Lesson 5: Switches and More
Recommended Article : Circuit Theory
1. Switch
2. Protective Device
3. Grounding
4. d’Arsonval Meter
a. How to Use a Digital Multimeter
c. How to Use a Function Generator
1. Switch
⑴ Switch Notation
① Pole, Throw
○ Pole : Number of movable arms
○ Throw : Number of contact points
② Notation Types
Figure. 1. Types of Switch Notation
○ Example : SPST stands for single-pole single-throw
○ Example : NOPB stands for normally-open push-button
③ Initial State (Example : SPST)
○ Initially open
Figure. 2. Notation for initially open
○ Initially closed
Figure. 3. Notation for initially closed
④ Simultaneity of Connection and Disconnection
○ Disconnection from one throw and connection from another throw cannot occur simultaneously.
○ break before make (Example : SPDT)
Figure. 4. Notation for break before make
○ make before break (Example : SPDT) : Note the hooked metal between a and b is fixed
Figure. 5. Notation for make before break
⑵ Switch-Resistor Circuit : Floating occurs when only the switch is connected to the load terminal
① Pull-up resistor : Resistor placed above the switch. Pressing the switch gives LOW, releasing gives HIGH
Figure. 6. Pull-up resistor
② Pull-down resistor : Resistor placed below the switch. Pressing the switch gives HIGH, releasing gives LOW
Figure. 7. Pull-down resistor
⑶ Switch Mechanism
① Toggle Switch (Example : SPDT)
Figure. 8. Structure of a Toggle Switch
Three conductors are shown on the left, where wires are connected. The right bar is manually moved, and the right bar’s middle part is fixed. The illustrated state shows wires connected to the upper and middle conductors. If the right bar is lifted, the opposite side goes down, and the hooked metal at the end of the bar pulls the metal in the shape of へ downward. Wires are connected to the middle and lower parts. A spring within the right bar firmly secures it.
② Other Mechanical Devices
Figure. 9. Different Types of Mechanical Switch Devices
③ Switching with Transistors in Semiconductor Circuits
⑷ Switch Performance
① Breakage : How effectively the switch blocks signals when two conductors are disconnected
○ As semiconductor circuits shrink, blocking performance deteriorates
② Signal Loss : How much energy is lost as signals pass through the switch
○ Some circuits require amplifiers due to signal loss
2. Protective Device
⑴ Fuse
① As excessive current flows through the wire and power increases, it melts and becomes irreusable
② Type F (fast-acting fuse) : For situations requiring quick response
③ Type T (time-delay fuse) : Disconnects after a specific time of accumulated power
④ Fuse Notation
Figure. 10. Notation for Fuses
⑤ Types of Fuses
Figure. 11. Types of Fuses
⑵ Circuit Breaker
① Reusable, but must be reset mechanically
② Circuit breaker using the heating effect of current : A bimetal spring opens the circuit
③ Circuit breaker using the magnetic field of current : Lorentz force opens the circuit
④ Circuit Breaker Notation
Figure. 12. Notation for Circuit Breakers
⑤ Circuit Breaker Types
Figure. 13. Circuit Breaker Types
3. Ground
⑴ Reference point in an electrical circuit
⑵ Earth Ground : A metal conductor 8 feet long connected to the earth for current flow
⑶ Reference Ground : Zero potential point. Often denoted as COM or COMM
⑷ Notation for Grounding
Figure. 14. Notation for Grounding
4. d’Arsonval Meter
⑴ Overview
d’Arsonval meter is an instrument where the needle rotates proportionally to the flowing current. If the rated current is 1 mA and the rated voltage is 50 mV, when 1 mA flows through the meter, the voltage drop across the meter is 50 mV, and the needle points to full scale. (Thus, resistance = 50 Ω)
⑵ Mechanical Mechanism of d’Arsonval Meter Movement
Figure. 15. Structure of a d’Arsonval Meter
The mechanical mechanism of the d’Arsonval meter utilizes Lorentz force. A constant magnetic field is provided by a permanent magnet. When current passes through the moving coil, torque is generated, causing the pointer to rotate clockwise. Control springs above and below, however, provide counteracting torque, resulting in an equilibrium state. The value read from the meter represents this equilibrium state.
Figure. 16. Structure of a d’Arsonval Meter
Figure. 17. Structure of a d’Arsonval Meter
Looking more closely, even a small angle of movement of the moving coil quickly points the pointer to full scale. The direction of the moving coil’s plane vector (pointer direction) is relatively perpendicular to the magnetic field direction. For further analysis, refer to the following diagram.
