On 2005-03-01 said:
Newsgroups: alt.home.repair,alt.energy.homepower
wrote:
[snip]
... The really cheap meters will say something like 2 Kohm /
volt or 3 K ohm / volt on the face. A better quality one will say
something like 20K ohm / volt. These ratings are usually based on 1
milliamp giving full scale deflection of the needle.
No, not 1 mA.
2 Kohm/volt = .5 mA
3 Kohm/volt = .33 mA
20 Kohm/volt = 50 microAmps

[snip]
Getting my first Field Effect Transistor powered meter was a reall
blessing. It had something like 2 MEG ohms / volt sensitivity. Most
of the digital stuff now probably has 10 or 20 Meg ohms / volt.
No, not "per volt". That's the input resistance for DC measurements.

My Beckman 3020 DMM is 22 megohms input resistance.

Fluke 27 and 73 read 11 megs on the Beckman
Fluke 36 DMM and clamp-on AC/DC ammeter reads 2 megs.


Tom Willmon
near Mountainair, (mid) New Mexico, USA

Net-Tamer V 1.12.0 - Registered

wrote:

If you remove the battery from Nick's charge pump circuit proposal, you
have what's commonly called a "voltage doubler"... as long as the battery
is there and can accept the current... you have 1.5V regulated by the
battery and no additional regulator is necessary.

I would modify this design by replacing the diode whose anode connects
to ground with a zener rated for 2.5 to 5 volts... if the battery should
open-circuit, the downstream components won't see the 370V spike...

Good idea. Two caps in series might also help with single point failures.

Similar "transformerless stepdown" circuits are used to supply regulated
12VDC to motion detectors.

UL-approved, in plastic boxes. I've heard California is banning
the transformer ilk because of the standby power.

The question of whether the negative terminal of the battery is
connected to an identifiable "common" node in the VOM's internal
circuit is left for further consideration.

That's only a problem for alt.home.repair nitwits, IMO :-)

Nick

wrote:


... The really cheap meters will say something like 2 Kohm /
volt or 3 K ohm / volt on the face. A better quality one will say
something like 20K ohm / volt. These ratings are usually based on 1
milliamp giving full scale deflection of the needle.

No, not 1 mA.
2 Kohm/volt = .5 mA
3 Kohm/volt = .33 mA
20 Kohm/volt = 50 microAmps


You forget that there are different scales on the meter face. Full scale
deflection on one scale may be reading 250 volts and 10 volts on another. The
meter movement HAS to see the same amperage in order to get fullscale
deflection. The amount of resistance you have to add will, of course, vary with
each scale, but at no time can the current EXCEED the 1 ma (or whatever) at full
scale.






[snip]
Getting my first Field Effect Transistor powered meter was a reall
blessing. It had something like 2 MEG ohms / volt sensitivity. Most
of the digital stuff now probably has 10 or 20 Meg ohms / volt.

No, not "per volt". That's the input resistance for DC measurements.

You are right. I had ohms per volt on the brain from the previous paragraph. I
was just trying to show how much less circuit loading there was with a vtvm (or
FET) type of meter.

please, shoot me now...






mike

On 2005-03-02 said:
Newsgroups: alt.home.repair,alt.energy.homepower
wrote:
... The really cheap meters will say something like 2 Kohm /
volt or 3 K ohm / volt on the face. A better quality one will
say something like 20K ohm / volt. These ratings are usually
based on 1 milliamp giving full scale deflection of the
needle.
No, not 1 mA.
2 Kohm/volt = .5 mA
3 Kohm/volt = .33 mA
20 Kohm/volt = 50 microAmps
You forget that there are different scales on the meter face.
No I didn't. Unless my simple, ohm's law calculations were wrong, meter
movement sensitivity is 1 / ohms per volt. To set measuring range of
meter you select a multiplier resistor which supplies full scale meter
current at your desired full scale voltage.

A 1Kohms/volt meter would use 10K to read 10V full scale.
A 20Kohms/volt would need 200K for the same range.


Full
scale deflection on one scale may be reading 250 volts and 10 volts
on another. The meter movement HAS to see the same amperage in
order to get fullscale deflection. The amount of resistance you
have to add will, of course, vary with each scale,
Yup.
but at no time
can the current EXCEED the 1 ma (or whatever) at full
scale.
Huh? Sorry, not clear to me what your point is here.
[snip]

please, shoot me now...
mike
Naw - you're doing OK now.

Cheers.


Tom Willmon near Mountainair, (mid) New Mexico, USA

Net-Tamer V 1.12.0 - Registered

wrote:


... The really cheap meters will say something like 2 Kohm /
volt or 3 K ohm / volt on the face. A better quality one will
say something like 20K ohm / volt. These ratings are usually
based on 1 milliamp giving full scale deflection of the
needle.

No, not 1 mA.
2 Kohm/volt = .5 mA
3 Kohm/volt = .33 mA
20 Kohm/volt = 50 microAmps

You forget that there are different scales on the meter face.


No I didn't. Unless my simple, ohm's law calculations were wrong, meter
movement sensitivity is 1 / ohms per volt. To set measuring range of
meter you select a multiplier resistor which supplies full scale meter
current at your desired full scale voltage.


I thought I had said something similar to that, in my own, obviously unclear
manner. We're stuck with the designed current rating of the meter, however, so
we can't vary the current for full deflection, Your formula is therefore better
stated as R = V/1ma, that is supposing a 1ma movement, as before. R is the
resistance needed to allow the needle to move all the way over on that
particular V scale.


A 1Kohms/volt meter would use 10K to read 10V full scale.
A 20Kohms/volt would need 200K for the same range.

Yes...and 10K and 200K ohms on the 100V scale.


Ah well..sorry for any misunderstanding I may have
caused/engendered/espoused/advocated or just plain originated.






mike

In alt.home.repair on Sat, 26 Feb 2005 03:10:30 -0330 "Terry"
posted:

There are some
applications where such a small analog meter can do better job than a more
sophisticated more sensitive one!

One thing is allowing you to watch a capacitor charge. Eventually the
needle reaches the resistance of the circuit, not counting the
capacitance.

Meirman
--
If emailing, please let me know whether
or not you are posting the same letter.
Change domain to erols.com, if necessary.

HvacTech2 wrote:



.. **FLASH** Eveready Bunny arrested, charged with battery.

***** Sentenced to 2-4 years in a dry cell.

Jeff


--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"