CA1101072A - Electrical circuit test devices - Google Patents

Abstract

A B S T R A C T
This invention relates to a test device for detecting an electric voltage and/or its polarity or the flow of current through an electrical conductor. Two luminescent diodes connected in antiparallel circuit are connected in series with as well a ceramic cold conductor thermistor, a keying switch, a battery and a protective diode. The battery provides a voltage when testing for flow of current is carried out. Further safety features guarantee that no damage to the device or to its user can occur when high voltages are tested. For example a zener diode may be connected across the keying switch.

Description

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. The invention relates to electrical circuit test deYices for detecting the presence and polarity of an ~'ectric potential or performing a continuity test, the de~ice comprising two test pro~es connected by a cable.
The German Patent Specification No. 2,060,884 describes a test device ~or indicating the polarity and the phase state of electric potentials relative to a given reference potential, whexein two luminescence diodes are connected in parallel with opposite polarities, and a series current limiting resistor is provided. The two luminescent diodes are disposed by a protective wlndow in a pencil-like housing of a test probe which also contains the series current limiting re.~istor and has a metal point at one end. The two luminescen~e diodes are connected via the current limiting resistor between the metal point and a test }ine which leads out of the pencil-like hous~ng~ When testing the presence o~ a d.c.
~oltage causes one or the other of the lumine~cence diodes to be illuminated, in accordance with the polarity, whereas ~0 when an a.c. voltage 15 present then both luminescence diodes are illuminated.
T the current limiting resis or is in the form o~ a ceramic cold conductor thermistor which, as is known, may consist of ferro-electric material on a base of n~doped and/or p-doped barium titanate, and has a temperature dependence such that in the ragion of the Curie temperature it exhibits a sudden increase ln its eIectrical xesistance by approximately 4 powers of ten, this device can be used ~L

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to check voltages ~rom approximately 2 V up to 500 V. The ceramic cold conductor exexts a protective function for ~he luminescence diodes~ as it limits the current if current flow causes self heating to a value above the Curie temperature, with the consequent considerable increase in resis~ancP.
However, in many cases it is not only necessary to determine whether a device is connected to a current source, i.e. to detect wheth~r a voltage is present and ascertain the nature of the polarity thereo~, but it is also desirable to be able to check whether there is continuity of an elec~ric conductor path, so that it can be ascertained whether an electric component, an electric assembly or the like load circuit may be traversed by current when connected to a lS source. Th~s chec~ing of con~inuity for the passage of current i~ often of particulax ~nterest, and may be desired when it is not necessary to check voltages.
Current continu~ty path checking devices operate in accordance with the principle that a current is supplied

2~ ~rom a test voltage source to ~low through tha electrical conductor path which is to be checked, and-an son~tic or optical signal is normally provided to indicate whether there ls a pa~sage of current or not.
The U.S. Patent Specification NoO 3,870,950 describes an input protection circuit for electric current path continuity che~king devi~e which contains a separate current source and a current flow indicator fbr testing voltage-free circuits, wi~h protection from al~en voltages which could jeopardise the current flow indicator, a semiconductor switch and a cold~conductor thermistor being connected in series, with the free-ends of probe pins serving as connecting terminals, the current source and the current flow indicator being connect-ed in series between the free ends, one free end being connected via the semiconductor switch and the other via the cold conduct-or thermistor.
One object of the present invention is to provide a voltage test device of a type known per se for detecting any electric voltage and/or the polarity thereof with further capabilities so that it can readily be used as a continuity test device by the passage of current through an electric conductor path, with adequate protection functions provided to increase the reliability of the device.
According to the invention there is provided a test device for .indicating an electrical voltage and/or the polarity thereof, as well as the passage of current through an electrical conductor, comprising two handles, which are connected by means of a cable and are provided with contact tips, and one of the handles contains two luminous diodes connected antiparallelly and preconnected to a ceramic cold conductor as a current limit-ing resistor and for testing for passage of current through an electrical conductor, the following features are present:
a~ a battery connected in series with a protective diode mounted in one of the handles, in which said cold conduc-tor and the luminous diodes are housed;
b) a keying switch and a Zener diode connected in parallel and mounted in said handle;

