Unit 2 Option A: Resistors, Thyristors, and the NPN Transistor
Welcome to your study notes for Electronic and Microelectronic Control Systems! Electronic circuits are everywhere—from the automatic night light in your hallway to the burglar alarm protecting a building. In this unit, we will explore three vital electronic components: Resistors, NPN Transistors, and Thyristors. By the end of this guide, you will understand how these components control current, make automatic switching decisions, and latch circuits in real-world systems.
---1. Resistors in Electronic Circuits
A resistor is a passive electronic component designed to regulate, limit, or divide the flow of electrical current and voltage in a circuit. Without resistors, sensitive components like light-emitting diodes (LEDs) and transistors would receive too much current and burn out!
Ohm's Law
The relationship between voltage, current, and resistance is defined by Ohm's Law:
\(V = I \times R\)
Where:
• \(V\) = Voltage in volts (\(\text{V}\))
• \(I\) = Current in amperes (\(\text{A}\))
• \(R\) = Resistance in ohms (\(\Omega\))
Types of Resistors
1. Fixed Resistors: These have a set, unchangeable resistance value (usually made from carbon film or metal film). They are commonly used as current-limiting resistors to protect LEDs or the base terminal of a transistor.
2. Variable Resistors (Potentiometers and Rheostats): These allow you to adjust the resistance manually by turning a dial or sliding a track. They are essential for setting sensitivity levels or thresholds in sensing circuits (for example, adjusting the darkness level at which an automatic light turns on).
The 4-Band Resistor Colour Code
Because fixed resistors are physically small, their values are marked with coloured bands instead of printed numbers. Standard resistors use a 4-band colour code system:
• Band 1: First significant digit
• Band 2: Second significant digit
• Band 3 (Multiplier): The power of ten to multiply by (\(10^n\))
• Band 4 (Tolerance): The accuracy range of the resistor value (e.g., Gold = \(\pm 5\%\), Silver = \(\pm 10\%\))
Resistor Combinations (Series and Parallel)
When multiple resistors are connected together, their combined total resistance (\(R_T\)) is calculated using specific formulae:
Series Resistors: Connected end-to-end in a single line. The total resistance increases because the current must pass through each resistor one after another:
\(R_T = R_1 + R_2 + R_3 + \dots\)
Parallel Resistors: Connected side-by-side across the same two points. The total resistance decreases because the current has multiple paths to flow through:
\(\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \dots\)
The Potential Divider Circuit
A potential divider (or voltage divider) uses two resistors connected in series across a power supply to split or divide the supply voltage (\(V_{in}\)) into a smaller, usable output voltage (\(V_{out}\)).
The Potential Divider Formula:
\(V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}\)
Where \(R_1\) is the top resistor connected to the positive supply, and \(R_2\) is the bottom resistor connected to \(0\text{ V}\) (ground), across which \(V_{out}\) is measured.
Analogy: Think of a potential divider like a pair of water valves sharing water pressure. If you increase the resistance of the bottom valve (\(R_2\)), more pressure (\(V_{out}\)) builds up across it!
Quick Review — Resistors:
• Resistors limit current and divide voltage.
• Series resistance adds up: \(R_T = R_1 + R_2\).
• Potential dividers provide a variable switching voltage for transistor circuits.
2. The NPN Bipolar Junction Transistor
The NPN transistor is one of the most important components in modern electronics. It acts as an electronic switch or amplifier. Unlike a mechanical switch that you press with your finger, a transistor is turned on and off by a small electrical voltage and current.
Transistor Terminals and Symbol
An NPN transistor has three terminals:
1. Base (B): The control terminal (the "trigger").
2. Collector (C): The input terminal where the main current enters from the positive supply.
3. Emitter (E): The terminal where the current exits to the ground line (\(0\text{ V}\)).
Memory Trick for the Circuit Symbol: Look at the arrow on the Emitter! For an NPN transistor, the arrow points outward (away from the Base) — Not Pointing iN (NPN).
How the NPN Transistor Works as a Switch
Don't worry if this seems tricky at first; think of the transistor as an electronically controlled tap:
• OFF State (Cut-off region): When the voltage between the Base and Emitter (\(V_{BE}\)) is below approximately \(0.6\text{ V} - 0.7\text{ V}\), the transistor stays OFF. No current flows from Collector to Emitter (\(I_C = 0\)).
• ON State (Saturation region): When \(V_{BE}\) reaches or exceeds \(0.7\text{ V}\), a small base current (\(I_B\)) flows into the Base terminal. This turns the transistor fully ON, allowing a much larger collector current (\(I_C\)) to flow from the Collector to the Emitter.
