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C I R C U I T L A B

Earth Leakage Tester

D. Mohan Kumar

Earth leakage from electrical wiring is a serious problem, especially during rainy season. Pipelines are more vulnerable toearth leakage and may cause an unexpected electric shock. Electrical appliances or faulty wiring may be the source ofleakage current through the wet wall to the earth. An ordinary AC tester cannot detect the earth leakage if current is nothigh enough to switch on a neon lamp.

This simple circuit can detect the electric current passing through earth wires. It is a remote-type tester and directcontact is not necessary to check the presence of current. It can also be used to check the presence of current in powercables, switches, etc.

The circuit picks up the electromagnetic field from the AC line through a coil sensor. Here the sensor used is the coil of anordinary 6V relay. The electromagnetic field from the AC generates a voltage in the coil through induction. The AC thus

generated is rectified by the bridge rectifier comprising 1N4007 diodes (D1 through D4). The rectified DC then passes tothe non-inverting input (pin 3) of IC1. The high input impedance makes IC1 very sensitive to minute voltage variations.

The open-loop voltage gain of TLO71 is around 100 dB. The inverting input of IC1 receives an adjustable voltage throughpreset VR1. Resistor R1 protects the inverting input if the wiper of preset VR1 is fully turned. R2 is the feedback resistor.

The output of IC1 is given through VR2 to the signal input (pin 5) of IC2. Resistor R3 also acts as a protective resistor.Monolithic IC LM3915 (IC2) is used to get an analogue display. The output of IC2 goes low one by one with an incrementof 0.5 volt in input pin 5. Pin 9 of IC2 is left unconnected to get a dot-mode indication. LED1 lights up when input pin 5receives 0.5 volt and LED2 lights up when the voltage increases to 0.75 volt. LED3 lights up when the voltage increases to

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Post Comment | 4 Comments

Rahul kumar 221 days ago

I made this circuit for my minor project and I have made all the connection properly but I have a problem that this circuit is onlyworking when I kept my fingers between the joint of D1 and D4 before there was no detection but accidently I found that while Iam keeping my hands on the joints it start detecting the live line and it works fine. so plz tell me that hoe can I get rid of thisbecause when I will pack this circuit in the plastic case I will be unable to touch the joint of D1 & D4. so plz give me a solution....

Reply

abhijit 207 days ago

D3 should be reversed i guess Reply

arslan 159 days ago

plz tell me about the specification of relay coil Reply

sanket 69 days ago

i have made all connection properly but output is not coming.plzz give some idea as early as possible. Reply

Electronics Buzz

receives 0.5 volt and LED2 lights up when the voltage increases to 0.75 volt. LED3 lights up when the voltage increases to1 volt. Thus it is easy to detect the strength of the electric current through the LED display. Resistor R5 controls thebrightness of the LEDs.

Calibration and setting require some trial and error. Keep the wiper of preset VR2 in the extreme anti-clockwise positionand slowly adjust VR1 until LED1 lights up. This indicates normal electromagnetic field around the circuit. Take the probenear the AC line. LED2 will light up to indicate the presence of electric current. When the distance becomes shorter, LED3lights up and LED2 turns off.

After setting, glue the wiper of VR1 and VR2 with adhesive. Take the probe near the earth-grounding pipe. If LED2 andLED3 light up, it indicates the drainage of current through leakage.

Assemble the circuit on a general-purpose PCB and enclose in a plastic case. Use a probe similar to that of the multimeter.Connect the probe to the circuit using a 30cm long flexible wire.

EFY note. Do not touch the probe to bare electric points or AC sockets for testing. It is not necessary to touch theprobe and a mere presence of electric current is sufficient for the circuit to get activated and be functional.

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