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c) the series connection of the battery and protective diode connected to a normally open contact of a keying switch, which is normally connected with a switch contact connected to a parallel line, which bridges -the battery and the protective diode;
d) the keying switch, Zener diode, battery and pro-tective diode connected in series with the antiparallel connect-ion of the luminous diodes;
e) the battery is housed with positive and negative terminals in a removablehousing which, by means of projections in the inserted state, engage the handle and is removeable only to an extent such that the opening of the recess in the handle remains closed.
Preferably the keying switch is arranged in front of a guard ring.
Preferably the keying switch is connected in parallel with a 2ener diode.
Advantageously, one handle may contain all the com-ponents and the second handle simply serves as a contact point probe.
The invention will now be described with reference to the drawings, in which:
Figure 1 schematically illustrates constructional details of one exemplary embodiment of the test device;
Figure 2 is a schematic circuit diagram of one exemplary embodiment of the invention;
Figure 3 is a schematic circuit diagram of a further exemplary embodiment of the invention;

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-Figure 4 schematically illustrates constructional details of another embodiment of the test device;
Figure 5 schematically illustrates details of yet another exemplary embodiment of the test device;
Figure 6 shows details of a preferred form of battery housing for use in embodiments of the invention; and Figure 7 is a section along the line IV-IV of Figure 5 showing the battery housing in the extracted position.
In the embodiment shown in Figure 1, the test device consists of two probes in the form of handles 1 and 2 which house the electrical components and electrical connection -5a-paths.
The handle 1 contains two luminescence diodes 11 and 12, whi~h are visi~le from the exterior. A guard ring 13 surrounds the handle 1 to serve as a protection against a user's finger slipping cnto a voltage source when a test is being carried out. ~ metal point l4 at one end of the handle 1 serves as a contact element tb connect the ~est devlce to a line, component or circuit point which is to be checked.
A contact point 15 is provided o-, the handle 2, and the two handles are connected together ~y a cable 16.
The handle 2 has a continuous guard rin~ 17 for protection, similar to the ring 13 of the handle 1. ~
keying switch 5 is arxanged between the metal point 15 and the ring 17. The handle 2 accommodates a battery 3 and a protective diode 4, and the circuit elements are wired together in accordance with the circuit diasram sho-~n i ~igure 2.
The battery, the pro~ctlve diode and the ~eying switch 5 can all be accommodated in the handle l, so that the handle 2 is a simple test probe, as shown in the embodimen~s illustra~ed in Figure 4 and 5.
In the circuit diagram shown in Figure 2, the terminal .. . .
points 14 and 15 correspond t~ the metal contact polnts of the test device. The ceramic cold conductor thermistor 18 is connected in series with the parallel arrangement of the oppositely poled luminescence diodes 11 and 12 to the keying switch 5, which in the rest position 9 is co~nected to a direct connection line leading ~o ~he metal contact point ~:~L0~L~7~
15. The keying switch S is ~ring-loaded, and is depressed manually to effect a continuity check so that contact is made with a switch terminal 8.to bring a battery 3 into circuit in series with a protective diode 4. The ~attery

3 has a positive pole 7 and a negative pole 6, and the diode

4 is poled to conduct current from the battery if there is a continuous conductive path present when making a continuity check.
The operation o the device is as follows. For a voltage check, the keying switch is left in its rest position, so that any voltage difference presented to.
the metal contact points flows via the thermistor and one of the luminescence diodes, to c~use it to glow, the particular diode indicating the polarity of the po~ential.
If an a.c. potential is present, then both diode~ glow. An excessively high potential wi:Ll cause the thermistor to be heated above its Curie temperilture so that its resistance rises rapidly~ and so protect~i the diodes from damage.
For a contlnuity chec~, the metal contact points are ~0 positioned on the path to be chec~ed, and the keying switch 5 is depre~sed so that current can flow from the battery 3 if th~re is continuity, and diode will glow. If an a.c. voltage or d.c. voltage is erroneously connected ~ia the circuit under test duriny a continuity check, this will be fed in via the metal points 14 and 15 and if of one polarity could produce a charging current and thus a gas discharge in the hattery, which ~ould lea~ to its destruction if it ls o~ encased design. The ceramic coId conductor 07;~