Figure. 18. Conceptual Diagram of a d’Arsonval Meter
The Lorentz force acts in the direction of the red arrow in this case. (Lorentz forces in other wires cancel out.) Thus, torque causes the pointer to rotate clockwise. The degree of freedom from above is as follows.
Figure. 19. Conceptual Diagram of a d’Arsonval Meter
The sizes of the red arrows marked here are labeled as Bil. The blue arrow represents the torque of the control spring, with a size of κ(θ0 - θ). (θ0 is the initial angle of the pointer.) Therefore, the following equation can be established.
Here, I is the inertia moment of the d’Arsonval meter, a purely mechanical element. While it’s difficult to solve this equation completely, it can be inferred that it undergoes simple harmonic motion.
However, due to friction, the amplitude of this motion will gradually decrease. As a result, it will reach an equilibrium state, where the right side of the equation becomes 0.
Considering the angle θ ranges around 90 degrees, the following equation is derived. Note that the value we read from the meter is θ0 - θ. Be aware that beyond the full scale, the meter’s reading is not linear with the current.
⑶ Voltmeter
① Construction
Figure. 20. Voltmeter Construction
The properties of the core’s material determine the rated current and voltage of the d’Arsonval meter. Consequently, the internal resistance Rv of the d’Arsonval meter is limited. Therefore, voltmeters are constructed in the manner shown above. When voltage V is applied across the terminals, the voltage across the d’Arsonval meter is as follows.
Thus, this system has a rated current of 1 mA, and the rated voltage is as follows. In other words, this system linearly displays voltages from 0 V to the mentioned voltage. (Different resistances are required for each full-scale count.)
② Circuit Connection
○ Connect in parallel to the terminals being measured
○ Always consider the internal resistance of the voltmeter : Note that the internal resistance of the voltmeter is very high
○ All resistances connected in parallel have the same voltage
Figure. 21. Circuit Connection of a Voltmeter
⑷ Ammeter
① Construction
Figure. 22. Ammeter Construction
The properties of the core’s material determine the rated current and voltage of the d’Arsonval meter. Consequently, the internal resistance Rv of the d’Arsonval meter is limited. Therefore, ammeters are constructed in the manner shown above. When voltage V is applied across the terminals, the voltage across the d’Arsonval meter is V. Additionally, the total current flowing through the terminals is as follows.
Thus, this system has a rated voltage of 50 mV, and the rated current is as follows. In other words, this system linearly displays currents from 0 A to the mentioned current. (Different resistances are required for each full-scale count.)
② Circuit Connection
○ Open the terminals to be measured and connect the ammeter in series
○ Always consider the internal resistance of the ammeter : Note that the internal resistance of the ammeter is very low
○ All resistances connected in series have the same current
Figure. 22. Circuit Connection of an Ammeter
⑸ Types of d’Arsonval Meters
① Digital Multimeter (DMM)
○ Measures voltage, current, resistance, etc.
○ Typically has an internal resistance of 10 to 11 MΩ
○ LCD Display : Long operation (2,000 hours or more, 9V), hard to see in the dark, slow response
○ LED Display : Short-term operation, visible even in the dark, high response
② LRC Meter
○ Measures physical quantities of capacitors and coils, unlike DMM
○ Capacitance meter : Measures capacitance C, considering polarity. If the capacitor is deteriorated, its resistance decreases and can be detected by a regular resistance meter
Figure. 23. LRC Meter
○ Inductance meter
③ Galvanometer
○ Needle moves only when current flows
④ Oscilloscope
○ Measures voltage changes over time and displays them on a screen
○ Control 1: VERTICAL : Adjusts voltage values (Position, VOLTS/DIV)
○ Control 2: HORIZONTAL : Adjusts time axis (Position, SEC/DIV)
○ Control 3: TRIGGER : Detects voltage changes
⑤ Function Generator
○ Generates signals like sine, square, triangle waves
⑹ Resolution and Accuracy
① Resolution
○ Most DMMs can display 3 digits and 1 decimal point (period)
○ 3 ½ digits, 4½ digits, 8½ digits are also available
○ Smaller measurement values have better resolution
○ Example : Up to 1.999 V, resolution is 0.001 V, from 2.00 V, resolution is 0.01 V
② Accuracy
○ Ordinary DMMs have accuracy from 0.01% to 0.5%
○ High-performance DMMs can have accuracy as low as 0.002%
Input: 2016-01-01 13:11
Updated: 2022-09-14 11:41