Test Contact Resistance of Dry Circuit Contacts

This invention is concerned with the testing of the contact resistance of dry circuit contacts, and is particularly concerned with low power contacts, that is low voltage, low current contacts. Such contacts are used in large numbers, particularly in telecommunications and other systems using solid state, semiconductor devices and components.

The primary requirement for dry circuit testing of contacts is that the voltage and current applied to the test contacts must never be allowed to exceed certain maximum limits. Standards are laid down for testing, of contacts, by the ASTM, and for dry circuit contacts, a maximum of 20 mV open circuit voltage and 100 mA short-circuit current are stipulated. This sets a maximum value of 0.2.OMEGA. resistance for the circuit.

In testing, long leads, use of relays to close switches to bring test contacts into the circuit, and number of contacts being tested and other features, result in a high circuit resistance over 0.2.OMEGA.. To maintain 100 mA, it is necessary to have a higher than 20 mV open circuit voltage, which is against the test specification. Alternatively, if a 20 mV limit is maintained, the short-circuit current drops, resulting in poor reading resolution.

The present invention provides for testing contacts by applying a 20 mV open circuit voltage, and 100 mA short-circuit current to the contacts, thus comforming to the test specification, then after closing the test contacts, if normally open, measuring the current in the circuit. If this is over a preset low value, for example 0.5 mA, which shows that the contacts are "good", then the test proceeds. If the current is below 100 mA, the voltage of the constant current source is increased until the current in the circuit is 100 mA. The voltage drop across the test contacts is then measured and the contact resistance calculated.

The invention will be readily understood by the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a diagramatic representation of the basic test circuit;

FIG. 2 is a diagramatic representation of a test system in accordance with the invention.

As illustrated in FIG. 1, the contacts to be tested, indicated generally at 10, are connected into a circuit having a constant current source, indicated at 11. For the purpose of the test, the constant current source 11 should supply electrical energy at a predetermined voltage. The current flowing in the circuit, when the contacts 10 are closed, is measured by a meter 12 across the contacts 10.

The contacts 10 may comprise a normal switch, having contacts moved together to make, or a switch in which the contacts are normally made, being moved to break contact. A further alternative is one in which a terminal pin, or similar member, is inserted into a female contact member, electrical contact occuring between pin and contact member on one or more surfaces. It may be desirable to maintain the circuit open until the contacts are in the circuit, particularly in the case of male and female contact members. To this extent, a further switch 14 can be provided. The contact to be tested can then be inserted into the test circuit and then by closing switch 14, the test carried out.

The test conditions, as set by the test standards, are that the contacts are subject to a maximum voltage of 20 mV open circuit voltage and maximum current of 100 mA, short-circuit. Such perameters give a maximum resistance to the circuit of 0.2.OMEGA., as stated above.

FIG. 2 is a diagramatic representation of a system for testing dry contacts. For the system as in FIG. 2, a voltage measuring device is required in the circuit of FIG. 1, for example at 20. Also a control device for controlling the current source, or power source 11, is required, as indicated at 21.

The system of FIG. 2 can be automated to test a plurality of contact sets or pairs by a system of stepping, but for the description of the invention, it will be described as for one set or pair. With a circuit as in FIG. 1, connections extend from the various items, current measuring device 12, voltage measuring device 20 and control device 21 to appropriate members of the system. Thus the current, voltage and control connections are indicated at 22, 23 and 24 respectively.

The operation of the system can be understood quite readily from FIG. 2. Assuming that test contacts 10 are in the circuit and switch 14 is open, the power source is set to give 20 mV by controller 25. This is checked by the meter 20, the result fed to the system at 26. If the voltage is less than 20 mV, then an indication can be provided to check the system. If the voltage is 20 mV, then the current loop is closed by device 27, which closes switch 14, as by a solenoid. At this time the contacts 10 will be closed.

The meter 12 measures the loop current, the value fed to the device 28. The system then makes two decisions, depending upon the current value. If less than a preset low value, e.g. 0.5 mA, it is assumed that the contacts are bad and a signal is sent from the meter 29 via connections 31, to print out "bad contact". If the current is over the preset low value, e.g. 0.5 mA, then the system determines at 30, if the current is equal to 100 mA. If the answer is yes, then the voltage drop is measured, by meter 20, the output fed to device 32.

The system then calculates the contact resistance, at 33, decreases the open-circuit voltage of the constant current source to 20 mV, and finally causes the switch 14 to open by instructions from device 34.

The system includes a device 35 which decides whether the contacts should be tested in both directions of current. If this is required, a signal is sent to device 36 which changes the current direction as by a changeover switch indicated at 40 in FIG. 1. The process is then repeated from the controllers 24 to the device 35. The device 35 then instructs the calculator 37 to calculate the average contact resistance and feed the result to a printer. Changeover switch 14 can be operated, for example, by a solenoid.

If the loop current is determined to be over the preset low value, e.g. 0.5 mA, but less than 10 mA, at 30, then a signal is fed to a control member 38 to increase the open circuit voltage of the constant current source (11 in FIG. 1) such that the current at 30 is increased to 100 mA.

The system is one which can be adapted to test a plurality of contact pairs. Thus, for example, in a multiple pin and socket connection, appropriate connections are made to the pins and to the sockets and then related pairs of pin and socket switched into the test sequence.

Describing the system generally, it provides a 20 mV supply to a test circuit, measures the open circuit voltage to check it is 20 mV, then closes the circuit to bring in the test contacts. The loop current, i.e. the short-circuit current, is measured and if less than the preset low value, e.g. 0.5 mA, rejects the contacts. If the loop current is 100 mA, then the voltage drop across the contacts is measured and contact resistance calculated, followed by opening of the circuit. If contact resistance is to be measured in both directions, the current direction is reversed and the voltage checking, open circuit voltage measurement, loop current measurement, voltage drop measurement and contact resistance calculation repeated. The final test result is the average contact resistance for both directions, if that is carried out, or the original contact resistance if current flow in one direction only is acceptable. The system includes a feedback control which adjusts the constant current source to produce a loop current of 100 mA if the loop current is below 100 mA, but above the preset low value, e.g. 0.5 mA. This latter feature provides automatic adjustment to the constant current source, to counterbalance changes in lead length and other variables in the test circuit.

It will be seen that the open circuit voltage is never more than 20 mV prior to test. This meets one of the parameters laid down by the test standard. It will also be seen that once the circuit is closed, the loop current is either at 100 mA, or adjusted up to this value. Again this meets another of the parameters laid down by the test standard. In the course of the test, the constant current source voltage is so adjusted as to maintain the 100 mA loop circuit, in the short circuit current, without exceeding the 20 mV drop over the test contacts. Obviously, this is possible only when the test contact is 200 mA maximum. When this condition is not satisfied, the 20 mV circuit will be maintained while the loop current is decreased proportionally. Therefore, all requirements of the test standard are met.

The test system has been described for the testing of dry circuit contacts in accordance with ASTM B539 test procedure. The system can readily be applied, using other voltage and current parameters, if desired.

The various measuring devices, calculators and the other devices, can be individual electronic circuits interconnected. However, a more convenient way is to program a micro-computer which then makes the various comparisons and calculations, issuing signals to the various items such as the loop closing switch, current source adjustment, and printer.