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G8MNY  > TECH     17.05.23 09:05z 149 Lines 6831 Bytes #15 (0) @ WW
BID : 47658_GB7CIP
Subj: Meter Movement Types
Sent: 230517/0854Z @:GB7CIP.#32.GBR.EURO #:47658 [Caterham Surrey GBR]

By G8MNY                                    (Updated Apr 23)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

There are several ways to move a meter from a current or voltage. The most
common type is the moving coil..


   /~   Horse    ~\                        Ţ
  │  Shoe Magnet   │     Needle Pointer  ((@))
 │   /~~~~~~~~~~\   │     ================/Ţ~/|
│   │            │   │                   /  / |
 │   │_        _│   │                   │~~│| |
  │    / /~~\ \    │                    │  │/ /Coil
   │  │ [COIL] │  │Pole                 │  │ /On former
    \  \ \__/ /  │Piece                 │__│/  
     ~~~  |   ~~~                          ŢAxle
          |                   Hair spring((@))
      Central                              Ţ
                                       Point Bearing

The coil is free to move in the gap between central iron & the shaped magnet
poles. It is electrically 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 that can bypass some 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                                            |torsion suspension
A pair of hair springs that wind up &        Pointer  |_
unwind are to compensate for temperature     ========/| /|
with end conical point bearings.                    /  / |
But sometimes a very fine twisting                 │~~│| |
torsion spring in used without bearings.           │  │/ /Coil
This means the meter has no friction,              │  │ /former
improving the accuracy. Drawback is                │__│/
these movements are more fragile!                     |
    /~  Horse ~\
   │ Shoe Magnet│
  │  /~~~~~~~~\  │            For some applications like
 │  /        _│   │           "light meters" the pole pieces 
│  │        [      │Shaped    can be shaped to produce a
 │  \   /~~\ \    │Pole       log or true square law scales.
  │  \ [COIL] \  │Piece
   │  \ \__/   \│
    \__] |

HIGH SENSITIVITY                                 ..--───--.,
Improvements to the basic design for            /   _..._   \
high sensitivity use powerful circular         │ N/~     ~\S │
ferrite magnets both on the outside &          │N│  /S~N\  │S│
inside of the coil, The penalty is             │N│ │S   N│ │S│
poor scale accuracy, but several times         │N│  \S_N/  │S│
the sensitivity can be achieved.               │ N\..___../S │
e.g. a large 6" meter with a 15uA 6k coil.      \,,__   __../
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. In an AF distion meter the tone is applied to
the electromagent & the filtered hamonics only tones to the moving coil, giving
a level independant read out, ideal for tape recorders with drop out & playback

Another approach is to use a magnet but use 2 coils & no return springs. this
gives high sensitivity.

This is a different system magnetic repulsion. It uses 2 pieces of iron inside
a coil. As the coil is energised the flux on each piece of iron causes them to
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        coil current is the same
Pointer  /   /  ▄▄\   \Bobbin      (inductance) & there is
==============@_▄  │   │           negligible iron losses at the
         \   \_  _/   /            AC frequency (e.g. DC to 1kHz?)
          \    ~~    /             The sensitivity is much lower
           ~─.____.─~              than moving coil meters.

The scale is non linear & a easy tell tale of a moving iron movement.

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        │ free/│     │
leaf electroscope. Versions           │leaf/ │     │
have been made calibrated in          │   ┴┴┴┴     │
kV. e.g. 0-50kV & of course           │  scale     │Glass
no current flows!                     └────────────┘Bottle

These are typicaly used on slow instruments like petrol gauges. A bimetal strip
has a heating element rapped around it. It's bending is geared up to work a
          Heater  /Pointer
    O>==[]======.o System
 Zero  Gain
clamp  Clamp

Two fixing clamps along the bimetal strip enable calibation. And if a simular
bimetal controlled voltage on/off switch (regulator) is used at the same
location as the display, to supply the referance power, then voltage &
temperature errors are all eliminated! But the switch can be an anoying source
RF noise in the dashboard!

METER STICKING (From feedback from Osvaldo LW1DSE)
Plastic meter movements can wrarp over time causing the needle pointer to foul
the glass or scale. Also I have found that static can be a problem (rubbed
glass), causing mis readings, the cure is a slow breath on the glass to
discharge it. Sometimes sticking is due to overtight bearings. More difficult
to fix is iron or rust particles in the magnet gap. These can be removed with
fine steel pin/wire, as the fine point concentrates the magnetism so the
magnetic dust is more attracted to the wire pin then the powerful pole pieces!

See meter Tech buls on... "Lafayete 57 Range Multimeter", "Meter Damping &",
Speed Up", "Marconi Distortion Meter TF2337","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

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