Circuit Continuity Tester Using Op-Amps

Figure 1 

The circuit was developed to produce a continuity tester with a low resistance mainly for checking the connections between soldered joints.

• Operational Amplifier (Op-Amp) – a DC coupled high gain electronic voltage amplifier with differential inputs and usually a single output
• Light Emitting Diode (LED) – a semiconductor diode that is commonly a source of light when electric current pass through it
• 741 Op-Amp – the most common and cheapest op-amp used in several circuits because of 1 MHz gain bandwidth product and is not prone to producing false oscillations due to tailored frequency response

The use of a 741 op-amp to fully function the circuit provides several features such as high input voltage range, excellent temperature stability, no latch up, short circuit protection, offset voltage null capability, and internal frequency compensation. As it operates in differential mode, it provides high input impedance and low noise amplification in the input stage. Specifically, the input being amplified comes from the voltage difference between the inverting and non-inverting inputs. The amplification is done by the full open loop gain of the op-amp which is developed when there is no feedback used in the circuit. However, in the presence of increasing frequency, the open loop gain of an operational amplifier falls very quickly.

The addition of 470K ohm and 10K ohm resistors in the circuit is very essential since they are responsible for creating a minimal voltage difference to be applied to the inputs of the op-amp. Without these resistors, if both resistors connected to the op-amp inputs are ideally equal, the circuit would be balanced wherein the output from the probes would be zero thus producing zero voltage difference. On the other hand, as this voltage difference is amplified, there will be a swing to full supply by the op-amp output which will cause the LEDs to shed light.

Before using the circuit for live testing, the probes should be connected initially to a resistor having a value between 0.22 ohm and 4 ohms. This is done to adjust the control until the LEDs give light while having this resistor across the probes. After the adjustment, the resistor must be removed while shorting the probes so that the LED light will disappear. The probes should be kept clean and free from dirt to avoid the increase in resistance and the circuit not to function well because the circuit itself has an extremely low resistance value.

I cases where the LEDs do not switch off, the 10K ohm preset resistor should be connected across the offset null terminals which is the Pin 1 and Pin 5 on the metal can package. These offset null terminals are responsible for eliminating the effects of internal component voltages on the output of the device. The wiper of the potentiometer or the control should be connected to the negative terminal of the battery terminal. This offset circuit will work at 0V and –Vcc and will behave as a comparator where it compares two voltages or currents and switches its output for indication of which is higher.

The 741 may be used in two ways. First, it can be used as an inverting amplifier where the negative input is coming from the Pin 2 and the output on Pin 6 with the polarity being reversed. Secondly, it can be used as a non-inverting amplifier where the positive input comes from Pin 3 and the out still at Pin 6 with the polarity being retained.

The continuity tester utilizes a test wire with normally alligator clip connected to the end. It is used mainly to find the cause of problem in a circuit or if a particular electronic component is containing electricity, but with the absence of the current. It is also useful in determining if an electrical path is possible on both ends, which is also similar to testing a bundle of wires to locate the two ends that belong to the same wire.

Before using the continuity tester, make sure that the power is unplugged or the power to the circuit is turned OFF to avoid unwanted injuries.

Source:www.zen22142.zen.co.uk/Circuits/Testgear/connectiontester.htm