Lesson Notes By Weeks and Term v4 - SHS 2
ELECTRONIC COMPONENTS AND CIRCUITS
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ELECTRONIC COMPONENTS AND CIRCUITS
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Subject: Applied Technology
Class: SHS 2
Term: 2nd Term
Week: 20
Grade code: 2.5.2.LI.11
Strand code: 5
Sub-strand code: 2
Content standard code: 2.5.2.CS.1
Indicator code: 2.5.2.LI.11
Theme: ELECTRICAL AND ELECTRONIC TECHNOLOGY
Subtheme: ELECTRONIC COMPONENTS AND CIRCUITS
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Welcome, future engineers and technicians! Have you ever wondered how the ECG official reads your home electricity meter, or how a mechanic knows if a car battery is working correctly? They use measuring instruments like ammeters and voltmeters. Many of the rugged, reliable analogue meters we see are based on the Moving Iron Instrument. These instruments, in their basic form, can only measure very small currents or voltages. This lesson is crucial because it teaches us the fundamental technique of modifying these simple instruments to measure much larger, more practical values of current and voltage.
This topic is broken down into two main parts: understanding the instrument itself, and then learning how to modify it. Part 1: The Moving Iron (MI) Instrument
A moving iron instrument is a type of meter that works on the magnetic effect of electric current. It does not have a permanent magnet.
A. Principle of Operation The core principle is simple: when a piece of soft iron is placed in a magnetic field, it becomes magnetized. The force produced by this magnetism (either attraction or repulsion) causes the iron to move. This movement is then used to deflect a pointer across a calibrated scale.
There are two main types: Attraction Type: A single piece of soft iron (the "vane") is attached to the pointer's spindle. When current flows through a nearby coil, it creates a magnetic field. This field attracts the soft iron vane, pulling it towards the centre of the coil. The stronger the current, the stronger the magnetic field, and the more the vane is pulled, causing a larger deflection of the pointer. Repulsion Type: This type has two soft iron vanes placed inside the coil. One vane is fixed, and the other is movable (attached to the spindle). When current flows through the coil, both vanes are magnetized with the same polarity (e.g., both become North poles on the same end). Since like poles repel, the movable vane is pushed away from the fixed vane. This repulsion force causes the pointer to move across the scale. The repulsion type is more common because it provides a more uniform scale.