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
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Simple Automatic Load Sensing Power Switch Circuit Diagram
Parts
| 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 |
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.
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