of specifying analog meters in terms of
ohms/volt is that one number told you the impedance on any range,
rather than having a table of impedances with one line per range. With
DVMs, the single "10 Mohm" spec applies to all ranges, so it is useful,
while an impedance expressed in ohms/volt would have to have a table of
values different for every range (and the product of range and
ohms/volt would always be 10 M).

None of that changes the *fact* that it is all just exactly the
same thing.

Even when comparing a DVM and analog meter, it makes more sense to
convert the analog spec in ohms/volt into an actual value in ohms on a
particular range, and compare that to the DVM impedance, than it does to
convert the DVM spec into an equivalent ohms/volt for comparison to the
analog meter. It's mathematically well-defined, just not very useful,
and not what is done in practice.

Either way is exactly the same thing, they are "mathematically
well-defined", they are useful, and some people don't even need
to have one converted to the other in writing to make them
useful and comparable! And other people do... ;-)

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

(Dave Martindale) writes:

My favourite example of this: Digital Equipment made a very popular line
of large minicomputers called Vax. At the same time, some UK
manufacturer of vacuum cleaners made one called Vax - I don't know if
that was the company name or just the model name.

Anyway, at one time the vacuum cleaner was featured in ads whose text
read "Nothing sucks like a Vax!". American computer-literate types were
amused. Apparently Digital Equipment was not.

Well, some of us were -- those in Large Computer Group software
engineering, for example.
--
David Dyer-Bennet, , http://www.dd-b.net/dd-b/
RKBA: http://www.dd-b.net/carry/
Pics: http://dd-b.lighthunters.net/ http://www.dd-b.net/dd-b/SnapshotAlbum/
Dragaera/Steven Brust: http://dragaera.info/