Abstract:
A retainer for a circuit board test probe includes a unitary member in which the test probe is inserted. The retainer may then be plugged into a desired opening in a test fixture plate, with a circumferential bead on the retainer and a circumferential groove in the plate opening providing a snap-in feature. Inward compression of the retainer moves the bead out of the groove to allow the retainer to be removed from the plate opening.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to circuit board test fixtures and, more particularly, to a snap-in retainer for a test probe to be used with such a fixture. 
     Circuit boards are commonly tested by placing them in a test fixture and using one or more test probes at predetermined locations relative to the circuit board to evaluate circuit conditions at those locations. Toward that end, the test fixture typically includes a plate which is fixed relative to the circuit board. The plate is formed with a plurality of openings at various locations in registration with circuit board locations at which testing is to be done. The test probes are inserted through selected ones of the openings to effect the desired measurements. In the past, holding a test probe in an opening has been done in different ways. For example, it is known to press fit the test probe into an opening. With such an approach, it is difficult to assemble, remove and replace the test probe. It is also known to press fit the test probe into a retainer block and screw the retainer block to the test fixture plate. With this arrangement, tools are required to install and remove the test probe and it is also time consuming. In addition, both of the aforedescribed arrangements provide a risk that the test probe can be damaged when it is press fit into either the test fixture plate or the retainer block. It would therefore be desirable to provide a retainer for a test probe which overcomes the aforedescribed disadvantages. 
     In a particular application, the test fixture plate has a large number of openings closely spaced in a limited area. It is also required to move the test probes from opening to opening. The use of the aforedescribed arrangements are unsuitable for such an application, since the press fitting of the test probe into a plate opening presents obstacles to relocation of the test probe and the use of a retainer block takes up excessive space and is time consuming to move. It would therefore be desirable to provide an improved retainer which can be quickly moved from opening to opening without the use of any tools. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a retainer for removably securing a test probe or the like to a test fixture having a plate with at least one opening for receiving the test probe. The test probe has a cylindrical body with a forward end. The plate opening is circular and has a diameter larger tan the diameter of the cylindrical body of the test probe, and the plate opening is formed with a circumferential groove. The inventive retainer comprises a unitary member having a generally cylindrical central bore. The central bore has a diameter larger than the diameter of the test probe cylindrical body with a portion of the bore at a first end of the unitary member having a reduced diameter sized to snugly retain therein the forward end of the test probe cylindrical body. A forward section of the unitary member extends rearwardly from the first end and includes at least the reduced diameter portion of the bore. The forward section has an exterior configured for a snug fit within the plate opening. The unitary member has a circumferential bead on its exterior surface within the forward section and rearwardly of the reduced diameter portion of the bore, the bead being receivable in the circumferential groove of the plate opening. 
     In accordance with an aspect of this invention, the unitary member is further formed with an enlarged portion spaced rearwardly from the first end of the unitary member by a distance equal to the thickness of the plate. Accordingly, the enlarged portion functions as a stop during insertion of the retainer into the plate opening. 
     In accordance with another aspect of this invention, the unitary member is open to the central bore on opposed sides extending rearwardly from the rear of the reduced diameter portion to a region beyond the rear of the enlarged portion. Accordingly, compression of the enlarged portion toward the central bore moves the bead out of the groove to assist in removal of the member from the plate opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing will be more readily apparent upon reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein: 
     FIG. 1 is a perspective view of a test fixture plate holding a test probe retainer constructed according to the present invention with a second inventive test probe retainer being shown as separated from the plate, with each of the test probe retainers holding a respective test probe; 
     FIG. 2 is a side view showing an inventive test probe retainer held in a plate opening and holding a test probe; 
     FIG. 3 is an end view of the inventive test probe retainer shown in FIG. 2; and 
     FIG. 4 is a longitudinal cross sectional view of the inventive test probe retainer shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, FIG. 1 shows a test fixture plate  10  adapted to overlie a printed circuit board (not shown) undergoing testing. The relative positions of the plate  10  and the printed circuit board being tested are fixed, so that the openings  12  in the plate  10  are in registration with predetermined locations on the printed circuit board at which testing is to be effected. Such testing is effected utilizing a test probe  14 , illustratively a coaxial probe Model No. 100304-00, manufactured by Interconnect Devices Inc. of Kansas City, Kans. As best shown in FIG. 4, the test probe  14  has a cylindrical body  16  with a forward end  18 . The test probe  14  is connected to a coaxial lead  20  for conducting signals from the circuit board to test electronics (not shown). 
