[1]2 @ 8 6 5 @ 0 0 0 Unioted States Patent Office Patented Dec. 16, 1958 2,865,000 6 AUTOMATIC VOLT-OHMETER Harold R. Newel I 1, Bradford, N. H. Application February 15, 1954, Serial No. 410,246 10 8,CIaims. (Cl. 324-115) This invention relates to electrical measuring instru- 15 ments and more particularly to an automatic voltohmet er having:a self-adjusting range so that when connected for measurements, the meter automatically selects the correct range to bring reading within the scale calibrati,D ns of the meter simultaneously presented on the 20 face of the meter. This prevents damage to the meter and provid es ease and accuracy @of reading. Due to the construction of this device it may be employe d to automatcally indicate resistance or voltage 25 ranges , the device being equally applicable to - voltag,e ineasu reinents of alternating current or direct current. An object of this invention is to eliminate the necessity of a meter havin.- a muliiplier for computing the proper measured values.30 Another object of this invention is to provide an automatic volt-ohmeter having fewer parts than instruments of this nature heretofore available. Still another object Of this invention is to provide a multiranged device for measuring electrical values which has the flexibility to be applicable in part to existirig 35 measuring circuits and havin- sufficient sim@licity to permit construction of a single unitary device embodying the features of an ohmeter, a D. C. volt meter, and an A. C. volt meter. 40 Another object of this invention is to provide an electrical measuring device wherein the input to the device is employed to automatically chaiige the calibrations of a meter to conform to the range 6f the electrical values to be measured. 45 ,Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the accom@anying drawings, wherein: ,Fig. I is a schematic circuit diagram of one embodiment of the invention including elements necessary. for 50 D. C. voltage measurements; Fig. 2 schematicauy illustrates additional circuit c@mponents to be connected for r@sistance meast@rements; and, Fig. 3 schematically shows the circuit diagram of addi55 tional components which can be connected to the invention for A. C. measurements. Referring to Fig. 1, the illustrated invention - includes a balanced bridge circuit comprising a pair of triodes 11 ro and 12 connected in combination with resistance elements 13, 14, 15, 16, 17 and 18. The anodes and cathodes of tubes 11 and 12 are connected to a suitable source of I D. C. potential. Potentiometer 13 functions as a zero setting control. Potentiometer 14 is employed to set the operating point of potentiometer 13 and once adjusted, 65 it need not be adjusted again except when parts are replaced. Resistance elements 15 and 16 are dropping resistors w@ich make possible application of the correct potential to the terminals of potentiometers 13 and 14. Coupled to the grid of triode 12 by means of - resistance 20 and switch 43 is a voltage divider network comprising resistance elements 44, 45, 46, 47, 48 and 49. The rotating element;of switch 43 may be successively coupled to the junction points of the resistors of the divider network by rotating it. Capacitator 19 is connected between the grid of triode 12 and ground to filter alternating current from the input of triode 12. Meter M is connecied in 8eries across th6 output of the bridge circuit through resistance 22 and double-pole double th@ow switch 21. Switch 21 provides polarity reversal for D. C. measurements. The output of the bridge as it appears across resistance 22 and meter M is applied to the grids Df triodes 23 and 24. ;Coupled in the plate circuits of triodes 23 and 24 are relays 25 Eind 26. These relays are adapted to'be closed when no voltage i.s applied to the grids 6f tubes 23 -and 24. Potentionieters 27 and 28 operate to control the plate current of tubes 23 and 24. Resistors 29' and 30 are conn6cted in the cathode circuits of tubes 23 ahd 24 in order to provide the proper cathode potential. Rdlays 25 and @26 are connected to a source of D. C. potential. Relay 37 is similarly connected to the same source. Contacts 36 on the armature of relay 25 control direct current flow through relay 37 in series with limiting resistance 38. Contacts 40 of relay 37 control alternating current flow through range change racheting solenoid 41, actuating the solenoid when closed. When actuated the solenoid closes contacts 51. Contacts 40 close when contacts 26 of relay 25 open. A condenser 42 is connected across contacts 40 to suppress sparking at the contacts. Contacts 35 close to short meter M when