AC and meter movements

[Mike Cook]

How does one use AC to actuate a D'Arsonval (Weston) meter movement? Is a
single rectifier sufficient? Bridge?

Movement of interest says "FS = 45.8 uA DC".

Extra points for links to drive circuits.

Thanks.

 

[Phil Allison]

"Mike Cook"

How does one use AC to actuate a D'Arsonval (Weston) meter movement? Is a
single rectifier sufficient? Bridge?

Movement of interest says "FS = 45.8 uA DC".

Extra points for links to drive circuits.

** The simplest possible AC converter circuit use two germanium diodes, one
wired across and one in series with the meter. This is what most anlogue
multimeters use for the AC volts ranges.

However, the sky is the limit if you go for active circuits using FETs,
op-amps and even special AC to DC converter ICs that compute the "true rms"
value of all manner of AC waveforms.



..... Phil

 

[default]

On Fri, 9 Apr 2010 01:48:28 -0700, Mike Cook <mcham@NOTyahoo.com>
wrote:

How does one use AC to actuate a D'Arsonval (Weston) meter movement? Is a
single rectifier sufficient? Bridge?

Movement of interest says "FS = 45.8 uA DC".

Extra points for links to drive circuits.

Thanks.

If you have power to power the circuit something called a "precision
rectifier" works well. It uses an op amp to effectively cancel the
forward voltage drop of the diodes.

If you haven't got power . . . and need sensitivity, the customary way
is with a "copper oxide rectifier." These are old technology but
still manufactured for this purpose, (but not in your local radio
shack unless it is inside a meter you purchase there.)

If you are just interested in making a meter for line voltages and
starting out with a 50 micro amp movement, the drop of some forward
biased germanium diodes is relatively insignificant. If you want to
measure a few volts, germanium isn't going to work.

Can one make a copper oxide rectifier?
http://books.google.com/books?id=wN8DAAAAMBAJ&pg=PA582&lpg=PA582&dq=copper+oxide+meter+rectifiers&source=bl&ots=WK6TfGc8X-&sig=nfesu5FMzb19dv6Wcz2RYJ-ey7k&hl=en&ei=Txu_S5uaOYWClAf3htnnBg&sa=X&oi=book_result&ct=result&resnum=19&ved=0CFQQ6AEwEg

how to make a Cu rectifier
Popular Mechanics - Oct 1935 - Google Books Result
Vol. 64, No. 4 - 256 pages - Magazine
Dry-contact rectifiers are of various types; however, all of them
include a common junction between two dissimilar substances.

Some info on Cu rectifiers along with typical Fwd V drop:
http://tera-3.ul.cs.cmu.edu/NASD/4dcb85c3-9fee-4c83-9e6d-fe6ce5522b59/China/disk4/75/75-3/31006598/PDF/00000134.pdf
--

 

[Phil Allison]

"default"

If you are just interested in making a meter for line voltages and
starting out with a 50 micro amp movement, the drop of some forward
biased germanium diodes is relatively insignificant. If you want to
measure a few volts, germanium isn't going to work.

** Huh ?????????

Ge diodes begin conducting at less than 100mV.

Only a matter of calibrating the scale on the lowest AC range to accommodate
this.

Millions of analogue multimeters made since the 1930s cannot be all wrong -
eg:

http://www.claytonengineering.com/Training/myweb6/MElect/FM%2055-509-1%20Chptr%209%20Circuit%20Measurement_files/fig9-28.gif



..... Phil

 

[default]

On Fri, 9 Apr 2010 22:44:15 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"default"

If you are just interested in making a meter for line voltages and
starting out with a 50 micro amp movement, the drop of some forward
biased germanium diodes is relatively insignificant. If you want to
measure a few volts, germanium isn't going to work.

** Huh ?????????

Ge diodes begin conducting at less than 100mV.

Only a matter of calibrating the scale on the lowest AC range to accommodate
this.