     According to the present invention, there is provided a retainer for the test probe  14  which allows the test probe  14  to be inserted into, or removed from, any of the openings  12  in the plate  10  as a one-handed operation without requiring screws, brackets or any tools. The test probe retainer  22  is a unitary member which may be fabricated from many different materials. Thus, the retainer  22  can be molded plastic, either insulating or conductive, machined plastic, either insulating or conductive, or molded or machined metal. As shown, the retainer  22  has a generally cylindrical central bore  24  which has a diameter larger than the diameter of the test probe cylindrical body  16  along the length of the retainer  22  with the exception that at a first end  26  of the retainer  22  the bore  24  has a reduced diameter sized to retain snugly therein the forward end  18  of the test probe cylindrical body  16 . Extending rearwardly from the first end  26  of the retainer  22 , a forward section  28  of the retainer  22  has an exterior configured for a snug fit within the opening  12  of the plate. The opening  12  is formed with a circumferential groove  30  and the forward section  28  of the retainer  22  is formed with a circumferential bead  32  rearwardly of the reduced diameter portion of the bore  24 . The retainer  22  is further formed with an enlarged portion  34  spaced rearwardly from the first end  26  of the retainer by a distance equal to the thickness of the plate  10 . The retainer  22  is initially formed as a stepped cylinder, i.e., it is circular in cross section orthogonal to its longitudinal axis. However, the stepped cylindrical shape is modified by slicing the retainer  22  on opposed sides from the rear of the reduced diameter portion of the bore  22  rearwardly beyond the enlarged portion  34  so as to expose the bore  24 . This provides room for compression of the enlarged portion  34  toward the central bore  24 . Thus, the retainer  22  is generally planar on those two opposed sides. 
     In use, a test probe  14  is installed in a retainer  22  with its forward end captured in the reduced diameter portion of the bore  24 . When it is desired to install the test probe  14  in a plate opening  12 , the retainer  22  is inserted into the selected opening  12  and moved inwardly until the enlarged portion  34  abuts the surface of the plate  10 . The dimensions of the opening  12  and the retainer  22  are such that when the enlarged portion  34  abuts the surface of the plate  10 , the bead  32  is seated in the groove  30 . When it is desired to remove the test probe  14 , the opposed enlarged portions  34  are squeezed together, with play being provided by the open sides of the bore  24  and the space between the cylindrical body  16  of the test probe  14  and the wall of the bore  24 . This squeezing of the enlarged portion  34  causes the bead  32  to be moved out of the groove  30 , thereby allowing the retainer  22  to be pulled out of the opening  12 . 
     Accordingly, there has been disclosed an improved test probe retainer. This retainer possesses a number of advantages. Thus, the inventive retainer eliminates the need for screws and/or special brackets. It is of simple one piece construction and its slim profile fits into tight locations. It is easy to assemble and disassemble by hand, with no special tools being required. In addition, the retainer gives strain relief to the probe and the probe is not damaged when it is removed from the test fixture plate. The inventive retainer maintains a high level of tolerance at multiple locations and fewer test probes are required to test many locations, since the test probe can be plugged in and unplugged very conveniently. The use of the inventive retainer allows an easy replacement of worn or broken test probes. In addition to the use of the inventive retainer with a coaxial test probe of the type disclosed, it can also be used in other applications, such as, for example, inserting thermocouples. Thus, while an illustrative embodiment of the present invention has been disclosed herein, it is understood that various adaptations and modifications to the disclosed embodiment are possible and it is intended that this invention be limited only by the scope of the appended claims.