relay 25 opens. Solenoid 41 is coupled through contacts 40 to a source of alternating current. Connected to the solenoid is a pawl 71 which advances ratchet wheel 70 one position c,ach time solenoid 41 is energized by closing of r.elay 37. Ratchet wheel 70.is mounted on a shaft niechanically connecting meter M to switch 43. Ratchet wheel 70 as iuustrated has five points, thereby permitting the pawl to move the shaft through five positions. Meter M is provided with an indicating scale having five ranges, ea@h of thege ranges being successively positioned as the shaft is rotated. Ratchet wheel stop 72 prevents return of the ratchet wheel to its original position except when desired Also mounted on the mechanical shaft is a Spring 7@ which is biased in a direction such that it tends to return the shaft to its initial position. In this case the initial position is when the rotating element of switch 43 is in the position shown in Fig. 1. Also provided on the shaft is -a mechanical stop 75 which limits rotation of the shaft, the limits being indicated at "Low" and "High," in Fig. 1. The mechanical shaft is also provided with a cam 74 which operates upon rotation of the shaft to close contacts 33 in one position and to perniit contacts 33 to separate in a second position. Reset relay 32 operates through ratchet wheel stop 72 to hold each range until actuated. When actuated relay 32 allows spring 73 to return the shaft to its original position. Reset relay 32 is actuated only when contacts 33 are closed at the same time that contacts 31 of relay 26 is closed, operation of relay 26 being controlled by the signal applied to the grid of tube 24. A capacitor 34 is provided to suppress sparking at contacts 31 and 33. A capacitator 50 is provided to bring the negative connections of triodes 23 and 24 to the same A. C. potential as the ground terminal B. Operation of this device is as follows: The tubes of the circuit are allowed to heat, after which the indication on the meter provided by the conventional needle or pointer is brought to zero by adjusting zero set potentiometer 13. This causes the potent al
[2]2,8(35,000 3 on the grids of tubes 23, 24, to be zero due to the fact that the bridge is in a balanced condition. A D. C. voltage to be measured is then applied between terminals A and B. If the voltage applied between terminals A and B is not excessive for the range of the 5 scale in,position on the meter, the circuit causes no movement of ratchet wheel 70. However, if the voltage applied to terminals A and B is beyond a predetermined point on the scale or exceeds the range of the scale in p6,sition on the meter, the voltage applied to the grid 10 of tube 12 causes the bridg@a to become unbalanced, and the output of the bridge is applied to the grids of tubes 23 and 24. The voltage applied to the grid of tub,e 23 causes the plate current to change sufficiently to ppen the armature of relay 25. When relay 25 is closed, the 15 normal condition, a closed d@irect current circuit exists comprising a D. C. supply, resistor 38, contacts 36, relay 37, dnd the supply. However, when relay 25 opens, relay 37 closes, thereby permitting alternating current to flow through solenoid 41 and causing the pawl to advance 20 the rdtchet wheel 70 one position. Closing of solenoid 41 causes contacts 51 to short relay 37 and - condenser 39 through resistance 52. Shorting of relay 37 causes qontacts 40 to open. Opening of contacts 40 causes the solenoid to open and the short circuit across relay 37 25 is removed. As is apparent, the time duration for completion of this action is determined by the values of the @apacitator 39 and resistors 38 and 52. Rotation of ratchet wheel 70 causes the rotatable ele,ment.of switch 43 to rotate to the junction of resistors 45 30 and 46, changing the voltage applied to the grid of tube 12. Tf the pointer of meter M then reads on the scale which has been moved into position by advancement of the ratchet wheel, the voltage applied to the grid of tube 23 is i 35 insufficient to cause opening of relay 25, and relay 25 @em- ains closed to prevent further range changing action. if -m,eter M still does not read o n scale, the range changing action provided by the circuit continues until the meter reads on scale. I?referably When the voltage output of the bridge cir'@0 cuit reaches a value proximately equal to 1 /:to of the scale of the meter or greater, sufficient voltage is applied to the grid of tube 24 to o en relay 26. When relay 26 is p opened, the alternating current circuit of relay 32 is br,o.ken and the ratchet wheel is prevented from returning 45 to