Abstract:
Disclosed herein is a hand-held clamp-type multi-meter capable of measuring current and other electrical characteristics. The clamp includes a hinged jaw which is openable in the current detecting mode of the multi-meter, but not in other modes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not applicable) 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     (Not applicable) 
     BACKGROUND OF THE INVENTION 
     The present invention relates to meters for measuring electrical parameters such as current and in particular to multi-meters which include a clamp-type ammeter. 
     Meters for measuring current, voltage and resistance or to detect electrical continuity of an electrical device or line are well known. Such meters typically include sensing circuitry as known in the art to measure only one or more of these parameters. Multi-meters are capable of measuring multiple parameters and include a selector for switching the sensing circuitry to detect the desired parameter. 
     Some multi-meters have a conductive wire coil or metallic loop that can be positioned about an electrical line, cable or bus bar to measure the current passing therethrough by picking up the surrounding inductive field. A closed loop is necessary to provide a closed electrical path to the sensing circuitry of the meter. Some meters, known as clamp-type ammeters, have a split loop disposed in a C-shaped clamp that can be opened to fit around the line. When the clamp is closed the loop again defines a closed path. 
     The coil or conductive loop is connected in the sensing circuitry so as to only measure current. That is, the loop is inoperative for measuring voltage or resistance. The clamp is only meant to be used when the selector is in the current measurement position. Thus, if a user attempts to measure voltage, for example, with the loop, the multi-meter would not provide a measurement. The same is the case if the user intended to measure current with the loop but the selector was positioned in a voltage or resistance measurement mode. In either situation, this may be frustrating for the user and cause him or her to incorrectly believe that the multi-meter was broken or functioning improperly. This can harm the reputation of the manufacturer and result in returned merchandise or exercised warranties despite the fact that the meter operates properly. This process can be time consuming and expensive for both the user and the manufacturer. 
     Accordingly, a need exists in for an improved clamp-type multi-meter. 
     SUMMARY OF THE INVENTION 
     The present invention provides a hand-held clamp-type multi-meter in which the current clamp can be opened only when the meter is set to measure current. 
     Specifically, the present invention is a multi-meter at least capable of detecting the current and one other electrical characteristic of an electrical line connected to an electrical circuit. The multi-meter has an indicator coupled to sensing circuitry that includes an electrically conductive loop disposed in two jaws of a current clamp. The jaws are opposable so that the current clamp can be separated and fit around the line without disconnecting it from the circuit. The current clamp can be opened only when the multi-meter is set in the current measuring mode. 
     Thus, the present invention ensures that the conductive loop is used only for measuring current. The multi-meter includes a locking mechanism which prevents the current clamp from opening if the multi-meter is not set to measure current. When the current clamp is open, the locking mechanism also prevents the multi-meter from being set to measure another parameter, such as voltage or resistance. 
     In a preferred form, the multi-meter includes a palm-sized housing in part containing the sensing circuitry and to which is mounted a measurement selector for switching the sensing circuitry between voltage, current and other parameters. Rotation of the measurement selector to measure current operates the locking mechanism to unlock the current clamp. A thumb lever is connected integrally to a pivotable jaw of the current clamp that is biased to close the jaws. When the multi-meter is set to measure current, the locking mechanism is positioned to allow the thumb lever to be depressed inwardly, which rotates the pivotable jaw away from a fixed jaw and separates the loop. Preferably, both jaws are arcuate members defining a closed path with an opening therebetween. The jaws, which include the loop, may also include an electrically inert covering material, such as plastic, which houses the inductive pick-up loop. The mating ends of the fixed and pivotable jaws of the current clamp have openings that allow the loop to define a closed circuit when the current clamp is closed. 
    
    
     A preferred embodiment of the invention is stated in the following description and illustrated in the accompanying drawings which form a part hereof. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the hand-held multi-meter with a current clamp of the present invention clamped around an electrical line (shown in phantom); 
     FIG. 2 is a partial cut-away view of the multi-meter of FIG. 1 showing the current clamp locking mechanism; 
     FIG. 3 is a partial front plan view showing a measurement selector positioned to measure voltage and the current clamp locked closed; 
     FIG. 4 is cut-away view of FIG. 3 showing the locking mechanism; 
     FIG. 5 is a side cross-sectional view taken along line  5 — 5  of FIG. 4; 
     FIG. 6 is a side cross-sectional view taken along line  6 — 6  of FIG. 4; 
     FIG. 7 is partial front plan view similar to FIG. 3 showing a measurement selector positioned to measure current and the current clamp opened; 
     FIG. 8 is a cut-away view of FIG. 7 showing the locking mechanism; 
     FIG. 9 is a side cross-sectional view taken along line  9 — 9  of FIG. 8; and 
     FIG. 10 is a side cross-sectional view taken along line  10 — 10  of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a multi-meter  10  includes a palm-sized housing  12 , preferably made of a suitable rigid plastic containing current, voltage and resistance sensing circuitry (not shown), as known in the art, and a power supply (not shown) such as two AA sized batteries. The housing  12  also includes an analog gauge  14  electrically coupled to the sensing circuitry and viewable through a transparent window  16 , from which the value of the electrical characteristic (i.e., current, voltage, or resistance) can be read. All features of the multi-meter  10  except the locking feature of the clamp described below are conventional and therefore not described in detail. 
