DC Fundamentals Unit 5 – Switches and Switching Concepts
Unit 5 – Switches and Switching Concepts ................................................................................
UNIT OBJECTIVE
At the completion of this unit, you will be able to identify types of switches and demonstrate
switch circuit operation by using a multimeter and LEDs.
UNIT FUNDAMENTALS
Switches are devices used to close, open, or change the connections in an electric circuit. Open is
the OFF, or break, position, and closed is the ON, or make, position. The switch is usually placed
in series with the voltage source and its load.
In the ON position, the closed switch has very little resistance, allowing maximum current to
flow in the load with very little voltage (typically less than 0.5V) dropped across the switch.
When the switch is OFF, or open, the switch has infinite resistance, no current flows in the
circuit, and there is maximum voltage dropped across the switch.
NEW TERMS AND WORDS
pole - the current-carrying moveable part or parts of a switch that closes and opens an electrical
circuit.
contacts - the current-carrying part of a switch that closes and opens an electrical circuit.
SPST - abbreviation for Single Pole Single Throw switch; switch with two terminals used to
connect one terminal to the other terminal.
DPST - abbreviation for Double Pole Single Throw switch; a switch with four terminals used to
connect one pair of terminals to the other pair of terminals.
SPDT - abbreviation for Single Pole Double Throw switch; switch with three terminals used to
connect one terminal to either of the other two terminals.
DPDT - abbreviation for Double Pole Double Throw switch; a switch with six terminals used to
connect one pair of terminals to either of the other two pairs.
PBNO - abbreviation for PushButton Normally Open; switch whose terminals are connected
when its pushbutton is depressed.
PBNC - abbreviation for PushButton Normally Closed; switch whose terminals are connected
when its pushbutton is not depressed.
EQUIPMENT REQUIRED
F.A.C.E.T base unit
DC FUNDAMENTALS circuit board
Multimeter