Testers

A circuit tester is disclosed which automatically open-circuits a main lead to an input circuit in response to an excessive overcurrent. The circuit tester of the present invention will automatically sense the existence of an excessive current which, in turn, open-circuits a main lead to the input circuit, actuates a buzzer and energizes a light-emitting diode. Since the main-lead to the input circuit has been open-circuited, the base voltage at a first transistor drops. This renders this transistor non-conductive and tends to cause the main-lead to the input circuit to short-circuit. This tends to restore the circuit tester to its initial condition. However, if the excessive current is continuing, the above sequence will re-commence, the sequence continuing until the excessive current no longer exists or until the tester is adjusted to a higher range of operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to testers, especially those testers which 
include at least two input terminals, a galvanometer capable of visual 
indication of measured values, and a network which is located between the 
input terminals and the galvanometer and capable of converting signals 
generated at the input terminals to the level suitable for the operation 
of the galvanometer. 
2. Description of the Prior Art 
The circuit tester is in wide use for a variety of measurements on electric 
circuits, e.g., for voltage measurements in a wide range from several 
hundred mV to ca. 1000 V. Given such uses, it is inevitable that a tester, 
set for a lower voltage range, may be used, by mistake, to measure 
voltages higher than the maximum voltage setting on the tester. Such 
mistaken operations often result in damage to meters, circuit elements 
(such as resistors), or patterns, within the tester. 
As a protective means for circuits to guard against such mistaken 
operations, some conventional testers have fuses equipped in their input 
circuits. The use of fuses, however, results in disadvantages in that 
measurements of electric currents result in errors since the fuse 
possesses a finite resistance and the measurement accuracy is lowered 
since the fuse resistance is temperature dependent. When an excess current 
is caused to flow, it takes the fuse some period of time to burn out, 
sometimes resulting in damage to the internal circuits. The fuse needs to 
be replaced after burn-out. Thus, the use of a fuse has specific 
disadvantages from the standpoint of maintenance of the tester. 
Some testers have been designed to allow their relays to detect overcurrent 
and to cut off their input circuits for measurement. Japan Pat. No. 
53-38481/1978 describes an example tester of this sort which uses a 
latching relay. This latching relay has a temporary stable state and a 
semipermanent stable state, the transition from the former to the latter 
state being effected by a flow of current over the working range through 
the coil, the reverse transition by external mechanical force being 
exerted via manual operation, etc. In general, relays are suitable for 
protecting tester circuits since they have low contact resistance with low 
temperature dependence and are capable of rapid reaction. In addition, the 
use of relays permits the protective operation to be executed at the same 
multiplying factor among all the measurement ranges, so that they may 
protect not only the current measuring circuits but also the voltage 
measuring circuits as well. The tester using the latching relay may cut 
off its input circuit in response to an excessive current by shifting into 
the semipermanent state. However, this tester has operational 
disadvantages in that manual operation is required to reset the relay from 
the semipermanent to the temporary stable state, and in that, before the 
cause for the input of the excessive current is removed, the tester may 
reset its relay resulting in a closed input circuit having the same 
excessive input current to its measuring circuit. It has an additional 
structural disadvantage in that, since the temporary stable state is a 
so-called "unstable stable-state", where the relay has its set of springs 
under some tension, the tester, when in the temporary stable state, may 
have its input circuit cut off in response to an accidental external force 
such as vibration, shock, etc. Such a structure makes the relay so large 
that it is difficult to build such a relay in a small-sized tester. 
SUMMARY OF THE INVENTION 
One of the objects of the present invention is to eliminate the 
above-described disadvantages of the conventional techniques in order to 
provide testers having devices which are capable of rapidly and completely 
protecting its measuring circuits without exerting any reducing effects on 
its measurement accuracy. 
Another object of the present invention is to provide testers with improved 
manageability in that the restoring operation does not require any manual 
operation. 
Still another object of the present invention is to provide testers which 
are small in size, have high reliability, and are free of faulty operation 
due to accidental exertion of external forces. 
The above objects of the present invention have been fulfilled through 
construction of testers which have the first and second stable states and 
which are provided with two devices: an overcurrent detecting device which 
allows the transition from the first to the second stable state when the 
galvanometer detects a current which exceeds the specified value; a 
circuit cut-off device which, in response to transition from the first to 
the second stable state, cuts off the line between the input terminal and 
the network for the measuring unit. 
In one embodiment of the present invention, an example tester is provided 
wherein the transition to the second stable state is allowed by initiation 
of a time-limiting operation, which causes the overcurrent detecting 
device, after a lapse of a prescribed time, to shift into the first stable 
state, thereby closing the line between the input terminal and the network 
of the measuring unit. 
In another example tester of the present invention, the transition to the 
second stable state is followed by the actuation of an alarm on the 
outside of the tester. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, an example tester of the present invention is 
illustrated. The tester comprises a pair of input terminals 10 and 12 for 
receiving inputs to be measured, a measuring-device network 16 connected 
with the pair of input terminals via a fuse 14 and a break contact point 
r1, and a galvanometer 22 connected via the resistor R1 with the output 
terminals 18 and 20 which are also the leads of the network 16. The 
galvanometer 22 may be a common instrument, e.g., a meter, which gives a 
visual indication of value of voltage, current, or resistance 
corresponding to the flow of current therethrough. The measuring-device 
network 16, one of the major components of the tester, may be a common 
circuit which is capable of converting various forms of input signals 
generated at the input terminals 10 and 12 into those signals which are 
suitable in form and level for the action of the galvanometer 22. The fuse 
14 is provided as a redundant component to cut off overcurrent and may be 
omitted. The circuit of FIG. 1 employs the separate representation method 
to denote coils and contacts of relays, in such a way that the relay coil 
is denoted with the upper case letters of the alphabet and the contact 
driven by that coil is denoted by the corresponding lower case letter and 
that, if one coil operates more than one contact, serial numbers are 
attached. For example, the contact r1 is the first contact which is put 
into operation when the relay coil R is energized. 
