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G8MNY > TECH 30.08.08 01:40l 141 Lines 4574 Bytes #999 (0) @ WW
BID : 1274_GB7CIP
Read: GUEST
Subj: Meter Movement Types
Path: IZ3LSV<IK2XDE<ON4HU<ON0BEL<GB7CIP
Sent: 080829/2237Z @:GB7CIP.#32.GBR.EU #:1274 [Caterham] $:1274_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Jun 08)
(8 Bit ASCII Graphics use code page 437 or 850)
METER PRINCIPLES
There are several ways to move a meter from a current or voltage. The most
common type is the moving coil..
MOVING COIL MOVEMENT
_...----..._
/~ Horse ~\ Ý
³ Shoe Magnet ³ Needle Pointer ((@))
³ /~~~~~~~~~~\ ³ ================/Ý~/|
³ ³ ³ ³ / / |
³ ³_ _³ ³ ³~~³| |
³ / /~~\ \ ³ ³ ³/ /Coil
³ ³ [COIL] ³ ³Pole ³ ³ /On former
\ \ \__/ / ³Piece ³__³/
~~~ | ~~~ ÝAxle
| Hair spring((@))
Central Ý
Iron
Point Bearing
The coil is free to move in the gap between central iron & the shaped magnet
poles. It is connected via the 2 hair springs which also give the return to
zero force. The light aluminium needle is attached to the coil & is counter
balanced with tiny weights so that the meter reads zero in any position (when
it left the factory unbent.) Movement calibration is often achieved with a
movable iron link the can bypass come of the magnet's flux.
With this movement the deflection angle is proportional to the coil current,
with string return torque & magnet flux fixed.
SUSPENSION
A pair of hair springs that wind up & unwind are usual to compensate for
temperature, but sometimes a twisting torsion spring in used without pivot
bearing. This means the meter has no friction, improving the accuracy. Drawback
is these movements are fragile!
|torsion suspension
Pointer |_
========/| /|
/ / |
³~~³| |
³ ³/ /Coil
³ ³ /former
³__³/
|
|
NON LINEAR
For some applications like "light meters" the pole pieces can be shaped to
produce a log or true square law scales.
_.----._
/~ Horse ~\
³ Shoe Magnet³
³ /~~~~~~~~\ ³
³ / _³ ³
³ ³ [ ³Shaped
³ \ /~~\ \ ³Pole
³ \ [COIL] \ ³Piece
³ \ \__/ \³
\__] |
|
Central
Iron
HIGH SENSITIVITY
Improvements to this design for high sensitivity use powerful circular ferrite
magnets both on the outside & inside of the coil, The penalty is poor scale
accuracy, but several times the sensitivity can be achieved. e.g. a large 6"
meter with a 15uA 6kê coil.
..--ÄÄÄ--.,
/ _..._ \
³ N/~ ~\S ³
³N³ /S~N\ ³S³
³N³ ³S N³ ³S³
³N³ \S_N/ ³S³
³ N\..___../S ³
\,,__ __../
~~~
MULTIPLICATION MOVEMENTS
If the magnet is replace with an electromagnet, then the deflection is
proportional to the product of the two. e.g. a true power meter where the AC or
DC current produces the magnetising flux. This flux must not be near saturation
if the effect is to be faithful. It is the only AC meter using moving coil
principle without rectification.
Another approach is to use a magnet but use 2 coils & no return springs. this
gives a high sensitivity.
MOVING IRON
This is a different system. It uses 2 pieces of iron inside a coil. As the coil
in is energised the flux on each piece of iron causes them repel each other.
One piece is held fixed & the other on a hair spring balanced arm & pointer.
This effect works for DC & true RMS AC current as long as the coil current is
the same (inductance) & there is negligible iron losses at the AC frequency
(e.g. ok DC to 1kHz?)
.-Ä~~~~Ä-,
/ _.._ \ Coil
Pointer / / ÜÜ\ \Bobbin
==============@_Ü ³ ³
\ \_ _/ /
\ ~~ /
~Ä.____.Ä~
If the coil is replace by coupled in flux from a power cable then the AC/DC
current can be read by a clip on meter. But 1 turn currents of about 20 Amps
are needed.
ELECTROSTATIC VOLTMETERS
These work in a similar way to the moving iron, but high voltage static is
used on 2 conductors & they try to repel. The normal example is the gold leaf
electroscope. Versions have been made calibrated in kV e.g. 0-50kV & of course
no current flows!
ÄÄÂÄÄ
ÚÄı³±Ä¿
³ ³ ³
/ ³ \
/ ³ \
³ free/³ ³
³leaf/ ³ ³
³ ÁÁÁÁ ³
³ scale ³Glass
ÀÄÄÄÄÄÄÄÄÄÄÄÄÙBottle
See Tech buls on "Lafayete 57 Range Multimeter", "Meter Damping & Speed Up" &
"Marconi Distortion Meter TF2337" for more related information.
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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