Millions of analogue multimeters made since the 1930s cannot be all wrong -
eg:

http://www.claytonengineering.com/Training/myweb6/MElect/FM%2055-509-1%20Chptr%209%20Circuit%20Measurement_files/fig9-28.gif



.... Phil

Curves of semiconductor diodes:
http://www.tcc.edu/faculty/webpages/AKoon/cd/ETR%20104%20Labs%20and%20Handouts/Example%20Lab%20Report.pdf

Probably loads cheaper to make meters with semiconductor diodes, but
they still make copper oxide rectifiers for that purpose.

Please put me back on your plunk list - you are an insufferable
egomaniacal idiot Phyllis dear.
--

 

[Phil Allison]

"default = one ignorant, stinking cunt "

If you are just interested in making a meter for line voltages and
starting out with a 50 micro amp movement, the drop of some forward
biased germanium diodes is relatively insignificant. If you want to
measure a few volts, germanium isn't going to work.

** Huh ?????????

Ge diodes begin conducting at less than 100mV.

Only a matter of calibrating the scale on the lowest AC range to
accommodate
this.

Millions of analogue multimeters made since the 1930s cannot be all
rong -
eg:

http://www.claytonengineering.com/Training/myweb6/MElect/FM%2055-509-1%20Chptr%209%20Circuit%20Measurement_files/fig9-28.gif



Curves of semiconductor diodes:
http://www.tcc.edu/faculty/webpages/AKoon/cd/ETR%20104%20Labs%20and%20Handouts/Example%20Lab%20Report.pdf

** Nice one, Mr Know Nothing Cunt

- that page proves my point just fine.


Now - get back to you kiddie porn induced masturbation,

as per fucking usual.



..... Phil

 

[Mike Cook]

** The simplest possible AC converter circuit use two germanium diodes, one
wired across and one in series with the meter. This is what most anlogue
multimeters use for the AC volts ranges.
...
.... Phil

Thanks Phil for the simple answer.

This is for measuring output of a simple variac, so yes, the germaniums seem
to be the answer.

Regards,
M

 

[John Larkin]

On Fri, 9 Apr 2010 08:19:34 -0700, Mike Cook <mcham@NOTyahoo.com>
wrote:

** The simplest possible AC converter circuit use two germanium diodes, one
wired across and one in series with the meter. This is what most anlogue
multimeters use for the AC volts ranges.
...
.... Phil

Thanks Phil for the simple answer.

This is for measuring output of a simple variac, so yes, the germaniums seem
to be the answer.

Small schottkies will behave about the same. If you use a big series
resistor, as you will in this case, any diode will work.

John

 

[Phil Allison]

"Mike Cook" <

** The simplest possible AC converter circuit use two germanium diodes,
one
wired across and one in series with the meter. This is what most anlogue
multimeters use for the AC volts ranges.


Thanks Phil for the simple answer.

This is for measuring output of a simple variac, so yes, the germaniums
seem
to be the answer.

** To prevent over any voltage condition on the Ge diodes, the parallel
one must be connected across the meter and series diode combination.



...... Phil

 

[Phil Allison]

"Phil Allison"

** Additional info:

Ge diodes begin conducting at less than 100mV.

** Just tested an AAZ15 Ge diode - at 1uA the forward voltage is only
13mV.

A BAT46 Schottky showed 60 mV at the same current.

As the OP had a 45 uA meter movement, this makes the Ge diode a useful
rectifier to around 10mV AC.

Only a matter of calibrating the scale on the lowest AC range to
accommodate this.

..... Phil

 

[Mike Cook]

** To prevent over any voltage condition on the Ge diodes, the parallel
one must be connected across the meter and series diode combination.

Thanks Phil. It's newbie tips like this that I need.

Cheers.

 

[Mike Cook]

Small schottkies will behave about the same. If you use a big series
resistor, as you will in this case, any diode will work.

John

- meter movement: 968 ohms; 45.8ľA DC full-scale
- for 140V AC full scale, V = 0.443344, in V mode: series R = 3046120.5 ohms
- for 15A AC full scale, V = 0.443344, in A mode: shunt R =29.5 milliohms

I didn't take into account the diode drops. Should I? Where?

This is a variac in "boost" mode (hence the max value of 140V).

Thanks.