thermistor provided for the protection o~ the luminescence diodes also serves to protect the battery, and thus has a combined effect which virtually eliminates the danger of destr~ction J but as the cold conductor requires a specific length o~ time in order to ~ecome high-ohmic ~y self heatiny the protective diode 4 is incorporated into the protective circuit to hIock excessi~e c~rren~ flow until the thermistor comes into operation.
The battery 3 may ~e a primary cell structure or a secondaxy cell structure, e.g. a 12 V monocell. In place of a pxotective diode~ it is also possible to use another semiconductor circuit element, for example a transistor or a triac arranged to provide a similar protective function.
In the modified circuit sh~wn in Figure 3, which may lS be used in place of the c.trcult illu~trated ln Figure 2 is expanded by the addition of a Zener diode l9 connected ac~oss the keying switch 5 to link the bridglng parallel line 10 to the adjacent ~unction of the parallel diode comblna~ion.
In practice the Zener diode 19 provldes three further pxotecti~e functions. Pirst~y, the keylng swltch 5 is protected from high voltages. Secondly, duriny the - tes~ing of high voltages, mistaken actuation of the keying switch 5 is prevented from leading to breakdown o~ the battery 3 and/or the protecti~e diode 4. Thixdly, pxovision i5 made for the event in which there is no battery 3 inserted, or in which the battery is discharged, so that misunderstandings by mistake~ act~atio~ of the keyi~g switsch

5 in positio~ for co~tinuity testing are elimlnatedO
~8-The ~orwards voltage o~ the Zener diode 19 should be selected in accordance wlth the voltage of the battery 3.
A zener diode for 15 V is particularly suitable in the case of battery voltages of less than 15 V, e.g. 12 V~
In the further embodiment shown in Figure 4, the test device again consists of two handles 1 and 2~ but ~he handle 2 contains no circuit components, and merely has the function of ~earlng the contact poin~ lS, and is provided with a guard ring 17, whereas the handle 1 connected to the handle 2 ~ia the line 16 contains all the components of the circuit illustrated in Figuxe 2 or Figure 3.
The luminescent diodes ll and 12 are arranged between the contact poin~ 1`4 and the guard ring 13, so that they are easily visible. They may be mutually aligned parallel to the longitudinal axls, or t:ransverse thereto.
The handle 1 also pos~esses the keying switch 5, which ls positioned on that side of the guard ring 13 opposite the luminescence diodes 11 and 12, i.e. on that side o~ the handle.l which is enclosed ~y the operators hand, when in use.
Within the handle l there is arranged the bat~ery 3, which lies with its poles 6 and 7 engaged between clamping cont~ct springs 20 and 21. The batte~y 3 i5 accommodated in a battery container 22 which ~tself is provided with an :
2S openiny 23 matched to the size of the battery.
A projection 24 ser~es to allow the extr2ctable housing 22 to be positiveIy engaged with the wall of the housing 1 when in the inserted state.

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In order to extract the battery 3 from the recess 23 of the container 22, the wall thereof is partially removed at one end 25 so that fxee areas are formed on ~oth sides of the ba~tery ~nd through which the battery can be gripped.
When the container 22 has ~ee~ extracted, an opening is provided through which the protective components, i.e.
the cold conductor 18~ protecti~e diode 4, and the Zener diode 19 if proYide~, are all accessi~le fro~-the exterior, ~or xepair or replacement.
In th~ alternative embodiment s~o~n ln Figure 5, the test device again consists of two handles l and 2 very similar to the embodiment shown in Figure 4.
Figure 6 lllustrates details of the battery container 22 with the cylindrical cavity 23 in which the battery 3 can be accommodated. The cavity 23 i~ partially closed at the left-hand end 30, as drawn, ~n such manner that only the positlve pole 7 o~ the battery passes through opening 31.
At the other end, the opening of the cavity 23 corresponds to the crDss-section o the battery 3. The lenyth of the cavlty 23 between the ends 30 and 32 corresponds exactly to the length of the battery less its positive pole 7, so that i~ the battery ls incoxrectly inserted into the recess 23, the positive pole 7 project~ towards the right. If the housing 22 wer then impressed into the opening 26 (Figure 5) o~ the handle l, the positi~e pole 7 would prevent complete insertion, ~ecause ~t projects beyond the outer dimensions o the container 220 ... . . .. .. . . . .