The Base Protection Resistor
The Base-Emitter junction inside a transistor is delicate. If connected directly to a high voltage, excessive current would destroy it immediately. Therefore, a fixed base protection resistor is always placed in series with the Base terminal to limit the base current (\(I_B\)) to a safe level.
Practical Applications
NPN transistors are used to interface low-power sensors (such as Light Dependent Resistors / LDRs or thermistors in a potential divider) with higher-current output devices like buzzers, motors, lamps, or relay coils.
Key Takeaway for Transistors: An NPN transistor requires at least \(0.7\text{ V}\) at its Base to switch ON. As soon as the Base voltage drops below \(0.7\text{ V}\), the transistor instantly switches OFF.
---3. The Thyristor (Silicon-Controlled Rectifier / SCR)
A thyristor is a semiconductor switching component with a special feature called latching. While a transistor turns off the moment its control signal drops, a thyristor stays switched on until it is manually reset!
Thyristor Terminals and Symbol
A thyristor has three terminals:
1. Anode (A): The positive terminal where the main load current enters.
2. Cathode (C): The negative terminal where the main current exits.
3. Gate (G): The control terminal used to trigger the device.
Circuit Symbol: The symbol looks like a standard diode (triangle pointing to a vertical bar) with an angled terminal line attached near the cathode side for the Gate.
Operating Principle: The Latching Action
1. Normal State (OFF): Initially, the thyristor blocks current. No current flows from Anode to Cathode.
2. Triggering (ON): When a small positive pulse or current is supplied to the Gate terminal, the thyristor turns ON. Current flows freely from Anode to Cathode, powering the output device (e.g., a siren).
3. Latching (Staying ON): Even if the Gate pulse is completely removed or disconnected, the thyristor remains ON (latched)!
Everyday Analogy: Think of a mouse trap. A tiny tap on the trigger snaps the trap shut. Once it has snapped shut, taking your hand away won't open it again—it stays locked shut until you physically reset it!
How to Reset (Unlatch) a Thyristor
Because the Gate terminal completely loses control once conduction starts, you cannot turn off a thyristor by grounding or disconnecting the Gate. To turn it OFF, you must stop the current flowing between the Anode and Cathode using one of two methods:
Method 1 (Series Interruption): Place a normally closed (NC) push switch in series with the Anode. When pressed, the switch opens, breaking the circuit path and stopping the current.
Method 2 (Parallel Shorting): Place a normally open (NO) push switch in parallel across the Anode and Cathode. When pressed, the switch temporarily diverts all current around the thyristor. With no current flowing through the thyristor, it unlatches and resets to OFF.
Practical Applications of the Thyristor
• Burglar Alarms: If an intruder steps on a pressure mat for just a split second, a gate pulse triggers the thyristor. The alarm siren latches ON and keeps sounding even after the intruder steps off the mat.
• Steady-Hand Games: Touching the metal loop against the wire track sends a momentary pulse to the Gate. The buzzer turns ON and stays latched ON to indicate a fail until the reset button is pressed.
4. Summary Comparison: Transistor vs. Thyristor
Understanding the difference between these two switching components is a favourite topic for CCEA examiners!
NPN Transistor:
• Terminals: Base (B), Collector (C), Emitter (E)
• Control Action: Non-latching. Needs a continuous Base voltage of \(\ge 0.7\text{ V}\) to stay ON.
• How it turns OFF: Automatically turns OFF when the Base voltage drops below \(0.7\text{ V}\).
• Typical Use: Automatic night lights, temperature-controlled fan switches.
Thyristor:
• Terminals: Anode (A), Cathode (C), Gate (G)
• Control Action: Latching. A brief positive pulse on the Gate locks it ON permanently.
• How it turns OFF: Requires an external reset switch to break or divert the Anode-Cathode current.
• Typical Use: Security alarms, latching warning systems, steady-hand games.
5. Common Pitfalls & Examiner Warnings
• Mistake 1: Confusing Terminals. Remember: Transistors have Base, Collector, Emitter. Thyristors have Anode, Cathode, Gate. Do not mix them up in circuit diagrams!
• Mistake 2: The Inward Arrow. On the NPN transistor symbol, the arrow must always point outward on the Emitter lead (Not Pointing iN).
• Mistake 3: Trying to reset a thyristor using the Gate. Once latched, the Gate has zero control. Grounding or cutting the Gate wire does NOT switch off a thyristor in a DC circuit.
• Mistake 4: Inverting Potential Divider Resistors. Always check which resistor is \(R_1\) (connected to positive rail) and which is \(R_2\) (connected to \(0\text{ V}\)). In the formula \(V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2}\), \(V_{out}\) is measured across \(R_2\).
• Mistake 5: Forgetting the Base Resistor. Always include a protective fixed resistor in series with the transistor's Base terminal when drawing circuit diagrams.