 

[Phil Allison]

"Mike Cook"

- meter movement: 968 ohms; 45.8ľA DC full-scale

** Very typical numbers.

- for 140V AC full scale, V = 0.443344,

** Should be 0.0443... volts DC.

in V mode: series R = 3046120.5 ohms

** Only true for DC voltages.

For AC, you need to consider that the rectifier is half wave and the voltage
is 198 volts peak and YOU want to see the RMS value on the meter. Meters
like yours, however, only respond to the *average value* of the imposed
current wave.

For half wave rectified sine waves, the *average rectified* current value is
1/pi the peak current - so the dropping resistor needs to pass pi times
the peak current of the meter when the voltage is 140 V rms.

Makes the current: pi x 45.8 = 143.9 uA

- so the needed resistor is 1.38 Mohms.

- for 15A AC full scale, V = 0.443344,

** Should be 0.0443... volts DC.

in A mode: shunt R =29.5 milliohms

** Only works for DC current and should be 2.95 milliohms.

I didn't take into account the diode drops. Should I? Where?

** Oh dear.

The AC voltmeter is easy to get right but a 15 amp AC current meter is not
so easy.

The scale will be very non-linear for amps cos the applied AC voltage from
the shunt will be so small.

Better invest in a Moving Iron meter for measuring AC amps - these need no
rectifier at all.




..... Phil

 

[Mike Cook]

The AC voltmeter is easy to get right but a 15 amp AC current meter is not
so easy.

The scale will be very non-linear for amps cos the applied AC voltage from
the shunt will be so small.

Better invest in a Moving Iron meter for measuring AC amps - these need no
rectifier at all.

Hmm... how non-linear would it be? It's for a variac so approximate current
would be fine (best to be more accurate at max 15A).

Meter was included with the cabinet I'm installing the variac, and since it
was free, at first I wasn't considering adding any metering at all.

Thanks for your help.

 

[The Phantom]

On Sat, 10 Apr 2010 10:18:18 -0700, Mike Cook <mcham@NOTyahoo.com> wrote:

The AC voltmeter is easy to get right but a 15 amp AC current meter is not
so easy.

The scale will be very non-linear for amps cos the applied AC voltage from
the shunt will be so small.

Better invest in a Moving Iron meter for measuring AC amps - these need no
rectifier at all.

Hmm... how non-linear would it be? It's for a variac so approximate current
would be fine (best to be more accurate at max 15A).

Meter was included with the cabinet I'm installing the variac, and since it
was free, at first I wasn't considering adding any metering at all.

Thanks for your help.

You can find moving iron AC ammeters on eBay for reasonable prices.

Beware, many are actually 5 amp FS (full scale) meters, designed to be used with
a current transformer. They may have a printed scale that indicates a range
other than 5 amps, but they are really 5 amp meters. For example, this one:

http://cgi.ebay.com/2324-Simpson-AC-Amperes-panel-meter-gauge-range-0-100_W0QQitemZ310201154064QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item48396dfa10

You can see in the picture in the lower right it says "FS = 5A". You can't pass
100 amps through that meter; you have to use a current transformer with it.
However, if you wanted a 5 amp meter, you could open it up and change the
lettering on the scale, or just remember that it's really 5 amps, and mentally
adjust the reading.

But, you can sometimes find meters that are really what their scale says. For
example:

http://cgi.ebay.com/Vintage-Amperes-Weston-A-C-Test-Panel-Meter-Model-476_W0QQitemZ310184055385QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item4838691259

I think this is a meter that can measure up to 20 amps without a current
transformer.

Here's one where it's not clear whether it needs a current transformer or not:

http://cgi.ebay.com/YOKOGAWA-GE-PANEL-METER-A-C-AMPERES-AMP-METER_W0QQitemZ200379812453QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item2ea7914a65

You'd be taking a chance if you bought it expecting it to measure 30 amps
without using a current transformer.
----------------------------------------------------

You can also get moving iron voltmeters which will read true RMS. For example:

http://cgi.ebay.com/WESTON-MODEL-476-PANEL-METER-0-15-0-150-A-C-VOLTS_W0QQitemZ390176487347QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5ad854b7b3

This one would be perfect for you. It even has a lower range for better
resolution at low voltages.

