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Thursday, February 20, 2014

Simple Automatic Load Sensing Power Switch

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).
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 Simple Automatic Load Sensing Power Switch Circuit Diagram


Simple Automatic Load Sensing Power Switch

Parts


Part            


  Total Qty.


Description



C1, C3               2               10uF 35V Electrolytic Capacitor
C2     1               1uF 35V Electrolytic Capacitor
R1     1               0.1 Ohm 10W Resistor
R2     1               27K 1/2W Resistor
R3, R4     1               1K 1/4W Resistor
R5     1               470K 1/4W Resistor
R6     1               4.7K 1/2W Resistor
R7     1               10K 1/4W Resistor
D1, D2, D4     3               1N4004 Rectifier Diode
D3     1               1N4744 15V 1 Watt Zener Diode
U1     1               LM358N Dual Op Amp IC
Q1     1               2N3904 NPN Transistor
K1     1               Relay, 12VDC Coil, 120VAC 10A Contacts
S1     1               SPST Switch 120AVC, 10A
MISC     1               Board, Wire, Socket For U1, Case, Mains Plug, Socket
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Notes
  • This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
  • A maximum of 5A can be used as the master unless the wattage of R1 is increased         S1 provides a manual bypass switch.
  • THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.


Kirish

Author & Editor

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