A pair of zener diodes ZD1 and ZD2 in opposed-parallel connection and the 
diode rectifying circuit 24 are connected with the leads 18 and 20. The 
terminal 26, one of the output terminals of the circuit 24, is connected 
with the negative common splice 26' of the dc power source E and the other 
output terminal 28 is connected with the base electrode 30 of the NPN 
transistor Q1 via the resistor R2. The emitter electrode 32 of the 
transistor Q1 is connected with the common splice 26' and the collector 
electrode 34 is connected with the base electrode 36 of the PNP transistor 
Q2 via the resistor R3. The emitter electrode 40 of the transistor Q2 is 
connected with the positive pole of the power source E. The resistor R2 is 
connected via the diode D1 to the parallel combination of the condenser C1 
and the break contact r2 and is further connected via the resistor R4 to 
the collector electrode 38 of the transistor Q2. This collector electrode 
38 is also connected via the resistor R5 to the base electrode 44 of the 
NPN transistor Q3. The emitter electrode 46 of the transistor Q3 is 
connected to the common splice 26' and the collector electrode 48 is 
connected to the coil of the relay R. The resistor R6, R7, and R8 are the 
bias resistors of the transistors Q1, Q2, and Q3, respectively. When both 
the poles of the power source E are connected, via the make-contact r3 of 
the relay R, the buzzer BZ and the light-emitting diode D2 are actuated. 
The sequence of the operation of this tester is as follows. A current 
flowing through the galvanometer 22 and resistor R1 gives a voltage in 
relation to the sum of the resistances of resistor R1 and the internal 
resistance of the galvanometer. 
If this voltage exceeds the zener voltage of the zener diode ZD1 or ZD2, it 
is clamped. When a mistaken operation has given the leads 18 and 20 a 
voltage having a reverse polarity, the rectifying circuit 24 prevents any 
unfavorable effects from propagating beyond the circuit 24 to the right 
section. 
When an overcurrent happens to flow through the galvanometer 22, causing 
the rectifying circuit via the resistor R2 to pass a current over the 
prescribed threshold value, the transistor Q1 will pass the current. This 
passage of current lowers the voltages of the collector electrode 34 of 
the transistor Q1 and the base electrode 36 of the transistor Q2, causing 
the transistor Q2 to pass current, which in turn raises the voltages of 
the collector electrode 38 of the transistor Q2 and the base electrode 44 
of the transistor Q3, causing the transistor Q3 to pass current. Thus, the 
relay R is energized. 
The excitation of the relay R causes the break contact r1 to be put into 
operation with a successive cut-off of the input circuit of the tester. 
Thus, the overcurrent is prevented from entering the measuring-device 
network 16 and the galvanometer 22. Simultaneously, the second break 
contact r2 is caused to open, allowing the condenser C1 to be gradually 
charged by the use of the path connecting the positive pole 42 of the 
power source E, the emitter-collector route of the transistor Q2 in 
operation, the resistor R4, the diode D1, and the resistor R6. Until the 
condenser C1 has been completely charged, both the positive voltage 
supplied from the source E via this charging path and the voltage 
accumulated at the condenser C2 serve to keep the transistor Q1 
conductive, successively permitting the transistor Q2 to be kept 
conductive. However, when the contact r1 is put into operation resulting 
in the cut-off of the input into the lead 18 and the complete charging of 
the condenser C1, this lowers the base voltage of the transistor Q1, the 
transistor Q1 is put into a cut-off (non-conductive) state and, 
simultaneously, the transistor Q2 is made non-conductive. This sequence 
successively causes the transistor Q3 to become non-conductive, resulting 
in the restoration of the relay R. 
The restoration of the relay R is accompanied by the closing of the break 
contact r2 resulting in the conduction of current therethrough, which is 
followed by an instantaneous discharge of the accumulated charges of the 
condenser C1. 
When the condenser C1 has completed its discharge, the relay R is restored 
and the apparatus is returned to its initial state. No problems are left 
if, by this time, the cause for making the current excessive has been 
removed, for example, by selecting a higher measurement range; otherwise, 
the apparatus will repeat the same troubled operation. 
The example tester shown in the drawing is equipped with a circuit capable 
of attracting a user's attention to the detected overcurrent state. This 
circuit comprises the buzzer BZ issuing an audible alarm and a 
light-emitting element for providing a visual display, such as the 
light-emitting diode D2 or a lamp. These are all driven in connection with 
the make contact r3 of the relay R. 
Having the above-described structure, the tester from the present invention 
may respond rapidly to the flow of overcurrent with the effective 
protection afforded to the measurement circuit of the present invention; 
no overcurrent will be imposed continuously for a long period of time on 
the measurement circuit. The apparatus is manageable since it requires no 
manual reset operation. In addition, it requires no special mechanical 
parts such as a latching relay, and may be packaged on a printed-circuit 
board since it uses electronic circuits composed of common separate parts 
and/or integrated circuits. The present invention may, therefore, offer 
for small-sized high-reliability testers a fault-free operation, providing 
adequate protection against actions caused by an accidentally applied 
external force. 
The present invention has been described in detail by the use of an 
appropriate example application. The scope of application of the present 
invention is, however, not limited to this example application, but may of 
course cover a variety of modifications within the range in conformity 
with the spirit of the present invention.