Or this one, larger and a different style:

http://cgi.ebay.com/Simpson-Analog-Panel-Meter-0-150-AC-Volts-L15_W0QQitemZ400060424316QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5d2575a07c

If you're not in a hurry, watch ebay for "panel meter" for a few months and
you'll find more of these sorts of meters.

 

[Phil Allison]

"The Phantom"

You can also get moving iron voltmeters which will read true RMS.

** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

The force on the pointer is proportional to the average of the *square* of
the applied current ( hence it automatically rectifies AC) - and the scale
on the face is somewhat non linear as it indicates actual current.


..... Phil

 

On Sun, 11 Apr 2010 12:32:23 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

"The Phantom"


You can also get moving iron voltmeters which will read true RMS.

** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

Why are you repeating what I already told the OP? I already explained
to him that moving iron voltmeters read true RMS.

The force on the pointer is proportional to the average of the *square* of
the applied current ( hence it automatically rectifies AC) - and the scale
on the face is somewhat non linear as it indicates actual current.


.... Phil

 

[Phil Allison]

<phantom@aol.com>

You can also get moving iron voltmeters which will read true RMS.


** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

Why are you repeating what I already told the OP? I already explained
to him that moving iron voltmeters read true RMS.

** Fraid you told the world that only * SOME* of them did.

Read your own words - dickhead.



..... Phil

 

[Fred Abse]

On Sun, 11 Apr 2010 05:05:01 -0500, phantom wrote:

On Sun, 11 Apr 2010 12:32:23 +1000, "Phil Allison" <phil_a@tpg.com.au
wrote:


"The Phantom"


You can also get moving iron voltmeters which will read true RMS.

** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

Why are you repeating what I already told the OP? I already explained
to him that moving iron voltmeters read true RMS.

No, you said "You can also get moving iron voltmeters which will read true
RMS."

"also"

The force on the pointer is proportional to the average of the *square*
of the applied current ( hence it automatically rectifies AC) - and
the scale on the face is somewhat non linear as it indicates actual
current.

Phil is correct.

--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)

 

[Paul E. Schoen]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:82cqm0FjbgU1@mid.individual.net...

"The Phantom"


You can also get moving iron voltmeters which will read true RMS.

** Moving iron meters always read "true RMS" - within the limits of their
frequency responses.

The force on the pointer is proportional to the average of the *square* of
the applied current ( hence it automatically rectifies AC) - and the
scale on the face is somewhat non linear as it indicates actual current.

I have used and calibrated iron vane meters, and I don't remember them as
true RMS for all waveforms. Particularly, they have a slightly different
response to a DC component, which is part of the true-RMS value (although
some meters have an option of RMS and AC+DC RMS). Here is some information
about older RMS metering devices:
http://ca.fluke.com/Fluke/usen/Community/Fluke+Plus/ArticleCategories/DMMs/True-rms+Facts.htm

The other types mentioned, electrodynamic and thermocouple, AFAIK are true
RMS with any waveform and DC component. The thermocouple type is especially
good for high frequency into the RF range.

Here is more information that may be helpful for the OP:
http://www.allaboutcircuits.com/vol_2/chpt_12/1.html
http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

It also mentions the electrostatic voltmeter, which also responds to AC as
well as DC. But they are very specialized, expensive, and usually for very
high voltages.

I found other references that state true-RMS performance of iron-vane
instruments, and perhaps they do respond accurately, although they are
definitely limited in frequency response to power-line and low audio
signals. Without actually performing tests I will accept that iron vane
types are at least better than rectifier types for RMS readings. Here is a
discussion with much detail that seems to prove your assertion:
http://yarchive.net/electr/analog_voltmeters.html

For the OP's purposes of voltage measurement, a silicon or germanium diode
bridge will work well, and can be used with most commonly available meters.
AC current is best measured with a CT, and there are some small inexpensive
ones that will handle currents of 5 to 30 amps with a ratio of about 500:1
such as this:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1103-ND

The output can be rectified and provides a current of 10 mA for 5 A with
enough voltage to produce a linear output through a silicon diode bridge.

Paul