Control Devices – Switches
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Table of Contents
Control devices come in many form factors but they all do the same thing, they turn on, or off, an electrical load by closing or opening a circuit. A closed circuit has a complete path from a power source through an electrical load and back to the power source. An open circuit does not have a complete path from a power source through an electrical load and back to the power source.
A switch is the most fundamental of all control devices and is a good place to start when learning the basics. This section will often reference toggle switches; however, the features and functions of toggle switches apply to almost all other switch form factors and other complex control devices.
In the illustration below, the control device (switch) is turned on. This closed the circuit and the motor will run. An older term for creating a closed circuit is to “make” the circuit.
In the illustration below, the control device (switch) is turned off. The circuit is now open and the motor will not run. An older term for creating an open circuit is to “break” the circuit.
Power Vs Ground Controlled Circuits
A control device can be placed in the power side of the circuit (between the battery’s positive post and the load) as shown in the previous two illustrations. This is called a power controlled circuit. The control device can also be placed in the ground side of the circuit (between the load and the ground post of the battery) as shown in the image below. This is called a ground controlled circuit.
When is Ground Control Better Than Power Control?
Most circuits are power controlled and when you first see a ground controlled circuit it just seems odd. So why would an engineer use ground control over power control? It is all about managing power and simplifying circuits. Horn circuits were one of the first applications of ground controlled circuits before relays were used.
Let’s compare the two circuit types using the ground controlled horn circuit, as shown below, as an example.
Current Flow For a Power Controlled Horn Circuit
- From the battery to the underhood fuse
- From the underhood fuse through the firewall
- Up the steering column to a slip ring (or clock spring) to the horn button
- From the horn button back through the slip ring (or clock spring) and back down the steering column
- Back through the firewall to the horn
- Through the horn to ground
Current Flow For a Ground Controlled Horn Circuit
- From the battery to the underhood fuse
- From the underhood fuse to the horn
- From the horn through the firewall
- Up the steering column to a slip ring (or clock spring) to the horn button to ground (through the steering column)
Inputs (Poles) / Outputs (Throws)
With a two terminal on/off switch, it is irrelevant which terminal is used for the input or output, but when you go past two terminals it makes a difference. The control device industry has standardized the name for inputs and outputs as follows:
- Input (Pole) – An input into a switch is called a pole.
- Output (Throw) – The outputs of a switch are called throws.
- A switch that can direct a single input (pole) to a single output (throw) is called a single pole / single throw (SPST) switch.
- A switch that can direct a single input (pole) to two different outputs (throws) is called a single pole / double throw (SPDT) switch.
- A switch that can direct two different inputs to two different outputs is called a double pole / double throw (DPDT) switch. A DPDT switch contains two SPST switches in one unit and controls both switches with a single lever (bat).
The contact box contains the actual contacts that make and break the circuit. Current flows in and out of the contact box through contacts that are connected by a “shoe” that is moved by the switch’s lever.
Contact boxes will contain two contacts for a SPST switch, three contacts for a SPDT switch and six contacts for a DPDT switch.
The input for a toggle switch is the handle, often called a “bat”, that travels in a linear motion. Most toggle switches are two-position switches, but some have a third, center (off) position. The bat could be in the shape of a paddle or even a rocker input but they all work the same way on the inside.
Wires are attached to toggle switches four ways as listed below:
- Screw – The terminals of the switch have a small threaded hole so a wire with a ring terminal could be attached with a screw.
- Solder Lugs – The terminals have lugs with a small hole for the wire to go through. The wire would then to be soldered to the lug.
- Push Connector – The terminals are a flat male electrical connector (as shown in the two above images). A wire would attach to the terminal with a female electrical crimp connector.
- Wire Leads – The switch has attached wires (4″-6″) long, that could be attached by crimp connectors or soldering directly to a circuit.
Switch Power Flow
SPST – “ON” Position
A single pole / single throw (SPST) switch will only have one output terminal as shown in the image below. The switch will be “On” when the bat “points” towards the output connector.
SPST – “OFF” Position
When the bat is in the off position the contact shoe doesn’t touch the output contact and opens the circuit.
SPDT Internal Shoe – “ON” Circuit 1
The input to a toggle switch is always the center terminal and a copper shoe connects the input to an output terminal on either side. The image below is an example of how a SPDT switch connects the center terminal to one of the outer terminals.
SPDT Internal Shoe – “ON” Circuit 2
The image below shows how a single input (pole) can alternate between two outputs (throws). Also, you can use a SPDT switch as a SPST switch by just using the input and one of the output terminals.
SPDT Internal Shoe – “OFF”
Some SPDT switches offer an optional “OFF” in the center. The image below shows how that is accomplished. When the bat is in the center position, the contact shoe doesn’t touch either output contact.
Single Pole Vs Double Pole Contact Box
The image below is the bottom of a DPDT switch’s contact box. It can switch two inputs (poles). This switch is actually two SPDT switches (one outlined in green and one outlined in red) mounted inside one contact box and controlled by a single bat. The two circuits are not connected internally in any way.
Additional Switch Features
Switches are offered in waterproof versions for extreme conditions. It is also possible to add a rubber boot to the bat to make a standard switch more water resistant.
Explosion Proof Switch
Switches are available that have a completely sealed contact box so that no outside air can enter. This type of switch should be used where there is any risk of flammable gas fumes.
Switch Block Out (Security) Covers
A security cover can be added to a toggle switch to help prevent accident switch manipulation. Security covers are often used on race car switch panels to block out accidental switch movement.
Some switches are offered with momentary “On” positions. For example, the switch could be as simple as an SPST with momentary “On” or more complex, such as a double throw switch with “Off” in the center and a momentary feature in either or both “On” positions.
Switch Symbols Video
Switch Schematic Symbols
|Single Pole Single Throw (SPST) Switch|
This switch is used to control a single circuit. It is drawn in the closed (On) position.
|Single Pole Single Throw (SPST) Switch|
This switch is used to control a single circuit. It is drawn in the open (Off) position.
|Single Pole Double Throw (SPDT) Switch|
This switch is used to connect a single input to one of two outputs and toggle between the two outputs.
|Single Pole Single Throw (SPST) Push Button Switch – Normally Open|
This is a push-button type switch that is used to control a single circuit. It is normally held open by a spring. This type of switch is also called a momentary on switch.
|Single Pole Single Throw (SPST) Push Button Switch – Normally Closed|
This is a push-button type switch that is used to control a single circuit. Is is normally held closed by a spring. This type of switch is also called a momentary off switch.
|/||Double Pole Single Throw (DPST) Switch|
This switch combines two SPST switches into one contact box. The dotted line between the two SPST switches means the two switches are “ganged” together and move as one.
|Double Pole Double Throw (DPDT) Switch|
This switch combines two SPDT switches into one contact box. The dotted line between the two SPDT switches means the two switches are “ganged” together and move as one.
This switch connects a single input to one of five outputs by rotating a shaft. More complex rotary switches can have multiple layers of switches that share a common shaft and can support multiple inputs and a large number of outputs.