1. SWITCHES, FUSES, & CIRCUIT BREAKERS

2. OVERVIEW
Switches
Fuses
Circuit Breakers

3. Switches
A switch is a device used to open and close electrical paths, thereby either stopping the current flow or allowing the current to flow in an electrical circuit or through an electrical component.

4. Switches
There are a wide variety of switches. The wide variety of switches available differ mechanically, electrically, and in the arrangement of switch contacts.
Arm
Single-Pole, Single Throw Switch (SPST) A single pole switch is a single circuit switch– it closes or opens only one circuit path. The single pole switch has one moving arm and one stationary contact. Example: ON/OFF Switch
(SPST)
Contact
A schematic representation of the simplest, most common arrangement of switch contacts.

5. Switches
There are a wide variety of switches. The wide variety of switches available differ mechanically, electrically, and in the arrangement of switch contacts.
Arm
Contact 1
Single-Pole, Double Throw Switch (SPDT) A single pole switch is a single circuit switch– it closes or opens only one circuit path. The single pole switch has one moving arm and two stationary contacts. Example: High/ Low beams in your car.
(SPDT)
Contact 2
A schematic representation of the simplest, most common arrangement of switch contacts.

6. Switches
There are a wide variety of switches. The wide variety of switches available differ mechanically, electrically, and in the arrangement of switch contacts.
Contact 1
(throw)
Double-Pole, Single Throw Switch (DPST) This double pole switch is a single circuit switch– it closes or opens only one circuit path. The double pole switch has two moving arms and two stationary contacts. The dotted line indicates the two arms move simultaneously and are part of the same assembly. Example: Dual ON/OFF switch.
Arms
(DPST)
Contact 2
(throw)
A schematic representation of the simplest, most common arrangement of switch contacts.

7. Switches
There are a wide variety of switches. The wide variety of switches available differ mechanically, electrically, and in the arrangement of switch contacts.
Contacts 1
(throw)
Double-Pole, Double Throw Switch (DPDT) The multi-pole switch has two moving arms and two sets of stationary contacts. The dotted line indicates the two arms move simultaneously and are part of the same assembly. Example: polarity reversal
Arms
(DPDT)
Contacts 2
(throw)
A schematic representation of the simplest, most common arrangement of switch contacts.

8. Switches
When looking at a schematic, switches are shown as normally open (NO) or normally closed (NC) position.

9. Switches
There is a large variety of switches. The most common switches are toggle switches, slide switches, rotary switches and push button switches.
The toggle switch is probably the most familiar switch. Most of the wall switches for the lights in our home are toggle switches.

10. Switches
There is a large variety of switches. The most common switches are toggle switches, slide switches, rotary switches and push button switches.
The slide switch is another very popular mechanical device. Usually slide switches are used on instruments directly mounted on printed circuit boards where material costs must be kept low.

11. Switches
There is a large variety of switches. The most common switches are toggle switches, slide switches, rotary switches and push button switches.
The rotary switch is primarily used as a selector switch. They are very popular in electronics because they can be designed in many different ways, which lets the design engineer choose many options as how to perform switching operation.

12. Switches
There is a large variety of switches. The most common switches are toggle switches, slide switches, rotary switches and push button switches.
The push button switch comes in various combinations of contacts, in many shapes and sizes as well as in ganged configurations. These switches are primarily used for control applications on the front panel of instruments or other electronic equipment.

13. Fuses and Circuit Breakers
Protection devices protect you and your equipment from electrical damage. Most electrical equipment has built in protection that will sense an increase in current. The protection device immediately shuts down the circuit.
There are two types of protection devices used in the electronics industry: Fuses and Circuit Breakers. These two types of devices are different, but the principle of their operation is the same, and they produce the same results.

14. Fuses
A fuse is the simplest and least expensive circuit protection device available. A fuse is a metallic component which is placed within an electrical circuit to monitor the current that passes through the circuit.
A shorted circuit in an electrical system can cause excessive current to flow through some of the components. These components heat up and eventually burn up, perhaps destroying the whole electrical system. Fortunately, having an inexpensive built-in protection device, such as a fuse, to quickly sense the short circuit condition and immediately shut the electrical circuit will prevent equipment damage.
Schematic symbol

15. Fuses
A fuse is the simplest and least expensive circuit protection device available. A fuse is a metallic component which is placed within an electrical circuit to monitor the current that passes through the circuit.
If the current in a protected circuit becomes excessive the metal part inside of the fuse heats up and melts, opening the electrical circuit the fuse is protecting.
Blown Fuses

16. Fuses
A fuse is the simplest and least expensive circuit protection device available.
Fuses have ratings. The most important rating is the current rating, which is stated in amps. The amperage value is marked on every fuse. This value is the maximum current level that the fuse can carry.
In addition to ratings, we have two categories of fuses: fast-acting and slow-blow. A fast-acting fuse blows within about one second after it senses an overload condition. A slow-blow fuse blows between one and thirty seconds after it senses an overload condition, depending on the amount of the overload.

17. Circuit Breakers
A circuit breaker is really two things in one: a switch and an indestructible fuse. The function of the circuit breaker is the same as a fuse.
There are two different kinds of circuit breakers: Thermal and magnetic
Schematic symbol

18. Circuit Breakers HEATING ELEMENT
THERMAL Circuit Breakers work on the principle of temperature rise in the activators sensing element. The sensor is usually a thermal bimetallic strip which heats up when current flow is excessive. Thermal breakers are not as convenient as magnetic breakers, because thermal breakers must cool down before they can be reset.
A1 (NC)
Input
B1 (NO)
HEATING ELEMENT

19. Circuit Breakers
MAGNETIC Circuit Breakers work on the principle of magnetism. The current flows through a coil in the circuit breaker housing. As the current flows, it develops a magnetic field. The strength of the magnetic field depends on the amount of current that flows through the circuit and the coil. If the current exceeds the rating of the breaker, the magnetic field becomes strong enough to produce a force to trip the breaker. This force attracts the pole away from the throw. They are very common in the electrical field, especially in house wiring circuits.

20. Summary
Understanding switch, fuse, and circuit breaker operation and construction are essential requirements for an electronics technician. Some of the key points of the lesson are listed below:
A switch is a mechanical device used to open and close electrical current paths.
The wide variety of switches available differ mechanically, electrically, and in the arrangement of switch contacts. SPST, SPDT, DPST, DPDT, etc.
The most common switches are toggle, slide, rotary and push button.
There are two types of protection devices used in the electronics industry: Fuses and Circuit Breakers.
Fuses are low cost metallic protection devices, which heat up, melt and are destroyed when the current rating is exceeded.
Circuit Breakers are indestructible fuses which open or trip a circuit when they sense an overload condition.