Senses high current-flow into power cables
No wire-cutting, three versions
R1,R2,R8____________1K 1/4W Resistors
R3,R4_____________220K 1/4W Resistors
R5________________100R 1/4W Resistor (See Notes)
R6_________________10K 1/2W Trimmer Cermet
R7,R10______________1M 1/4W Resistors
R9_________________22K 1/2W Resistor
R11 to R17__________1K 1/4W Resistors
C1,C3_____________100µF 25V Electrolytic Capacitors
C2,C4_______________1µF 63V Electrolytic Capacitors
D1________________5mm. Red LED
D3,D4___________1N4002 100V 1A Diodes
D2,D5,D6,D7_______LEDs (Any color and size)
Q1_______________BC327 45V 800mA PNP Transistor
IC1______________TL061 Low current BIFET Op-Amp (First version)
IC1______________LM358 Low Power Dual Op-amp (Second version)
IC1______________LM324 Low Power Quad Op-amp (Third version)
L1________________10mH miniature Inductor (See Notes)
RL1______________Relay with SPDT 2A @ 220V switch
Coil Voltage 12V. Coil resistance 200-300 Ohm
J1_______________Two ways output socket
This circuit was designed on request, to remotely monitor when a couple of
electric heaters have been left on. Its sensor must be placed in contact with
the feeder to be able to monitor when the power cable is drawing current, thus
causing the circuit to switch-on a LED.
The circuit and its sensor coil can
be placed very far from the actual load, provided an easy access to the power
cable is available.
Any type of high-current load or group of loads can be
monitored, e.g. heaters, motors, washing machines, dish-washers, electric ovens
etc., provided they dissipate a power comprised at least in the 0.5 - 1KW
This design features three versions. The basic one illuminates a LED
when the load is on. The second version drives a Relay when a pre-set current
value flows into the power cable. The third version switches-on D7 when the load
power is about 1KW, D6 when the load power is about 2KW and D5 when the load
power is about 3KW.
The basic circuit is shown top left in the drawing and must be used in all
three versions. IC1 acts as a differential amplifier having a gain of 220. The
small AC voltage picked-up by L1 is therefore amplified to a value capable of
driving the LED D1.
The second version is drawn bottom left, must be
connected to the basic circuit and uses a dual op-amp, therefore IC1 will be
labeled IC1A and its pin connection varies slightly. IC1B acts as a voltage
comparator and its threshold voltage can be precisely set by means of trimmer
R6. Q1 is the Relay driver and D2 illuminates when the Relay is on. You can use
the Relay contacts to drive an alarm or a lamp when the AC load exceeds a
pre-set value, e.g. 2KW.
The third version is shown to the right of the
drawing, must be connected to the basic circuit and uses a quad op-amp,
therefore IC1 will be labeled IC1A and its pin connection varies slightly. IC1B,
C and D are wired as comparators. They switch on and off the LEDs, referring to
voltages at their non-inverting inputs set by the voltage divider resistor chain
The pick-up coil L1 is a common 10mH miniature inductor, having the shape
of a small rectangular plastic box of 10x7x4 mm. with radial leads.
This inductor must be placed tightly against one wire of the power cable,
leaving the other wire some centimeters apart.
The sensitivity will be doubled if the inductor is placed tightly between
the two wires as shown in the diagram, top left. On the contrary, do not place
the inductor against paired wires as the signal tends to cancel and the
circuit will not work.
The LED limiting resistor R5 should have a value comprised in the 100R -
1K range, depending on the output voltage obtained.
LED D1 and its limiting resistor R5 can be omitted in versions two and
three of the circuit.
Versions one and three draw a small current, thus allowing possible 9V