Switch Circuit Types, SPDT,  SPST and DPDT

Switches are basic methods for opening and closing an electrical circuit. The circuits vary from simple make/break circuits to multi-makes and multi-break circuits. Though the logic is quite simple, it is important that the proper circuit is chosen for the application. So lets look at each circuit style and better understand the mechanics of the switch and the best applied application for each circuit.

 

SPDT Circuits – Single Pole, Double Throw

One of the most common style of switches is the SPDT Mechanical switch. The SPDT stands for Single Pole, Double Throw. Simply stated, there is on common terminal on the switch where the voltage and current is applied and that voltage and current can be either directed to the normally open or normally closed terminal. The direction of the current flow is typically directed by a mechanical roller lever that actuates the switch.

Benefits of the SPDT Mechanical Switch

• Low cost
  
• Proven reliability for most applications
  
• Switches ability to function with high voltages and currents
  
• Higher switching currents allow the device to control
  • Relays
    
  • Plant devices such as pumps
    
  • And drive motors
    

Mechanics of the Switch

The SPDT switch has three connections – the NO (Normally Open), NC (Normally Closed) and the C (Common). Input current enters the switch through the common and makes the circuit either with the NO (normally open) or the NC (normally closed) contacts.

Applications

In control rooms, two lights are used for each of the valve positions. One light for “on” and one for “off”, thus assuring that the switch is functioning regardless of its position.

Look for switches that are specified with stiffer contact springs and engineered materials of construction. These small details will provide the switch with the ability to better resist dirt and debris and assist in preventing freeze-up in cold conditions. A common problem where the mechanical internals of the switch are frozen in place. This condition is prevalent with “off the shelf” switches.

 

DPDT Circuits – Double Pole, Double Throw

Benefits of the DPDT Mechanical Switch

• Ability to carry two independent signals
  
• Fail over capabilities
  
• Switches ability to function with high voltages and currents
  
• Higher switching currents allow the device to control
  • Relays
    
  • Plant devices such as pumps
    
  • And drive motors
    

Mechanics of the DPDT Switch

The DPDT switch is, in effect, the equivalent of two SPDT switches that operate in tandem. The DPDT switch is two SPDT circuits which is located internally to the switch housing and these two circuits are actuated in tandem by a common lever mechanism on inside the switch housing. There are typically six terminals on a DPDT mechanical switch.  Two C (Common),  Two NO (Normally Open) and Two NO (Normally Closed).

DPDT Applications

Because of this design, mechanical DPDT switches are often used for simultaneous switching two independent signals that must operate in tandem. One example frequently used by the DPDT design is when one switch circuit is used to send a low level signal back to the control room for PLC or DCS processing, while at the same moment the second internal switch circuit is used to drive a relay for another device. With both circuits being completely isolated from each other, the application may switch two different signal voltage and currents.

Another important application that is handled simply with the DPDT mechanical switch is for redundant signaling applications where a fail over circuit is already setup in the event that there is signal loss from the first cicuit.

 

SPST Circuits – Single Pole, Single Throw

Benefits of the SPST Mechanical Switch

• Simple design
  
• Easier to setup and wire too
  
• Less required cabling
  
• Not a common design
  

Mechanics of the SPST Switch

The SPST mechanical switch has two terminal connections – the NO (Normally Open) and the C (common). When the switch is actuated, the circuit is closed (or made) and current flows from the common terminal through to the normally open terminal. When the SPST mechanical switch is not actuated, the circuit is not closed, rather it is an open circuit (not made). The design is analogous to a simple light switch that is either on (lighted) or off (dark).

SPST Applications

Advanced process control systems can use SPST mechanical switches in place of two wire sensors. As is commonly done, one can use two of the three contacts of a standard SPDT switch to mimic an SPST.