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Annex 1 (BE01) — Lesson BE.01.05 • TLO: 28XX-MAIN-1002 / 2800-ACT-1303 • ELOs: 28XX-MAIN-1002j, 1002k, 1002l

Switches

Mechanical switches control current flow by intentionally opening or closing conductive paths between power sources and loads. Identifying switch configurations by their poles, throws, and normal resting state—and recognizing the voltage, current, and resistance signatures of open versus closed contacts—is foundational to circuit diagnostics.

1. Switch Terminology: Poles, Throws & Categories

Every mechanical switch is defined by two structural parameters:

Poles (Movable Contacts / Arms): The number of electrically independent movable wipers or contact arms inside the switch. Each pole connects to its own separate input conductor/circuit.
Throws (Output Positions): The number of distinct stationary contact positions to which each pole can be moved in order to complete a path to a load.

Two Primary Switch Classifications in `BE.01.05`

Toggle & Slide Switches: Maintained-contact mechanical switches designated by their number of poles and throws (SPST, SPDT, DPST, DPDT). Once actuated, the switch remains in that position until physically toggled back.
Push-Button Switches: Momentary-contact Single-Pole Single-Throw (SPST) switches designated by their normal (unpressed) resting condition:
- PBNO (Push Button Normally Open): Contacts remain open at rest and close only while the button is depressed.
- PBNC (Push Button Normally Closed): Contacts remain closed at rest and open only while the button is depressed.

2. Switch Types, Schematic Symbols & Applications

<!-- 2. SPDT -->
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<circle cx="295" cy="65" r="4.5" />
<line x1="299" y1="63" x2="346" y2="47" />
<circle cx="350" cy="45" r="4.5" />
<line x1="355" y1="45" x2="390" y2="45" />
<circle cx="350" cy="85" r="4.5" />
<line x1="355" y1="85" x2="390" y2="85" />

<!-- 3. DPST -->
<line x1="480" y1="45" x2="515" y2="45" />
<circle cx="520" cy="45" r="4.5" />
<line x1="524" y1="43" x2="572" y2="22" />
<circle cx="575" cy="45" r="4.5" />
<line x1="580" y1="45" x2="615" y2="45" />
<!-- Bottom Pole -->
<line x1="480" y1="95" x2="515" y2="95" />
<circle cx="520" cy="95" r="4.5" />
<line x1="524" y1="93" x2="572" y2="72" />
<circle cx="575" cy="95" r="4.5" />
<line x1="580" y1="95" x2="615" y2="95" />
<!-- Mechanical Ganging Line -->
<line x1="548" y1="33" x2="548" y2="83" stroke-dasharray="4,4" stroke-width="1.8" />

<!-- 4. DPDT (Toggle & Slide) -->
<!-- Toggle DPDT -->
<line x1="25" y1="195" x2="55" y2="195" />
<circle cx="60" cy="195" r="4" />
<line x1="64" y1="193" x2="106" y2="180" />
<circle cx="110" cy="180" r="4" />
<line x1="114" y1="180" x2="142" y2="180" />
<circle cx="110" cy="210" r="4" />
<line x1="114" y1="210" x2="142" y2="210" />
<line x1="25" y1="250" x2="55" y2="250" />
<circle cx="60" cy="250" r="4" />
<line x1="64" y1="248" x2="106" y2="235" />
<circle cx="110" cy="235" r="4" />
<line x1="114" y1="235" x2="142" y2="235" />
<circle cx="110" cy="265" r="4" />
<line x1="114" y1="265" x2="142" y2="265" />
<line x1="85" y1="187" x2="85" y2="242" stroke-dasharray="4,4" stroke-width="1.8" />

<!-- Slide DPDT -->
<circle cx="185" cy="190" r="4" />
<circle cx="185" cy="222" r="4" />
<circle cx="185" cy="254" r="4" />
<rect x="172" y="186" width="6" height="40" fill="currentColor" />
<circle cx="235" cy="190" r="4" />
<circle cx="235" cy="222" r="4" />
<circle cx="235" cy="254" r="4" />
<rect x="242" y="186" width="6" height="40" fill="currentColor" />
<line x1="178" y1="206" x2="242" y2="206" stroke-dasharray="4,4" stroke-width="1.8" />

<!-- 5. PBNO -->
<line x1="300" y1="230" x2="340" y2="230" />
<circle cx="345" cy="230" r="4.5" />
<circle cx="395" cy="230" r="4.5" />
<line x1="400" y1="230" x2="440" y2="230" />
<!-- Plunger suspended above -->
<line x1="338" y1="212" x2="402" y2="212" stroke-width="3" />
<line x1="370" y1="188" x2="370" y2="212" stroke-width="2.5" />

<!-- 6. PBNC -->
<line x1="495" y1="220" x2="535" y2="220" />
<circle cx="540" cy="220" r="4.5" />
<circle cx="590" cy="220" r="4.5" />
<line x1="595" y1="220" x2="635" y2="220" />
<!-- Plunger bridging contacts from below/resting -->
<line x1="533" y1="229" x2="597" y2="229" stroke-width="3" />
<line x1="565" y1="190" x2="565" y2="229" stroke-width="2.5" />

1. SPST (1 Pole, 1 Throw) 2. SPDT (1 Pole, 2 Throws) 3. DPST (2 Poles, 1 Throw Ea.) 4. DPDT (Toggle & Slide Symbols) 5. PBNO (Normally Open) 6. PBNC (Normally Closed)

