Low resistance connection tester circuit diagram sharing

The low resistance connection tester here can be used as a cable or wire tester, solder joints and other types of connections with resistance values ​​between 0.25 and 4 ohms.

Circuit notes:

This simple circuit uses a 741 op amp in differential mode as a continuity tester.

The voltage difference between the non-inverting and inverting inputs is amplified by the op amp's full open-loop gain. Ignore the 470k and 10k controls for now and look at the op amp input. If the resistors are perfectly matched, the voltage difference is zero and the output is zero. However, using the 470k and 10k controls allows a small potential difference to be applied to the op amp inputs and disrupts the balance of the circuit. This is amplified, causing the op amp output to swing to full supply voltage and light up the LED.

Circuit Setup and Testing

The probe should first be connected to a resistor with a resistance between 0.22 ohms and 4 ohms. Adjust the control until the LED just lights up and there is resistance across the probe. The resistor should then be removed and the probes shorted and the LED should go out. Since low resistance values ​​are extremely low, it is very important to keep the probe (whether it is an alligator clip, needle, etc.) clean, otherwise dirt will increase the contact resistance and cause the circuit to malfunction. The circuit should also be used with MOSFET type op amps such as the CA3130, CA3140 and JFET type such as the LF351. If the LED won't go off, you should connect the 10k preset to the offset zero terminal, pins 1 and 5, and the controller's tap to the negative battery terminal.