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As shown in the cross~section shown in Figure 7, the container 22 can be fully removed ~rom the handle 1, for example, when the device requires to be serviced. However, the contalner cannot ~e readily extracted during nonmal use, because this is prevented by the projections 27 (Fig. 6).
In place of ~he projections 27 it ls also possible to provide a bead 28 which extends along the entire length of the conta~ner 22. Since, when the container 22 is in this limited state of extractlon, in accordance with Figure 7, the opening 26 reamins closed to prevent arcess to the interior, so that lt is not normally possible to touch the contact 20 or the contact 21. The projections serve to lock the container 22 in the fully inserted state.
It is most advantageous to install the container ~2 into the handle 1 when this handle is itsel assembled. The handle 1 in this embodiment consists of two halves 33 and 34, as indicated in Figure ~ a:nd Figure 7, which are screwed together o~ flrmly ~lu,ed to one another in a plane 35. Prior to this assembly, t!he handle half 33 can be elastically deformed ln the region o~ the opening 26 to such an extent that the battery container can be insertedO The glu~ing of.the two.handle halves 33 and 34 along the lin~
35 cancels this elasticity, so that the container 22 can no longer be rPadily extracted if the handle halves are not simultaneously dlsassembled.
Naturally, it is also possible to bevel the projactions as indicated by the line 36 in ~igure 7, so that the projections act, as it were~ as barbs. In this case the .

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hatter~ container can be installed following. the assembly o~
the handle 1 by force~ul pressure, but cannot be full~
extracted without the use of a sulta~le tool.
This embodiment o~ the invention ensures that when S the test device is used to indicate a voltage, then in the event of any misactuation of the keying switch 5, and pos~ibly a breakdown Gf the protective diode 19 which bridges this keying switch 5, two possible dangers are avoided~ namely the touching of the contacts 20 and 21 or the possibillty that due to the misinsertion o~ the battery into the cavity 23 the negative pole 6 of the battery 3 lie~ in the recess in such manner that it incorractly touches the contact 21 and doe~ not as is correct - touch the contact 20. Naturally the touchin~
o~ the contacts 20 and 21 by a user in the e~ent of misactuation of the key 5 during ~he display of a voltage could be a serious and dangerous condit~on.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A test device for indicating an electrical voltage and/
or the polarity thereof, as well as the passage of current through an electrical conductor, comprising two handles, which are connected by means of a cable and are provided with contact tips, and one of the handles contains two luminous diodes con-nected antiparallelly and preconnected to a ceramic cold con-ductor as a current limiting resistor and for testing for pass-age of current through an electrical conductor, the following features are present:
a) a battery connected in series with a protective diode mounted in one of the handles, in which said cold con-ductor and the luminous diodes are housed;
b) a keying switch and a Zener diode connected in parallel and mounted in said handle;
c) the series connection of the battery and protect-ive diode connected to a normally open contact of a keying switch, which is normally connected with a switch contact connected to a parallel line, which bridges the battery and the protective diode;
d) the keying switch, Zener diode, battery and pro-tective diode connected in series with the antiparallel connection of the luminous diodes;
e) the battery is housed with positive and negative terminals in a removable housing, which, by means of projections in the inserted state, engage the handle and is removable only to an extent such that the opening of the recess in the handle remains closed.

2. A test device according to claim 1, characterized in that the housing contains a cylindrical cavity for receiving of the battery, and said cavity has one side which is limited by an opening, into which only the positive terminal of the battery fits, and the other side of said cavity has an opening which corresponds to the diameter of the battery, the length of said cavity between the two sides corresponds to the length of the battery less said positive terminal, the length of the opening in the handle is less than the length of the housing, and the housing is provided on its bottom side with projections which limit the distance it can be removed from said handle.