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).
Schematic
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 |
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.