Lesson Notes By Weeks and Term v4 - SHS 3
ELECTRONIC COMPONENTS AND CIRCUITS
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ELECTRONIC COMPONENTS AND CIRCUITS
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Applied Technology
Class: SHS 3
Term: 1st Term
Week: 1
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! In our daily lives in Ghana, from the ECG meter outside our homes to the work of the auto-electrician at Abossey Okai, measuring electricity is crucial. How do we know if the voltage from the socket is correct? How do we know how much current a phone charger is drawing? We use measuring instruments. Today, we will study a very common and robust type of instrument called the Moving Iron (MI) Instrument. More importantly, we will learn the practical skill of modifying a basic MI instrument to measure a wide range of voltages and currents, turning it into a useful voltmeter or ammeter for real-world tasks.
Part 1: The Moving Iron (MI) Instrument
A Moving Iron instrument is an electromechanical device used for measuring AC or DC voltage or current. Its operation is based on the magnetic effect of electric current.
Principle of Operation: The core principle is that a piece of soft iron, when placed in a magnetic field, will be acted upon by a force. The magnetic field is produced by the current flowing through a coil. The strength of this force depends on the strength of the magnetic field, which in turn depends on the magnitude of the current. This force causes the soft iron to move, which then moves 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 and is drawn or *attracted* into the magnetic field of the coil when current flows. The stronger the current, the stronger the attraction, and the further the pointer moves. Repulsion Type (more common): Two soft iron vanes are placed inside the coil. One is fixed, and the other is movable (attached to the pointer). When current flows, 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 repulsive force moves the pointer. The deflection is proportional to the square of the current (I²), which is why it can measure both AC and DC. For AC, it measures the RMS (Root Mean Square) value.