     The housing  12  also mounts a rotary measurement selector  18  for switching the sensing circuitry as known in the art to measure current, voltage or resistance in a circuit as well as between various sensitivity levels of amperage and voltage. At one end of the housing  12  is an inductive pick-up current clamp  20  having a fixed jaw  22  and a pivotable jaw  24  rotatable about a pivot pin  26 . As is well-known, the jaws  22  and  24  include a conductive loop of laminated steel sheets electrically connected to the sensing circuitry and housed in plastic sheaths. When closed, the laminations of the jaws  22  and  24  form a closed magnetic inductive pick-up loop in well-known fashion. 
     Referring to FIG. 2, the jaws  22  and  24  are connected to the housing  12  at pivot pin  26 . Pivotable jaw  24  is freely rotatable about the pivot pin  26  while the fixed jaw  22  is stationary. Integrally molded to the plastic sheath of the pivotable jaw  24  is a thumb-operated lever  28  for opening the current clamp  20  by depressing the thumb lever  28  downward. As shown in FIG. 4, the thumb lever  28  is biased outward by a torsion spring  30  fixed to the housing  12 , which biases the current clamp  20  closed. The movement of the thumb lever  28  is controlled by a locking mechanism  32  also mounted in the housing  12 , the operation of which is controlled by the measurement selector  18 . 
     Referring to FIGS. 2,  4  and  5 , the locking mechanism  32  consists of a cam disk  34 , a lever bar  36  and a torsion spring  38 . The disk  34  is rotatably mounted inside the housing  12  in registration with the measurement selector  18 . The disk  34  has a square recess  40  that receives a square peg  42  extending axially from the measurement selector  18  so that the disk  34  rotates with measurement selector  18 . The disk  34  has a raised ridge  44  extending around approximately ⅔ of its circumference ramping down to an arcuate recessed surface  46 . 
     The lever bar  36  is pivotably mounted via a pin  45  connection to a fixed fulcrum member  47  extending inwardly from the inside of the housing  12 . The lever bar  36  has a rounded cam follower end  48  and an opposite flat stopper end  50 . The lever bar  36  is connected to the torsion spring  38  so that the cam follower end  48  is biased against the circumferential ridge  44  or recess  46  of the disk  34 , depending upon the angular position of the disk  34 . Thus, as the measurement selector  18  is turned to the desired setting, the disk  34  rotates and the cam follower end  48  of the lever bar  36  rides along the disk  34 . 
     When the measurement selector  18  is positioned so that the multi-meter  10  is set to measure resistance or voltage (as in FIGS.  3 - 6 ), the disk  34  is positioned so that the cam follower end  48  contacts the raised ridge  44 . In this position, the lever bar  36  is pivoted so that the stopper end  50  lies a stop  52  on the thumb ever  28  and a backstop member  54  integral to the housing  12  so as to be pinched between them when the lever  28  is depressed. This prevents the thumb lever  28  from being depressed and the pivotable jaw  24  from being rotated. In this way, the current clamp  20  is locked so that it cannot be opened in the voltage or resistance measurement modes at the multi-meter  10 . The backstop member  54  supports the stopper end  50  of the lever bar  36  and prevents it from bending or breaking in the event a user attempts to depress the thumb lever  28  in the locked position. 
     Referring to FIGS. 7-10, as the measurement selector  18  is turned to measure current, the cam follower end  48  rides down the ramp  35  to the recessed surface  46 . The lever bar  36  is pivoted so that the stopper end  50  does not interfere with the stop  52  of the thumb lever  28 . As such, depressing the thumb lever  28  pivots the pivotable jaw  24  to open the current clamp  20 . The current clamp  20  can then be fit around an electrical line to measure the current in it. 
     Illustrative embodiments of the invention have been described in detail for the purpose of disclosing a practical, operative structure whereby the invention may be practiced advantageously. However, the apparatus described above is intended to be illustrative only, and the novel characteristics of the invention may be incorporated in other structural forms without departing from the scope of the invention. For example, other locking mechanisms may be employed to prevent the clamp from opening when not measuring current. Moreover, the clamp may be of a different configuration as long as it provides for a closed path for the loop. Accordingly, to apprise the public of the full scope of the invention, the following claims are made.