Switch Identification & Functional Summary

Switch Designation	Full Name	Poles	Throws (per Pole)	Circuit Connection & Operational Function	Practical Example
`SPST`	Single Pole Single Throw	$1$	$1$	Connects one conductor/circuit to one load; controls current in a single path.	Standard wall light switch or power `ON/OFF` switch.
`SPDT`	Single Pole Double Throw	$1$	$2$	Connects one conductor/circuit to either of two different loads (one load energized at a time).	`HIGH / LOW` power selector or `A / B` mode selector.
`DPST`	Double Pole Single Throw	$2$	$1$	Connects two electrically independent circuits to one load each using a single mechanically ganged actuator (dashed line).	Dual-line `ON/OFF` breaker simultaneously switching both hot legs.
`DPDT`	Double Pole Double Throw	$2$	$2$	Connects two electrically independent circuits to either of two loads each (functions as two mechanically ganged `SPDT` switches; common in slide switches).	Polarity-reversing switch or multi-channel audio/signal router.
`PBNO`	Push Button Normally Open	$1$	$1$ (Momentary)	Circuit is open when unpressed; depressing the plunger temporarily closes the contacts to energize the load.	Doorbell button, keyboard key, or horn switch.
`PBNC`	Push Button Normally Closed	$1$	$1$ (Momentary)	Circuit is closed when unpressed; depressing the plunger temporarily opens the contacts to de-energize the load.	Refrigerator door light switch or automobile dome-light jamb switch.

3. Switching Concepts: Open vs. Closed & Slide Switch Teasing

Electrical Characteristics of Open vs. Closed Switches

At any given instant, a switch path is either Open (electrical equivalent of a break) or Closed (electrical equivalent of a short):

Switch State	Electrical Equivalent	Resistance Across Switch ( $R_{\text{SW}}$ )	Circuit Current ( $I$ )	Conductance ( $G$ )	Voltage Dropped Across Switch ( $V_{\text{SW}}$ )	Voltage Across Load ( $V_L$ )
OPEN Switch	Break (Open Circuit)	Maximum ( $\infty$ / `OL`)	Minimum ( $0,\text{A}$ )	Minimum ( $0,\text{S}$ )	Maximum ( $\approx V_A$ ) — Full applied source voltage is felt across the open contacts	Minimum ( $0,\text{V}$ )
CLOSED Switch	Short Circuit	Minimum ( $\approx 0,\Omega$ ) (internal contact wire only)	Maximum (limited by load $R_L$ )	Maximum	Minimum ( $\approx 0,\text{V}$ ) — No potential difference across closed contacts	Maximum ( $\approx V_A$ )

Why an Open Switch Reads Full Source Voltage ( $V_A$ ): By Ohm’s Law ( $V = I \times R$ ), when a switch is open, circuit current is $0,\text{A}$ , so the series load resistor drops $V_L = 0,\text{A} \times R_L = 0,\text{V}$ . By Kirchhoff’s Voltage Law, the entire applied source voltage ( $V_A$ ) appears across the open switch terminals—one switch terminal is connected through the load to ground ( $0,\text{V}$ ) while the other terminal is connected directly to the power supply ( $V_A$ ).

Teasing a Slide Switch

Definition: Unlike toggle switches with snap-action mechanisms, a slide switch can be teased—meaning its sliding actuator is carefully nudged to an intermediate position where the movable bridging contact touches both throw contacts simultaneously.
Circuit Effect: When a slide switch is teased, both loads (leads) connected to the switch are energized at the same time (placed in parallel with the pole), rather than switching cleanly from one load to the other.

4. Bench & Troubleshooting Application (`BE.01.05` Lab)

1. Probing `PBNO` Open vs. Closed States with the Fluke 77/BN

In the BE.01.05 breadboard lab, a 4-pin tactile PBNO switch is wired in series with a $330,\Omega$ resistor across the DC supply:

Unpressed (OPEN State):
- Resistance (Power OFF): Meter displays 1. / OL (Maximum resistance).
- Voltage Across Switch (Power ON): Meter displays full supply voltage ( $\approx 5.0,\text{V}$ or battery voltage $\approx 8.68,\text{V}$ — Maximum voltage).
Depressed & Held (CLOSED State):
- Resistance (Power OFF): Meter displays $\approx 0.1,\Omega\text{ to }1.1,\Omega$ (Minimum resistance, representing only the internal metal contacts and test lead resistance).
- Voltage Across Switch (Power ON): Meter displays 0.00 V (Minimum voltage) because voltage drop is directly proportional to resistance, and nearly $100%$ of the source voltage now drops across the $330,\Omega$ load resistor.

2. Visual Ohm’s Law & `SPDT` Dual-LED Switching

LED Polarity Rule: Light-Emitting Diodes (LEDs, covered in depth in BE03) are polarity-sensitive: the longer lead is the Anode (Positive $+$ , toward the positive supply) and the shorter lead / flat cathode edge is Negative ( $-$ , toward ground). Always keep the $330,\Omega$ current-limiting resistor in series to prevent burning out the LED.
SPDT Load Routing: When an SPDT slide switch has its center pole connected to ground and its two outer throws connected to a Red LED branch and a Green LED branch, toggling the slide switch routes the return current path exclusively through one LED at a time—visually demonstrating how a single-pole double-throw switch selects between two independent loads.

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