A relatively simple test circuit immediately and automatically provides indications of (1) the continuity (or the lack thereof) of the wires of a two-wire cable, plus (2) a short circuit (if any) between the wires. In the case of a coaxial cable, one of the wires can be the shield, the other wire, or the center conductor.
Some prior cable-testing circuits indicate whether a cable is defective, but do not immediately and automatically indicate whether the defect is a short or an open circuit. Other prior cable-testing circuits fail to indicate which wire is the defective one in the event of an open circuit. Still other prior cabletesting circuits require separate tests for continuity and shorts. Of course, it is possible to test a cable by manual probing with an ohmmeter or a simple continuity checker, but this procedure is cumbersome and is time-consuming in situations in which there are many cables to test. The present cable tester overcomes the limitations of manual probing and of prior cabletesting circuits.
The tester (see figure) is equipped with sockets that mate with the plugs on both ends of a cable to be tested. There can be several such pairs of sockets (e.g., three as shown in the figure), each pair accommodating one of several different types of plugs that may be in use on cables to be tested. The tester contains a pair of batteries, current-limiting resistors, and light-emitting diodes (LEDs); these components are connected such that current flowing along one of the desired or undesired paths through the cable causes one of the LEDs to light up.
As soon as both ends of the cable are plugged into the tester, the cable completes the circuits through the aforementioned components. Then LED 1 lights or does not light up, indicating continuity or discontinuity, respectively, in one wire (the center conductor in the case of a coaxial cable). Similarly, LED 2 does or does not light up, indicating continuity or discontinuity, respectively, in the other wire (the shield in the case of a coaxial cable). If LED 3 lights up, then this is an indication that there exists an undesired current path (a short circuit) between the two wires in the cable.
The circuit also includes a switch and a pair of banana jacks or other convenient auxiliary connectors for testing LEDs or testing external circuits for continuity. If the switch is in "diode check" position, then an LED should light up when connected to the auxiliary terminals with forward polarity and should not light up when connected with reverse polarity: An LED that passes this test is suitable as a replacement for one of the LEDs in this or a similar tester. If the switch is in "continuity check" position and an external circuit is connected to the auxiliary terminals, the illumination or lack of illumination of LED 3 indicates continuity or lack of continuity, respectively, in the external circuit. If nothing is connected to the auxiliary terminals, then for the purpose of testing a cable, the position of the switch does not matter.
This work was done by Arthur R. Hayhurst of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronic Components & Systems category.