| |
G8MNY > TECH 28.11.11 11:41l 124 Lines 5268 Bytes #999 (0) @ WW
BID : 18352_GB7CIP
Read: GUEST
Subj: Meter Movement Types
Path: IZ3LSV<IK2XDE<DB0RES<DK0WUE<7M3TJZ<UA6ADV<GB7CIP
Sent: 111128/1016Z @:GB7CIP.#32.GBR.EU #:18352 [Caterham Surrey GBR] $:18352_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Dec 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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 |torsiupension
A pair of hair springs that wind up Pointer |_
& unwind are usual to compensate for ========/| /|
temperature with end conical point / / |
bearings, but sometimes a twisting ³~~³| |
torsion spring in used without bearing. ³ ³/ /Coil
This means the meter has no friction, ³ ³ /former
improving the accuracy. Drawback is ³__³/
these movements are more fragile! |
|
NON LINEAR
_.----._
/~ Horse ~\
³ Shoe Magnet³
³ /~~~~~~~~\ ³ For some applications like
³ / _³ ³ "light meters" the pole pieces
³ ³ [ ³Shaped can be shaped to produce a
³ \ /~~\ \ ³Pole log or true square law scales.
³ \ [COIL] \ ³Piece
³ \ \__/ \³
\__] |
|
Central
Iron
HIGH SENSITIVITY ..--ÄÄÄ--.,
Improvements to the basic design for / _..._ \
high sensitivity use powerful circular ³ N/~ ~\S ³
ferrite magnets both on the outside & ³N³ /S~N\ ³S³
inside of the coil, The penalty is ³N³ ³S N³ ³S³
poor scale accuracy, but several times ³N³ \S_N/ ³S³
the sensitivity can be achieved. ³ N\..___../S ³
e.g. a large 6" meter with a 15uA 6kê coil. \,,__ __../
~~~
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 high sensitivity.
MOVING IRON
This is a different system magnetic repulsion. 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
.-Ä~~~~Ä-, RMS AC current as long as the
/ _.._ \ Coil coil current is the same
Pointer / / ÜÜ\ \Bobbin (inductance) & there is
==============@_Ü ³ ³ negligible iron losses at the
\ \_ _/ / AC frequency (e.g. ok DC to 1kHz?).
\ ~~ / The sensitivity is much lower
~Ä.____.Ä~ than moving coil meters.
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 ÄÄÂÄÄ
These work in a similar way ÚÄı³±Ä¿
to the moving iron, but high ³ ³ ³
voltage static is used on 2 / ³ \
conductors & they try to repel. / ³ \
The normal example is the gold ³ free/³ ³
leaf electroscope. Versions ³leaf/ ³ ³
have been made calibrated in ³ ÁÁÁÁ ³
kV. e.g. 0-50kV & of course ³ scale ³Glass
no current flows! ÀÄÄÄÄÄÄÄÄÄÄÄÄÙBottle
See meter Tech buls on...
Lafayete 57 Range Multimeter
Meter Damping & Speed Up
Edgcume Peebles Earth Loop Z
Edgcume Peebles Megger & Low R
PEP Meter modification
Mains Peak/RMS/Mean Meter
Marconi Distortion Meter TF2337
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
