Analog Panel Meters (PMMC / Moving Iron / Galvanometer)
Test Instrument
The needle instruments of the Measurements syllabus: the PMMC (D'Arsonval) movement behind every classic ammeter and voltmeter, its moving-iron sibling for AC, and the galvanometer they all descend from. Torque equations, shunts and multipliers — exam bread and butter.
What it looks like
Analog Panel Meters (PMMC / Moving Iron / Galvanometer)
A typical test instrument — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Full-scale deflection
µA–A / mV–VThe movement's basic sensitivity — a typical PMMC movement is 50 µA/few-hundred mV FSD; everything bigger is shunts and multipliers.
Shunt / multiplier
ΩAmmeter = movement + parallel shunt (Ish = I − Im); voltmeter = movement + series multiplier (Rs = V/Im − Rm). The two most-set numericals in the subject.
Accuracy class
class 0.5–2.5Percent-of-FSD error class marked on the dial — class 1.0 panel meters are standard switchboard grade.
Ohms per volt
Ω/VVoltmeter sensitivity = 1/Ifsd. A 20 kΩ/V meter loads circuits far less than a 1 kΩ/V one — the loading-effect exam question.
Damping & control
—Spring control + eddy-current damping (PMMC) or air damping (MI): the deflecting/controlling/damping torque trio every viva asks.
Markings
Dial symbols tell all: a horseshoe magnet symbol = PMMC (DC), a curved iron piece = moving iron (AC/DC), class number, mounting position symbol, and the scale's own linearity reveals the type (square-law cramped start = MI).
Standard values
Switchboard meters: 96×96 mm or 72×72 mm squares, 0–500 V and CT-fed 0–5 A ranges (dial rescaled to the CT primary, e.g. 0–100 A via 100/5 CT). Education kits: 50 µA and 1 mA movements for building meters.
How to choose
1) DC circuits: PMMC — linear scale, high sensitivity, but reads average (wrong on AC unless rectifier type). 2) AC mains panels: moving iron — true RMS regardless of waveform, rugged, cheap. 3) Panel retrofits: digital LED meters with the same cutout size. 4) For lab meter-building: pick the movement, then compute shunt/multiplier. 5) CT/PT-fed meters must match the transformer secondary (5 A/110 V).
Pinout & package
Two terminals (+/− on DC types); switchboard cases mount through square panel cutouts with rear studs. Galvanometers add a mirror-and-lamp scale in lab versions. Shunts for large currents are external manganin blocks with millivolt leads.
Example circuits
- Convert a 50 µA, 2 kΩ movement into a 0–10 V voltmeter (multiplier ≈ 198 kΩ) — the standard numerical
- Ammeter + external 75 mV shunt reading 0–200 A on a battery charger
- Ballistic galvanometer measuring charge in the magnetism lab
- MI voltmeter + MI ammeter + dynamometer wattmeter around a single-phase load — the classic measurement bench
Common failures
Bent/sticking pointer from shock, open coil after gross overload (needle slams and dies), weakened magnet or fatigued springs reading low, and moisture-fogged dials. MI meters survive abuse that kills PMMC movements.
How to test
Zero-check mechanically (adjust screw), then compare against a trusted DMM at several points across the scale. A PMMC movement can be gently tested with a resistor-limited cell — full deflection at its rated µA. Check shunt/multiplier resistors when a rebuilt meter reads proportionally wrong.
Substitutes
Digital panel meters replace analog in the same cutouts (mind CT/shunt compatibility). PMMC-with-rectifier substitutes MI at low currents but mis-reads distorted waveforms. For trend-watching (tuning, nulling), many still prefer the needle — hence analog multimeters survive.