its original position. If the voltage output is less than :y .toth, relay 26 remains closed. When the meter is at its lo@vest range position contacts 33 are open due to the position of cam 74. Therefore with relay 26 closed, due 50 to the fact that the voltage appearing on the tap of switch 43 is less than I/ 10, relay 22 is not actuated, and stop 72 remains in contact with the ratchet wheel. If the meter is not at its lowest range position, contacts 33 are closed due to the position of cam 74, and reset relay 32 is actu 55 ated when the reading drops below 1/10 of the scale since relay 26 is also closed. Stop 72 thereupon moves out of contact with wheel 70 and the spring retums the shaft to its original position. Relay 32 holds each range, i. e. prevents the shaft from returning to its original - position, 60 until actuated. In this case this occurs when the D. C. voltage to be measured is removed from terminals A and B. Solenoid 41, as is evident from the fi,-Ure, ratchets the scale of meter and the rotating element of switch 43 ahead o,ne position each time the solenoid is actuated. 6 5 Fig. 2 illustrates an alternative combination to be connected at terminal C in place of the volta,@e divider network and the switch element of Fi.-. 1. This alternative arrangement adapts the invention for measurement of resistance. In this case a circuit comprising switch 53 and 70 resistance elements 54 through 58 are substituted for 'fch 43 and re i swi s stances 44 through 49 with terminal C' of Fig. 2 connected to terminal C of Fig. 1, terminal B' of Fig. 2 connected to terminal B of Fig. 1, and corresponding connections of elements 44 through 49 being 7,5 4 removed. Terminal A is shown in dotted lines in Fig. 2 merely for the purpose of illustrating the relative position of the resistance measurement circuit in the device of Fig. 1. However, when the resistance measurement circuit of Fig. 2 is coupled to terminal C of Fig. 1, terminal A is not connected. A source of knownpotential, e. g. a battery 59, is connected through zero centering potentiometer 60 between terminals C' and D. The resistance to be measured is coupled between terminals D and B. Preferably a resistance 61 of larger value is substituted by suitable switch means (not shown) for smaller resistance 22 between ten-ninals F and G in Fig. 1 in order that the voltage applied to the grid of tube 23 will be increased to cause range scale adjustment with less deflection of the meter pointer. In Fi@-. 3 a third alternative embodiment is illustrated. This alternative construction is employed to adapt the device for A. C. voltage measurement. A plurality of resistance elements corresponding to resistors 45 th,rpugh 49 of Fig. 1 are employed togeiher with diodes 62 and 63, resistances 64, 65, 67 and 69, and capacitors 66 and 68 to provide a D. C. isolating and balanced rectirier c,ircuit. While the circuit of Fi-. 3 is shown as a unitary substitute for the circuit connected between terminals A and B of Fig. 1,1, it is obviolis that diodes 62 and ()3, resistances 64, 65, 67 and 69, and capacitors 66 and 68, may be connected by suitable switch means to Lhe same network of Fig. I to provide a rectifier circuit as in Fig. 3. For this reason numerals 45@49 are employed in both Figs. 1 and 3 to i-@idicate that the same resistor netWork is employable in both applications of the present invent,on. lvhile the network of Fig. I may beei-nployed unchaiigpd, preferably resistor 44 is omitted, sinc.- diodes 62 and 63 introduce sligiit additional resistances. Terminals E and B serve as input terminals, and switch 43 couples this circuit to terminal C of the device of Fig. 1. AlthoLigh the embodiments of Figs. 1, 2, and 3 are shown in disconnected relationship, the embodiments of Figs. 2 and 3 being mentioned hereinabove as substitutes for the voltage divider network of Fig. 1, it is obvious that all threeembodiments may be incorporated in a single volt-ohmeter unit, suitable switching means (not shown) being employed to switch in any one of the three embodiments for measurement. . Since this device is equally p icable to measurement of resistance and voltages, it is aP '@ preferred that ihe meter scale have calibrations providing for both reslistance and voltage readings. Such a meter - may have two groups of changeable calibrations for resistance and voltage on the sarti I e scale, the scale being rotatably mounted in the meter. ' Obviously many modifications alnd variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scopp of the a pended claims th in e @p e v ntion may be practised otherwise than as specifically described. I
