Automatic Load Sensing Power Switch

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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	Substitutions
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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Comments

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Brian Nebeker Automatic Load Sensing Power Switch Thursday, November 17, 2011 12:21:11 PM

Current sensing using a large resistor is wasteful of power and sensing power on the neutral side can cause dangerous conditions. If R1 fails it will leave the load side of the neutral line at the potential of the hot line. A far safer way of sensing current is to use a hall effort sensor such as those provided by Allegro. These sensors provide high voltage isolation and high power level handling due to the fact that they use a magnetic field instead of a resistance to sense the current. I have used these sensors on several projects and they make life much easier. If there is enough interest I could post of example of this circuit using a hall effect sensor.

anonymous Automatic Load Sensing Power Switch Monday, April 25, 2011 9:43:10 PM

anybody has got this working??

PsyPhi Automatic Load Sensing Power Switch Wednesday, November 04, 2009 12:42:46 AM

There's nothing in this circuit to keep the voltage across C3 from rising above its rated voltage when Q1 is off (i.e. when no current is being drawn by the master load). The capacitor will try to charge up to about 170V (the peak voltage of 120 VAC RMS), but it will eventually fail (and might literally explode!) once the voltage gets significantly above the nominal 35V. A zener or even a resistor in parallel with C3 could solve this problem.

Abdul Wahid Automatic Load Sensing Power Switch Sunday, March 01, 2009 5:42:16 AM

The values for 240VAC operation are R2 = 50K, 1W and R6 = 200K, 1W. these are calculated just using Ohms law by keeping the current at same level. if you want to increase the current then increase the wattage with same ratio, i.e doubling current will double the wattage of R2 and R6

anonymous Automatic Load Sensing Power Switch Sunday, February 15, 2009 3:42:05 PM

Not sure this will work. To keep a 12V relay energised would need something like 50mA. So the power dissipated in R6 would be 0.050 * 0.050 * 4700 = 11.75W!

Eliud kabuga Gichaiya Automatic Load Sensing Power Switch Wednesday, September 24, 2008 6:58:46 AM

1. the value of r2 r6 need to be changed if you are in 240v .what is the value of this resistors. 2. if u need islorate .that is high version, about 15amp.what is rate of resistor.

Loupitou Automatic Load Sensing Thursday, June 05, 2008 12:40:02 AM

To make a "high power version" capable of sustaining ~15A total (Master + slave peak ), I understand that I need to change the W rating of R1 and also the relay/switch model but anything else ?

gagan kapoor Automatic Load Sensing Power Switch Monday, May 26, 2008 2:40:53 AM

what should be the value of r2 & r6 when connected to 240vac

scofield Automatic Load Sensing Power Switch Monday, February 18, 2008 11:40:55 PM

what is the value of R2 and R6, when we want to use 240v as the supply???urgent...

anonymous Automatic Load Sensing Power Switch Thursday, February 14, 2008 11:10:27 PM

ohms law , would indicate half the resistance , not double and it would be 4 times the wattage(power dissapation---...---...---...G.R.. . . . .

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Brian Nebeker	Automatic Load Sensing Power Switch	Thursday, November 17, 2011 12:21:11 PM
Current sensing using a large resistor is wasteful of power and sensing power on the neutral side can cause dangerous conditions. If R1 fails it will leave the load side of the neutral line at the potential of the hot line. A far safer way of sensing current is to use a hall effort sensor such as those provided by Allegro. These sensors provide high voltage isolation and high power level handling due to the fact that they use a magnetic field instead of a resistance to sense the current. I have used these sensors on several projects and they make life much easier. If there is enough interest I could post of example of this circuit using a hall effect sensor.
anonymous	Automatic Load Sensing Power Switch	Monday, April 25, 2011 9:43:10 PM
anybody has got this working??
PsyPhi	Automatic Load Sensing Power Switch	Wednesday, November 04, 2009 12:42:46 AM
There's nothing in this circuit to keep the voltage across C3 from rising above its rated voltage when Q1 is off (i.e. when no current is being drawn by the master load). The capacitor will try to charge up to about 170V (the peak voltage of 120 VAC RMS), but it will eventually fail (and might literally explode!) once the voltage gets significantly above the nominal 35V. A zener or even a resistor in parallel with C3 could solve this problem.
Abdul Wahid	Automatic Load Sensing Power Switch	Sunday, March 01, 2009 5:42:16 AM
The values for 240VAC operation are R2 = 50K, 1W and R6 = 200K, 1W. these are calculated just using Ohms law by keeping the current at same level. if you want to increase the current then increase the wattage with same ratio, i.e doubling current will double the wattage of R2 and R6
anonymous	Automatic Load Sensing Power Switch	Sunday, February 15, 2009 3:42:05 PM
Not sure this will work. To keep a 12V relay energised would need something like 50mA. So the power dissipated in R6 would be 0.050 * 0.050 * 4700 = 11.75W!
Eliud kabuga Gichaiya	Automatic Load Sensing Power Switch	Wednesday, September 24, 2008 6:58:46 AM
1. the value of r2 r6 need to be changed if you are in 240v .what is the value of this resistors. 2. if u need islorate .that is high version, about 15amp.what is rate of resistor.
Loupitou	Automatic Load Sensing	Thursday, June 05, 2008 12:40:02 AM
To make a "high power version" capable of sustaining ~15A total (Master + slave peak ), I understand that I need to change the W rating of R1 and also the relay/switch model but anything else ?
gagan kapoor	Automatic Load Sensing Power Switch	Monday, May 26, 2008 2:40:53 AM
what should be the value of r2 & r6 when connected to 240vac
scofield	Automatic Load Sensing Power Switch	Monday, February 18, 2008 11:40:55 PM
what is the value of R2 and R6, when we want to use 240v as the supply???urgent...
anonymous	Automatic Load Sensing Power Switch	Thursday, February 14, 2008 11:10:27 PM
ohms law , would indicate half the resistance , not double and it would be 4 times the wattage(power dissapation---...---...---...G.R.. . . . .