Leakage tester

This is a remote sensing type tester that does not require direct contact with the surface where the current is present. It can also be used to check if there is electricity in objects such as power cords and power switches.

This circuit uses a coil detector to pick up the electromagnetic field emitted by the AC line. The coil is a coil on an ordinary 6V relay. The electromagnetic field of the AC line generates a voltage in the coil through induction. This voltage is rectified by the D1-D4 bridge and sent to the in-phase input pin ③ of the op amp comparator IC1. Since the input impedance of IC1 is very high, it is extremely sensitive to small voltage changes.

The voltage open loop gain of TL071 is about 100dB. IC1's inverting input pin ② is set to an adjustable voltage by VR1. R1 can protect against VR1 turning to the end, and R2 is the feedback resistor.

IC1 output is regulated by VR2 and sent to IC2 signal input pin ⑤. R3 is also a protective resistor. IC2 is a monolithic integrated circuit LM3915, which is used to obtain analog display. When IC2 input pin ⑤ increases by 0.5V, its output becomes lower in turn. When IC2 pin 9 is open, dot mode display can be obtained. When input pin ⑤ receives 0.5V, green LED1 lights up. When the voltage increases to 0.75V, yellow LED2 lights up. When the voltage increases to 1V, red LED3 lights up. Therefore, through the display status of LED, it is easy to detect the intensity of leakage current.

Circuit calibration and setting: Turn VR2 counterclockwise to the bottom, and then slowly adjust VR1 until LED1 lights up. This indicates that the electromagnetic field around the circuit is normal. Then bring the probe close to the AC wire. If LED2 lights up, it means there is current. If the probe is closer, LED3 lights up and LED2 goes out.

After setting, use glue to fix the center contact position of VR1 and VR2. Put the probe close to the ground pipeline line. If LED2 and LED3 light up, it means that there is current leakage through the leakage path.

The circuit can be mounted on a common PCB board and sealed in a plastic box. Use the probe of a multimeter as a probe and connect it to the circuit with a 30cm long soft wire.

Note: When testing, do not directly contact the probe with the exposed power point or AC power socket, because a slight leakage current can make the circuit work normally.